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5. String and Array Utilities

Operations on strings (or arrays of characters) are an important part of many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It's fairly common for beginning C programmers to "reinvent the wheel" by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you're less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too.


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5.1 Representation of Strings

This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.

A string is an array of char objects. But string-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else--in an array variable, a string constant, or dynamically allocated memory (see section Allocating Storage For Program Data). It's up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a null pointer in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

"string" normally refers to multibyte character strings as opposed to wide character strings. Wide character strings are arrays of type wchar_t and as for multibyte character strings usually pointers of type wchar_t * are used.

By convention, a null character, '\0', marks the end of a multibyte character string and the null wide character, L'\0', marks the end of a wide character string. For example, in testing to see whether the char * variable p points to a null character marking the end of a string, you can write !*p or *p == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

String literals appear in C program source as strings of characters between double-quote characters (`"') where the initial double-quote character is immediately preceded by a capital `L' (ell) character (as in L"foo"). In ISO C, string literals can also be formed by string concatenation: "a" "b" is the same as "ab". For wide character strings one can either use L"a" L"b" or L"a" "b". Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either. It's generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character.

A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.

Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see Introduction to Extended Characters). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.

But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes a call to strncpy could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte sequence in the target buffer.

To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see section Introduction to Extended Characters). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied.

The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available.


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5.2 String and Array Conventions

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.

Functions that operate on arbitrary blocks of memory have names beginning with `mem' and `wmem' (such as memcpy and wmemcpy) and invariably take an argument which specifies the size (in bytes and wide characters respectively) of the block of memory to operate on. The array arguments and return values for these functions have type void * or wchar_t. As a matter of style, the elements of the arrays used with the `mem' functions are referred to as "bytes". You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument. Parameters to the `wmem' functions must be of type wchar_t *. These functions are not really usable with anything but arrays of this type.

In contrast, functions that operate specifically on strings and wide character strings have names beginning with `str' and `wcs' respectively (such as strcpy and wcscpy) and look for a null character to terminate the string instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.) The array arguments and return values for these functions have type char * and wchar_t * respectively, and the array elements are referred to as "characters" and "wide characters".

In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size.

Some of the memory and string functions take single characters as arguments. Since a value of type char is automatically promoted into an value of type int when used as a parameter, the functions are declared with int as the type of the parameter in question. In case of the wide character function the situation is similarly: the parameter type for a single wide character is wint_t and not wchar_t. This would for many implementations not be necessary since the wchar_t is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types the wint_t type is used.


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5.3 String Length

You can get the length of a string using the strlen function. This function is declared in the header file `string.h'.

Function: size_t strlen (const char *s)

The strlen function returns the length of the null-terminated string s in bytes. (In other words, it returns the offset of the terminating null character within the array.)

For example,

 
strlen ("hello, world")
    ⇒ 12

When applied to a character array, the strlen function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character array that holds a string using the sizeof operator:

 
char string[32] = "hello, world";
sizeof (string)
    ⇒ 32
strlen (string)
    ⇒ 12

But beware, this will not work unless string is the character array itself, not a pointer to it. For example:

 
char string[32] = "hello, world";
char *ptr = string;
sizeof (string)
    ⇒ 32
sizeof (ptr)
    ⇒ 4  /* (on a machine with 4 byte pointers) */

This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.

It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and wcslen can be used or something like the following code can be used:

 
/* The input is in string.
   The length is expected in n.  */
{
  mbstate_t t;
  char *scopy = string;
  /* In initial state.  */
  memset (&t, '\0', sizeof (t));
  /* Determine number of characters.  */
  n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
}

This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.

The wide character equivalent is declared in `wchar.h'.

Function: size_t wcslen (const wchar_t *ws)

The wcslen function is the wide character equivalent to strlen. The return value is the number of wide characters in the wide character string pointed to by ws (this is also the offset of the terminating null wide character of ws).

Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.

This function was introduced in Amendment 1 to ISO C90.

Function: size_t strnlen (const char *s, size_t maxlen)

The strnlen function returns the length of the string s in bytes if this length is smaller than maxlen bytes. Otherwise it returns maxlen. Therefore this function is equivalent to (strlen (s) < n ? strlen (s) : maxlen) but it is more efficient and works even if the string s is not null-terminated.

 
char string[32] = "hello, world";
strnlen (string, 32)
    ⇒ 12
strnlen (string, 5)
    ⇒ 5

This function is a GNU extension and is declared in `string.h'.

Function: size_t wcsnlen (const wchar_t *ws, size_t maxlen)

wcsnlen is the wide character equivalent to strnlen. The maxlen parameter specifies the maximum number of wide characters.

This function is a GNU extension and is declared in `wchar.h'.


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5.4 Copying and Concatenation

You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file `string.h' while the `wstr' and `wmem' functions are declared in the file `wchar.h'.

A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.

Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like sprintf (see section Formatted Output Functions) and scanf (see section Formatted Input Functions).

Function: void * memcpy (void *restrict to, const void *restrict from, size_t size)

The memcpy function copies size bytes from the object beginning at from into the object beginning at to. The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

The value returned by memcpy is the value of to.

Here is an example of how you might use memcpy to copy the contents of an array:

 
struct foo *oldarray, *newarray;
int arraysize;
…
memcpy (new, old, arraysize * sizeof (struct foo));
Function: wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

The wmemcpy function copies size wide characters from the object beginning at wfrom into the object beginning at wto. The behavior of this function is undefined if the two arrays wto and wfrom overlap; use wmemmove instead if overlapping is possible.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

 
wchar_t *
wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
}

The value returned by wmemcpy is the value of wto.

This function was introduced in Amendment 1 to ISO C90.

Function: void * mempcpy (void *restrict to, const void *restrict from, size_t size)

The mempcpy function is nearly identical to the memcpy function. It copies size bytes from the object beginning at from into the object pointed to by to. But instead of returning the value of to it returns a pointer to the byte following the last written byte in the object beginning at to. I.e., the value is ((void *) ((char *) to + size)).

This function is useful in situations where a number of objects shall be copied to consecutive memory positions.

 
void *
combine (void *o1, size_t s1, void *o2, size_t s2)
{
  void *result = malloc (s1 + s2);
  if (result != NULL)
    mempcpy (mempcpy (result, o1, s1), o2, s2);
  return result;
}

This function is a GNU extension.

Function: wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

The wmempcpy function is nearly identical to the wmemcpy function. It copies size wide characters from the object beginning at wfrom into the object pointed to by wto. But instead of returning the value of wto it returns a pointer to the wide character following the last written wide character in the object beginning at wto. I.e., the value is wto + size.

This function is useful in situations where a number of objects shall be copied to consecutive memory positions.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

 
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}

This function is a GNU extension.

Function: void * memmove (void *to, const void *from, size_t size)

memmove copies the size bytes at from into the size bytes at to, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the bytes in the block at from, including those bytes which also belong to the block at to.

The value returned by memmove is the value of to.

Function: wchar_t * wmemmove (wchar *wto, const wchar_t *wfrom, size_t size)

wmemmove copies the size wide characters at wfrom into the size wide characters at wto, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the wide characters in the block at wfrom, including those wide characters which also belong to the block at wto.

The following is a possible implementation of wmemcpy but there are more optimizations possible.

 
wchar_t *
wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
          size_t size)
{
  return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
}

The value returned by wmemmove is the value of wto.

This function is a GNU extension.

Function: void * memccpy (void *restrict to, const void *restrict from, int c, size_t size)

This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.

Function: void * memset (void *block, int c, size_t size)

This function copies the value of c (converted to an unsigned char) into each of the first size bytes of the object beginning at block. It returns the value of block.

Function: wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size)

This function copies the value of wc into each of the first size wide characters of the object beginning at block. It returns the value of block.

Function: char * strcpy (char *restrict to, const char *restrict from)

This copies characters from the string from (up to and including the terminating null character) into the string to. Like memcpy, this function has undefined results if the strings overlap. The return value is the value of to.

Function: wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

This copies wide characters from the string wfrom (up to and including the terminating null wide character) into the string wto. Like wmemcpy, this function has undefined results if the strings overlap. The return value is the value of wto.

Function: char * strncpy (char *restrict to, const char *restrict from, size_t size)

This function is similar to strcpy but always copies exactly size characters into to.

If the length of from is more than size, then strncpy copies just the first size characters. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then strncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is specified by the ISO C standard.

The behavior of strncpy is undefined if the strings overlap.

Using strncpy as opposed to strcpy is a way to avoid bugs relating to writing past the end of the allocated space for to. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, size may be large, and when it is, strncpy will waste a considerable amount of time copying null characters.

Function: wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is similar to wcscpy but always copies exactly size wide characters into wto.

If the length of wfrom is more than size, then wcsncpy copies just the first size wide characters. Note that in this case there is no null terminator written into wto.

If the length of wfrom is less than size, then wcsncpy copies all of wfrom, followed by enough null wide characters to add up to size wide characters in all. This behavior is rarely useful, but it is specified by the ISO C standard.

The behavior of wcsncpy is undefined if the strings overlap.

Using wcsncpy as opposed to wcscpy is a way to avoid bugs relating to writing past the end of the allocated space for wto. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, size may be large, and when it is, wcsncpy will waste a considerable amount of time copying null wide characters.

Function: char * strdup (const char *s)

This function copies the null-terminated string s into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, strdup returns a null pointer. Otherwise it returns a pointer to the new string.

Function: wchar_t * wcsdup (const wchar_t *ws)

This function copies the null-terminated wide character string ws into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, wcsdup returns a null pointer. Otherwise it returns a pointer to the new wide character string.

This function is a GNU extension.

Function: char * strndup (const char *s, size_t size)

This function is similar to strdup but always copies at most size characters into the newly allocated string.

If the length of s is more than size, then strndup copies just the first size characters and adds a closing null terminator. Otherwise all characters are copied and the string is terminated.

This function is different to strncpy in that it always terminates the destination string.

strndup is a GNU extension.

Function: char * stpcpy (char *restrict to, const char *restrict from)

This function is like strcpy, except that it returns a pointer to the end of the string to (that is, the address of the terminating null character to + strlen (from)) rather than the beginning.

For example, this program uses stpcpy to concatenate `foo' and `bar' to produce `foobar', which it then prints.

 
#include <string.h>
#include <stdio.h>

int
main (void)
{
  char buffer[10];
  char *to = buffer;
  to = stpcpy (to, "foo");
  to = stpcpy (to, "bar");
  puts (buffer);
  return 0;
}

This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.

Its behavior is undefined if the strings overlap. The function is declared in `string.h'.

Function: wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

This function is like wcscpy, except that it returns a pointer to the end of the string wto (that is, the address of the terminating null character wto + strlen (wfrom)) rather than the beginning.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

The behavior of wcpcpy is undefined if the strings overlap.

wcpcpy is a GNU extension and is declared in `wchar.h'.

Function: char * stpncpy (char *restrict to, const char *restrict from, size_t size)

This function is similar to stpcpy but copies always exactly size characters into to.

If the length of from is more then size, then stpncpy copies just the first size characters and returns a pointer to the character directly following the one which was copied last. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then stpncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the strncpy is used. stpncpy returns a pointer to the first written null character.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap. The function is declared in `string.h'.

Function: wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is similar to wcpcpy but copies always exactly wsize characters into wto.

If the length of wfrom is more then size, then wcpncpy copies just the first size wide characters and returns a pointer to the wide character directly following the last non-null wide character which was copied last. Note that in this case there is no null terminator written into wto.

If the length of wfrom is less than size, then wcpncpy copies all of wfrom, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the wcsncpy is used. wcpncpy returns a pointer to the first written null character.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap.

wcpncpy is a GNU extension and is declared in `wchar.h'.

Macro: char * strdupa (const char *s)

This macro is similar to strdup but allocates the new string using alloca instead of malloc (see section Automatic Storage with Variable Size). This means of course the returned string has the same limitations as any block of memory allocated using alloca.

For obvious reasons strdupa is implemented only as a macro; you cannot get the address of this function. Despite this limitation it is a useful function. The following code shows a situation where using malloc would be a lot more expensive.

 
#include <paths.h>
#include <string.h>
#include <stdio.h>

const char path[] = _PATH_STDPATH;

int
main (void)
{
  char *wr_path = strdupa (path);
  char *cp = strtok (wr_path, ":");

  while (cp != NULL)
    {
      puts (cp);
      cp = strtok (NULL, ":");
    }
  return 0;
}

Please note that calling strtok using path directly is invalid. It is also not allowed to call strdupa in the argument list of strtok since strdupa uses alloca (see section Automatic Storage with Variable Size) can interfere with the parameter passing.

This function is only available if GNU CC is used.

Macro: char * strndupa (const char *s, size_t size)

This function is similar to strndup but like strdupa it allocates the new string using alloca see section Automatic Storage with Variable Size. The same advantages and limitations of strdupa are valid for strndupa, too.

This function is implemented only as a macro, just like strdupa. Just as strdupa this macro also must not be used inside the parameter list in a function call.

strndupa is only available if GNU CC is used.

Function: char * strcat (char *restrict to, const char *restrict from)

The strcat function is similar to strcpy, except that the characters from from are concatenated or appended to the end of to, instead of overwriting it. That is, the first character from from overwrites the null character marking the end of to.

An equivalent definition for strcat would be:

 
char *
strcat (char *restrict to, const char *restrict from)
{
  strcpy (to + strlen (to), from);
  return to;
}

This function has undefined results if the strings overlap.

Function: wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom)

The wcscat function is similar to wcscpy, except that the characters from wfrom are concatenated or appended to the end of wto, instead of overwriting it. That is, the first character from wfrom overwrites the null character marking the end of wto.

An equivalent definition for wcscat would be:

 
wchar_t *
wcscat (wchar_t *wto, const wchar_t *wfrom)
{
  wcscpy (wto + wcslen (wto), wfrom);
  return wto;
}

This function has undefined results if the strings overlap.

Programmers using the strcat or wcscat function (or the following strncat or wcsncar functions for that matter) can easily be recognized as lazy and reckless. In almost all situations the lengths of the participating strings are known (it better should be since how can one otherwise ensure the allocated size of the buffer is sufficient?) Or at least, one could know them if one keeps track of the results of the various function calls. But then it is very inefficient to use strcat/wcscat. A lot of time is wasted finding the end of the destination string so that the actual copying can start. This is a common example:

 
/* This function concatenates arbitrarily many strings.  The last
   parameter must be NULL.  */
char *
concat (const char *str, …)
{
  va_list ap, ap2;
  size_t total = 1;
  const char *s;
  char *result;

  va_start (ap, str);
  /* Actually va_copy, but this is the name more gcc versions
     understand.  */
  __va_copy (ap2, ap);

  /* Determine how much space we need.  */
  for (s = str; s != NULL; s = va_arg (ap, const char *))
    total += strlen (s);

  va_end (ap);

  result = (char *) malloc (total);
  if (result != NULL)
    {
      result[0] = '\0';

      /* Copy the strings.  */
      for (s = str; s != NULL; s = va_arg (ap2, const char *))
        strcat (result, s);
    }

  va_end (ap2);

  return result;
}

This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient:

 
char *
concat (const char *str, …)
{
  va_list ap;
  size_t allocated = 100;
  char *result = (char *) malloc (allocated);

  if (result != NULL)
    {
      char *newp;
      char *wp;

      va_start (ap, str);

      wp = result;
      for (s = str; s != NULL; s = va_arg (ap, const char *))
        {
          size_t len = strlen (s);

          /* Resize the allocated memory if necessary.  */
          if (wp + len + 1 > result + allocated)
            {
              allocated = (allocated + len) * 2;
              newp = (char *) realloc (result, allocated);
              if (newp == NULL)
                {
                  free (result);
                  return NULL;
                }
              wp = newp + (wp - result);
              result = newp;
            }

          wp = mempcpy (wp, s, len);
        }

      /* Terminate the result string.  */
      *wp++ = '\0';

      /* Resize memory to the optimal size.  */
      newp = realloc (result, wp - result);
      if (newp != NULL)
        result = newp;

      va_end (ap);
    }

  return result;
}

With a bit more knowledge about the input strings one could fine-tune the memory allocation. The difference we are pointing to here is that we don't use strcat anymore. We always keep track of the length of the current intermediate result so we can safe us the search for the end of the string and use mempcpy. Please note that we also don't use stpcpy which might seem more natural since we handle with strings. But this is not necessary since we already know the length of the string and therefore can use the faster memory copying function. The example would work for wide characters the same way.

Whenever a programmer feels the need to use strcat she or he should think twice and look through the program whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use strcat.

Function: char * strncat (char *restrict to, const char *restrict from, size_t size)

This function is like strcat except that not more than size characters from from are appended to the end of to. A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The strncat function could be implemented like this:

 
char *
strncat (char *to, const char *from, size_t size)
{
  to[strlen (to) + size] = '\0';
  strncpy (to + strlen (to), from, size);
  return to;
}

The behavior of strncat is undefined if the strings overlap.

Function: wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is like wcscat except that not more than size characters from from are appended to the end of to. A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The wcsncat function could be implemented like this:

 
wchar_t *
wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
         size_t size)
{
  wto[wcslen (to) + size] = L'\0';
  wcsncpy (wto + wcslen (wto), wfrom, size);
  return wto;
}

The behavior of wcsncat is undefined if the strings overlap.

Here is an example showing the use of strncpy and strncat (the wide character version is equivalent). Notice how, in the call to strncat, the size parameter is computed to avoid overflowing the character array buffer.

 
#include <string.h>
#include <stdio.h>

#define SIZE 10

static char buffer[SIZE];

main ()
{
  strncpy (buffer, "hello", SIZE);
  puts (buffer);
  strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
  puts (buffer);
}

The output produced by this program looks like:

 
hello
hello, wo
Function: void bcopy (const void *from, void *to, size_t size)

This is a partially obsolete alternative for memmove, derived from BSD. Note that it is not quite equivalent to memmove, because the arguments are not in the same order and there is no return value.

Function: void bzero (void *block, size_t size)

This is a partially obsolete alternative for memset, derived from BSD. Note that it is not as general as memset, because the only value it can store is zero.


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5.5 String/Array Comparison

You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See section Searching and Sorting, for an example of this.

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is "less" than the second, while a positive value indicates that the first string is "greater".

The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.

All of these functions are declared in the header file `string.h'.

Function: int memcmp (const void *a1, const void *a2, size_t size)

The function memcmp compares the size bytes of memory beginning at a1 against the size bytes of memory beginning at a2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

If the contents of the two blocks are equal, memcmp returns 0.

Function: int wmemcmp (const wchar_t *a1, const wchar_t *a2, size_t size)

The function wmemcmp compares the size wide characters beginning at a1 against the size wide characters beginning at a2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is a1 is smaller or larger than the corresponding character in a2.

If the contents of the two blocks are equal, wmemcmp returns 0.

On arbitrary arrays, the memcmp function is mostly useful for testing equality. It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers.

wmemcmp is really only useful to compare arrays of type wchar_t since the function looks at sizeof (wchar_t) bytes at a time and this number of bytes is system dependent.

You should also be careful about using memcmp to compare objects that can contain "holes", such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size. The contents of these "holes" are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison.

For example, given a structure type definition like:

 
struct foo
  {
    unsigned char tag;
    union
      {
        double f;
        long i;
        char *p;
      } value;
  };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

Function: int strcmp (const char *s1, const char *s2)

The strcmp function compares the string s1 against s2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as unsigned char objects, then promoted to int).

If the two strings are equal, strcmp returns 0.

A consequence of the ordering used by strcmp is that if s1 is an initial substring of s2, then s1 is considered to be "less than" s2.

strcmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use strcoll.

Function: int wcscmp (const wchar_t *ws1, const wchar_t *ws2)

The wcscmp function compares the wide character string ws1 against ws2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is ws1 is smaller or larger than the corresponding character in ws2.

If the two strings are equal, wcscmp returns 0.

A consequence of the ordering used by wcscmp is that if ws1 is an initial substring of ws2, then ws1 is considered to be "less than" ws2.

wcscmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use wcscoll.

Function: int strcasecmp (const char *s1, const char *s2)

This function is like strcmp, except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

strcasecmp is derived from BSD.

Function: int wcscasecmp (const wchar_t *ws1, const wchar_T *ws2)

This function is like wcscmp, except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

wcscasecmp is a GNU extension.

Function: int strncmp (const char *s1, const char *s2, size_t size)

This function is the similar to strcmp, except that no more than size characters are compared. In other words, if the two strings are the same in their first size characters, the return value is zero.

Function: int wcsncmp (const wchar_t *ws1, const wchar_t *ws2, size_t size)

This function is the similar to wcscmp, except that no more than size wide characters are compared. In other words, if the two strings are the same in their first size wide characters, the return value is zero.

Function: int strncasecmp (const char *s1, const char *s2, size_t n)

This function is like strncmp, except that differences in case are ignored. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related.

strncasecmp is a GNU extension.

Function: int wcsncasecmp (const wchar_t *ws1, const wchar_t *s2, size_t n)

This function is like wcsncmp, except that differences in case are ignored. Like wcscasecmp, it is locale dependent how uppercase and lowercase characters are related.

wcsncasecmp is a GNU extension.

Here are some examples showing the use of strcmp and strncmp (equivalent examples can be constructed for the wide character functions). These examples assume the use of the ASCII character set. (If some other character set--say, EBCDIC--is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.)

 
strcmp ("hello", "hello")
    ⇒ 0    /* These two strings are the same. */
strcmp ("hello", "Hello")
    ⇒ 32   /* Comparisons are case-sensitive. */
strcmp ("hello", "world")
    ⇒ -15  /* The character 'h' comes before 'w'. */
strcmp ("hello", "hello, world")
    ⇒ -44  /* Comparing a null character against a comma. */
strncmp ("hello", "hello, world", 5)
    ⇒ 0    /* The initial 5 characters are the same. */
strncmp ("hello, world", "hello, stupid world!!!", 5)
    ⇒ 0    /* The initial 5 characters are the same. */
Function: int strverscmp (const char *s1, const char *s2)

The strverscmp function compares the string s1 against s2, considering them as holding indices/version numbers. Return value follows the same conventions as found in the strverscmp function. In fact, if s1 and s2 contain no digits, strverscmp behaves like strcmp.

Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them:

 
strverscmp ("no digit", "no digit")
    ⇒ 0    /* same behavior as strcmp. */
strverscmp ("item#99", "item#100")
    ⇒ <0   /* same prefix, but 99 < 100. */
strverscmp ("alpha1", "alpha001")
    ⇒ >0   /* fractional part inferior to integral one. */
strverscmp ("part1_f012", "part1_f01")
    ⇒ >0   /* two fractional parts. */
strverscmp ("foo.009", "foo.0")
    ⇒ <0   /* idem, but with leading zeroes only. */

This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers.

strverscmp is a GNU extension.

Function: int bcmp (const void *a1, const void *a2, size_t size)

This is an obsolete alias for memcmp, derived from BSD.


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5.6 Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.

You can use the functions strcoll and strxfrm (declared in the headers file `string.h') and wcscoll and wcsxfrm (declared in the headers file `wchar') to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category; see Locales and Internationalization.

In the standard C locale, the collation sequence for strcoll is the same as that for strcmp. Similarly, wcscoll and wcscmp are the same in this situation.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.

The functions strcoll and wcscoll perform this translation implicitly, in order to do one comparison. By contrast, strxfrm and wcsxfrm perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm or wcsxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp or wcscmp.

Function: int strcoll (const char *s1, const char *s2)

The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Function: int wcscoll (const wchar_t *ws1, const wchar_t *ws2)

The wcscoll function is similar to wcscmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them. The actual sort algorithm is not written here; it comes from qsort (see section Array Sort Function). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using strxfrm.)

 
/* This is the comparison function used with qsort. */

int
compare_elements (char **p1, char **p2)
{
  return strcoll (*p1, *p2);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings (char **array, int nstrings)
{
  /* Sort temp_array by comparing the strings. */
  qsort (array, nstrings,
         sizeof (char *), compare_elements);
}
Function: size_t strxfrm (char *restrict to, const char *restrict from, size_t size)

The function strxfrm transforms the string from using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to. Up to size characters (including a terminating null character) are stored.

The behavior is undefined if the strings to and from overlap; see Copying and Concatenation.

The return value is the length of the entire transformed string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed string did not entirely fit in the array to. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call strxfrm again with a bigger output array.

The transformed string may be longer than the original string, and it may also be shorter.

If size is zero, no characters are stored in to. In this case, strxfrm simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what to is if size is zero; to may even be a null pointer.

Function: size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)

The function wcsxfrm transforms wide character string wfrom using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array wto. Up to size wide characters (including a terminating null character) are stored.

The behavior is undefined if the strings wto and wfrom overlap; see Copying and Concatenation.

The return value is the length of the entire transformed wide character string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed wide character string did not entirely fit in the array wto. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call wcsxfrm again with a bigger output array.

The transformed wide character string may be longer than the original wide character string, and it may also be shorter.

If size is zero, no characters are stored in to. In this case, wcsxfrm simply returns the number of wide characters that would be the length of the transformed wide character string. This is useful for determining what size the allocated array should be (remember to multiply with sizeof (wchar_t)). It does not matter what wto is if size is zero; wto may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings.

 
struct sorter { char *input; char *transformed; };

/* This is the comparison function used with qsort
   to sort an array of struct sorter. */

int
compare_elements (struct sorter *p1, struct sorter *p2)
{
  return strcmp (p1->transformed, p2->transformed);
}

/* This is the entry point---the function to sort
   strings using the locale's collating sequence. */

void
sort_strings_fast (char **array, int nstrings)
{
  struct sorter temp_array[nstrings];
  int i;

  /* Set up temp_array.  Each element contains
     one input string and its transformed string. */
  for (i = 0; i < nstrings; i++)
    {
      size_t length = strlen (array[i]) * 2;
      char *transformed;
      size_t transformed_length;

      temp_array[i].input = array[i];

      /* First try a buffer perhaps big enough.  */
      transformed = (char *) xmalloc (length);

      /* Transform array[i].  */
      transformed_length = strxfrm (transformed, array[i], length);

      /* If the buffer was not large enough, resize it
         and try again.  */
      if (transformed_length >= length)
        {
          /* Allocate the needed space. +1 for terminating
             NUL character.  */
          transformed = (char *) xrealloc (transformed,
                                           transformed_length + 1);

          /* The return value is not interesting because we know
             how long the transformed string is.  */
          (void) strxfrm (transformed, array[i],
                          transformed_length + 1);
        }

      temp_array[i].transformed = transformed;
    }

  /* Sort temp_array by comparing transformed strings. */
  qsort (temp_array, sizeof (struct sorter),
         nstrings, compare_elements);

  /* Put the elements back in the permanent array
     in their sorted order. */
  for (i = 0; i < nstrings; i++)
    array[i] = temp_array[i].input;

  /* Free the strings we allocated. */
  for (i = 0; i < nstrings; i++)
    free (temp_array[i].transformed);
}

The interesting part of this code for the wide character version would look like this:

 
void
sort_strings_fast (wchar_t **array, int nstrings)
{
  …
      /* Transform array[i].  */
      transformed_length = wcsxfrm (transformed, array[i], length);

      /* If the buffer was not large enough, resize it
         and try again.  */
      if (transformed_length >= length)
        {
          /* Allocate the needed space. +1 for terminating
             NUL character.  */
          transformed = (wchar_t *) xrealloc (transformed,
                                              (transformed_length + 1)
                                              * sizeof (wchar_t));

          /* The return value is not interesting because we know
             how long the transformed string is.  */
          (void) wcsxfrm (transformed, array[i],
                          transformed_length + 1);
        }
  …

Note the additional multiplication with sizeof (wchar_t) in the realloc call.

Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.


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5.7 Search Functions

This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file `string.h'.

Function: void * memchr (const void *block, int c, size_t size)

This function finds the first occurrence of the byte c (converted to an unsigned char) in the initial size bytes of the object beginning at block. The return value is a pointer to the located byte, or a null pointer if no match was found.

Function: wchar_t * wmemchr (const wchar_t *block, wchar_t wc, size_t size)

This function finds the first occurrence of the wide character wc in the initial size wide characters of the object beginning at block. The return value is a pointer to the located wide character, or a null pointer if no match was found.

Function: void * rawmemchr (const void *block, int c)

Often the memchr function is used with the knowledge that the byte c is available in the memory block specified by the parameters. But this means that the size parameter is not really needed and that the tests performed with it at runtime (to check whether the end of the block is reached) are not needed.

The rawmemchr function exists for just this situation which is surprisingly frequent. The interface is similar to memchr except that the size parameter is missing. The function will look beyond the end of the block pointed to by block in case the programmer made an error in assuming that the byte c is present in the block. In this case the result is unspecified. Otherwise the return value is a pointer to the located byte.

This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like

 
   rawmemchr (str, '\0')

will never go beyond the end of the string.

This function is a GNU extension.

Function: void * memrchr (const void *block, int c, size_t size)

The function memrchr is like memchr, except that it searches backwards from the end of the block defined by block and size (instead of forwards from the front).

This function is a GNU extension.

Function: char * strchr (const char *string, int c)

The strchr function finds the first occurrence of the character c (converted to a char) in the null-terminated string beginning at string. The return value is a pointer to the located character, or a null pointer if no match was found.

For example,

 
strchr ("hello, world", 'l')
    ⇒ "llo, world"
strchr ("hello, world", '?')
    ⇒ NULL

The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument. It would be better (but less portable) to use strchrnul in this case, though.

Function: wchar_t * wcschr (const wchar_t *wstring, int wc)

The wcschr function finds the first occurrence of the wide character wc in the null-terminated wide character string beginning at wstring. The return value is a pointer to the located wide character, or a null pointer if no match was found.

The terminating null character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide character string by specifying a null wude character as the value of the wc argument. It would be better (but less portable) to use wcschrnul in this case, though.

Function: char * strchrnul (const char *string, int c)

strchrnul is the same as strchr except that if it does not find the character, it returns a pointer to string's terminating null character rather than a null pointer.

This function is a GNU extension.

Function: wchar_t * wcschrnul (const wchar_t *wstring, wchar_t wc)

wcschrnul is the same as wcschr except that if it does not find the wide character, it returns a pointer to wide character string's terminating null wide character rather than a null pointer.

This function is a GNU extension.

One useful, but unusual, use of the strchr function is when one wants to have a pointer pointing to the NUL byte terminating a string. This is often written in this way:

 
  s += strlen (s);

This is almost optimal but the addition operation duplicated a bit of the work already done in the strlen function. A better solution is this:

 
  s = strchr (s, '\0');

There is no restriction on the second parameter of strchr so it could very well also be the NUL character. Those readers thinking very hard about this might now point out that the strchr function is more expensive than the strlen function since we have two abort criteria. This is right. But in the GNU C library the implementation of strchr is optimized in a special way so that strchr actually is faster.

Function: char * strrchr (const char *string, int c)

The function strrchr is like strchr, except that it searches backwards from the end of the string string (instead of forwards from the front).

For example,

 
strrchr ("hello, world", 'l')
    ⇒ "ld"
Function: wchar_t * wcsrchr (const wchar_t *wstring, wchar_t c)

The function wcsrchr is like wcschr, except that it searches backwards from the end of the string wstring (instead of forwards from the front).

Function: char * strstr (const char *haystack, const char *needle)

This is like strchr, except that it searches haystack for a substring needle rather than just a single character. It returns a pointer into the string haystack that is the first character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

For example,

 
strstr ("hello, world", "l")
    ⇒ "llo, world"
strstr ("hello, world", "wo")
    ⇒ "world"
Function: wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle)

This is like wcschr, except that it searches haystack for a substring needle rather than just a single wide character. It returns a pointer into the string haystack that is the first wide character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

Function: wchar_t * wcswcs (const wchar_t *haystack, const wchar_t *needle)

wcswcs is an deprecated alias for wcsstr. This is the name originally used in the X/Open Portability Guide before the Amendment 1 to ISO C90 was published.

Function: char * strcasestr (const char *haystack, const char *needle)

This is like strstr, except that it ignores case in searching for the substring. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related.

For example,

 
strcasestr ("hello, world", "L")
    ⇒ "llo, world"
strcasestr ("hello, World", "wo")
    ⇒ "World"
Function: void * memmem (const void *haystack, size_t haystack-len,
const void *needle, size_t needle-len)

This is like strstr, but needle and haystack are byte arrays rather than null-terminated strings. needle-len is the length of needle and haystack-len is the length of haystack.

This function is a GNU extension.

Function: size_t strspn (const char *string, const char *skipset)

The strspn ("string span") function returns the length of the initial substring of string that consists entirely of characters that are members of the set specified by the string skipset. The order of the characters in skipset is not important.

For example,

 
strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
    ⇒ 5

Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Function: size_t wcsspn (const wchar_t *wstring, const wchar_t *skipset)

The wcsspn ("wide character string span") function returns the length of the initial substring of wstring that consists entirely of wide characters that are members of the set specified by the string skipset. The order of the wide characters in skipset is not important.

Function: size_t strcspn (const char *string, const char *stopset)

The strcspn ("string complement span") function returns the length of the initial substring of string that consists entirely of characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

For example,

 
strcspn ("hello, world", " \t\n,.;!?")
    ⇒ 5

Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Function: size_t wcscspn (const wchar_t *wstring, const wchar_t *stopset)

The wcscspn ("wide character string complement span") function returns the length of the initial substring of wstring that consists entirely of wide characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

Function: char * strpbrk (const char *string, const char *stopset)

The strpbrk ("string pointer break") function is related to strcspn, except that it returns a pointer to the first character in string that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such character from stopset is found.

For example,

 
strpbrk ("hello, world", " \t\n,.;!?")
    ⇒ ", world"

Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Function: wchar_t * wcspbrk (const wchar_t *wstring, const wchar_t *stopset)

The wcspbrk ("wide character string pointer break") function is related to wcscspn, except that it returns a pointer to the first wide character in wstring that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such character from stopset is found.


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5.7.1 Compatibility String Search Functions

Function: char * index (const char *string, int c)

index is another name for strchr; they are exactly the same. New code should always use strchr since this name is defined in ISO C while index is a BSD invention which never was available on System V derived systems.

Function: char * rindex (const char *string, int c)

rindex is another name for strrchr; they are exactly the same. New code should always use strrchr since this name is defined in ISO C while rindex is a BSD invention which never was available on System V derived systems.


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5.8 Finding Tokens in a String

It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the strtok function, declared in the header file `string.h'.

Function: char * strtok (char *restrict newstring, const char *restrict delimiters)

A string can be split into tokens by making a series of calls to the function strtok.

The string to be split up is passed as the newstring argument on the first call only. The strtok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the newstring argument. Calling strtok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.

On the next call to strtok, the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to strtok.

If the end of the string newstring is reached, or if the remainder of string consists only of delimiter characters, strtok returns a null pointer.

Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Function: wchar_t * wcstok (wchar_t *newstring, const char *delimiters)

A string can be split into tokens by making a series of calls to the function wcstok.

The string to be split up is passed as the newstring argument on the first call only. The wcstok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same wide character string are indicated by passing a null pointer as the newstring argument. Calling wcstok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls wcstok behind your back (which would mess up this internal state information).

The delimiters argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string newstring is overwritten by a null wide character, and the pointer to the beginning of the token in newstring is returned.

On the next call to wcstok, the searching begins at the next wide character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to wcstok.

If the end of the wide character string newstring is reached, or if the remainder of string consists only of delimiter wide characters, wcstok returns a null pointer.

Note that "character" is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Warning: Since strtok and wcstok alter the string they is parsing, you should always copy the string to a temporary buffer before parsing it with strtok/wcstok (see section Copying and Concatenation). If you allow strtok or wcstok to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after strtok or wcstok has modified it, and it would not have the expected value.

The string that you are operating on might even be a constant. Then when strtok or wcstok tries to modify it, your program will get a fatal signal for writing in read-only memory. See section Program Error Signals. Even if the operation of strtok or wcstok would not require a modification of the string (e.g., if there is exactly one token) the string can (and in the GNU libc case will) be modified.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.

The functions strtok and wcstok are not reentrant. See section Signal Handling and Nonreentrant Functions, for a discussion of where and why reentrancy is important.

Here is a simple example showing the use of strtok.

 
#include <string.h>
#include <stddef.h>

…

const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *token, *cp;

…

cp = strdupa (string);                /* Make writable copy.  */
token = strtok (cp, delimiters);      /* token => "words" */
token = strtok (NULL, delimiters);    /* token => "separated" */
token = strtok (NULL, delimiters);    /* token => "by" */
token = strtok (NULL, delimiters);    /* token => "spaces" */
token = strtok (NULL, delimiters);    /* token => "and" */
token = strtok (NULL, delimiters);    /* token => "punctuation" */
token = strtok (NULL, delimiters);    /* token => NULL */

The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings.

Function: char * strtok_r (char *newstring, const char *delimiters, char **save_ptr)

Just like strtok, this function splits the string into several tokens which can be accessed by successive calls to strtok_r. The difference is that the information about the next token is stored in the space pointed to by the third argument, save_ptr, which is a pointer to a string pointer. Calling strtok_r with a null pointer for newstring and leaving save_ptr between the calls unchanged does the job without hindering reentrancy.

This function is defined in POSIX.1 and can be found on many systems which support multi-threading.

Function: char * strsep (char **string_ptr, const char *delimiter)

This function has a similar functionality as strtok_r with the newstring argument replaced by the save_ptr argument. The initialization of the moving pointer has to be done by the user. Successive calls to strsep move the pointer along the tokens separated by delimiter, returning the address of the next token and updating string_ptr to point to the beginning of the next token.

One difference between strsep and strtok_r is that if the input string contains more than one character from delimiter in a row strsep returns an empty string for each pair of characters from delimiter. This means that a program normally should test for strsep returning an empty string before processing it.

This function was introduced in 4.3BSD and therefore is widely available.

Here is how the above example looks like when strsep is used.

 
#include <string.h>
#include <stddef.h>

…

const char string[] = "words separated by spaces -- and, punctuation!";
const char delimiters[] = " .,;:!-";
char *running;
char *token;

…

running = strdupa (string);
token = strsep (&running, delimiters);    /* token => "words" */
token = strsep (&running, delimiters);    /* token => "separated" */
token = strsep (&running, delimiters);    /* token => "by" */
token = strsep (&running, delimiters);    /* token => "spaces" */
token = strsep (&running, delimiters);    /* token => "" */
token = strsep (&running, delimiters);    /* token => "" */
token = strsep (&running, delimiters);    /* token => "" */
token = strsep (&running, delimiters);    /* token => "and" */
token = strsep (&running, delimiters);    /* token => "" */
token = strsep (&running, delimiters);    /* token => "punctuation" */
token = strsep (&running, delimiters);    /* token => "" */
token = strsep (&running, delimiters);    /* token => NULL */
Function: char * basename (const char *filename)

The GNU version of the basename function returns the last component of the path in filename. This function is the preferred usage, since it does not modify the argument, filename, and respects trailing slashes. The prototype for basename can be found in `string.h'. Note, this function is overriden by the XPG version, if `libgen.h' is included.

Example of using GNU basename:

 
#include <string.h>

int
main (int argc, char *argv[])
{
  char *prog = basename (argv[0]);

  if (argc < 2)
    {
      fprintf (stderr, "Usage %s <arg>\n", prog);
      exit (1);
    }

  …
}

Portability Note: This function may produce different results on different systems.

Function: char * basename (char *path)

This is the standard XPG defined basename. It is similar in spirit to the GNU version, but may modify the path by removing trailing '/' characters. If the path is made up entirely of '/' characters, then "/" will be returned. Also, if path is NULL or an empty string, then "." is returned. The prototype for the XPG version can be found in `libgen.h'.

Example of using XPG basename:

 
#include <libgen.h>

int
main (int argc, char *argv[])
{
  char *prog;
  char *path = strdupa (argv[0]);

  prog = basename (path);

  if (argc < 2)
    {
      fprintf (stderr, "Usage %s <arg>\n", prog);
      exit (1);
    }

  …

}
Function: char * dirname (char *path)

The dirname function is the compliment to the XPG version of basename. It returns the parent directory of the file specified by path. If path is NULL, an empty string, or contains no '/' characters, then "." is returned. The prototype for this function can be found in `libgen.h'.


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5.9 strfry

The function below addresses the perennial programming quandary: "How do I take good data in string form and painlessly turn it into garbage?" This is actually a fairly simple task for C programmers who do not use the GNU C library string functions, but for programs based on the GNU C library, the strfry function is the preferred method for destroying string data.

The prototype for this function is in `string.h'.

Function: char * strfry (char *string)

strfry creates a pseudorandom anagram of a string, replacing the input with the anagram in place. For each position in the string, strfry swaps it with a position in the string selected at random (from a uniform distribution). The two positions may be the same.

The return value of strfry is always string.

Portability Note: This function is unique to the GNU C library.


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5.10 Trivial Encryption

The memfrob function converts an array of data to something unrecognizable and back again. It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text. The transformation is analogous to Usenet's "Rot13" encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, memfrob works on arbitrary binary data, not just text.

For true encryption, See section DES Encryption and Password Handling.

This function is declared in `string.h'.

Function: void * memfrob (void *mem, size_t length)

memfrob transforms (frobnicates) each byte of the data structure at mem, which is length bytes long, by bitwise exclusive oring it with binary 00101010. It does the transformation in place and its return value is always mem.

Note that memfrob a second time on the same data structure returns it to its original state.

This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See section DES Encryption and Password Handling.

Portability Note: This function is unique to the GNU C library.


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5.11 Encode Binary Data

To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.

Function: char * l64a (long int n)

This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 7 character buffer which contains an encoded version of n. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if n is zero, which is somewhat bizarre but mandated by the standard.
Warning: Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library.
Compatibility Note: The XPG standard states that the return value of l64a is undefined if n is negative. In the GNU implementation, l64a treats its argument as unsigned, so it will return a sensible encoding for any nonzero n; however, portable programs should not rely on this.

To encode a large buffer l64a must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this:

 
char *
encode (const void *buf, size_t len)
{
  /* We know in advance how long the buffer has to be. */
  unsigned char *in = (unsigned char *) buf;
  char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
  char *cp = out, *p;

  /* Encode the length. */
  /* Using `htonl' is necessary so that the data can be
     decoded even on machines with different byte order.
     `l64a' can return a string shorter than 6 bytes, so 
     we pad it with encoding of 0 ('.') at the end by 
     hand. */

  p = stpcpy (cp, l64a (htonl (len)));
  cp = mempcpy (p, "......", 6 - (p - cp));

  while (len > 3)
    {
      unsigned long int n = *in++;
      n = (n << 8) | *in++;
      n = (n << 8) | *in++;
      n = (n << 8) | *in++;
      len -= 4;
      p = stpcpy (cp, l64a (htonl (n)));
      cp = mempcpy (p, "......", 6 - (p - cp));
    }
  if (len > 0)
    {
      unsigned long int n = *in++;
      if (--len > 0)
        {
          n = (n << 8) | *in++;
          if (--len > 0)
            n = (n << 8) | *in;
        }
      cp = stpcpy (cp, l64a (htonl (n)));
    }
  *cp = '\0';
  return out;
}

It is strange that the library does not provide the complete functionality needed but so be it.

To decode data produced with l64a the following function should be used.

Function: long int a64l (const char *string)

The parameter string should contain a string which was produced by a call to l64a. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character not in the table, rather like atoi; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line characters.

The decoded number is returned as a long int value.

The l64a and a64l functions use a base 64 encoding, in which each character of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits:

0

1

2

3

4

5

6

7

0

.

/

0

1

2

3

4

5

8

6

7

8

9

A

B

C

D

16

E

F

G

H

I

J

K

L

24

M

N

O

P

Q

R

S

T

32

U

V

W

X

Y

Z

a

b

40

c

d

e

f

g

h

i

j

48

k

l

m

n

o

p

q

r

56

s

t

u

v

w

x

y

z

This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings.


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5.12 Argz and Envz Vectors

argz vectors are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters ('\0').

Envz vectors are an extension of argz vectors where each element is a name-value pair, separated by a '=' character (as in a Unix environment).


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5.12.1 Argz Functions

Each argz vector is represented by a pointer to the first element, of type char *, and a size, of type size_t, both of which can be initialized to 0 to represent an empty argz vector. All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified.

The argz functions use malloc/realloc to allocate/grow argz vectors, and so any argz vector creating using these functions may be freed by using free; conversely, any argz function that may grow a string expects that string to have been allocated using malloc (those argz functions that only examine their arguments or modify them in place will work on any sort of memory). See section Unconstrained Allocation.

All argz functions that do memory allocation have a return type of error_t, and return 0 for success, and ENOMEM if an allocation error occurs.

These functions are declared in the standard include file `argz.h'.

Function: error_t argz_create (char *const argv[], char **argz, size_t *argz_len)

The argz_create function converts the Unix-style argument vector argv (a vector of pointers to normal C strings, terminated by (char *)0; see section Program Arguments) into an argz vector with the same elements, which is returned in argz and argz_len.

Function: error_t argz_create_sep (const char *string, int sep, char **argz, size_t *argz_len)

The argz_create_sep function converts the null-terminated string string into an argz vector (returned in argz and argz_len) by splitting it into elements at every occurrence of the character sep.

Function: size_t argz_count (const char *argz, size_t arg_len)

Returns the number of elements in the argz vector argz and argz_len.

Function: void argz_extract (char *argz, size_t argz_len, char **argv)

The argz_extract function converts the argz vector argz and argz_len into a Unix-style argument vector stored in argv, by putting pointers to every element in argz into successive positions in argv, followed by a terminator of 0. Argv must be pre-allocated with enough space to hold all the elements in argz plus the terminating (char *)0 ((argz_count (argz, argz_len) + 1) * sizeof (char *) bytes should be enough). Note that the string pointers stored into argv point into argz--they are not copies--and so argz must be copied if it will be changed while argv is still active. This function is useful for passing the elements in argz to an exec function (see section Executing a File).

Function: void argz_stringify (char *argz, size_t len, int sep)

The argz_stringify converts argz into a normal string with the elements separated by the character sep, by replacing each '\0' inside argz (except the last one, which terminates the string) with sep. This is handy for printing argz in a readable manner.

Function: error_t argz_add (char **argz, size_t *argz_len, const char *str)

The argz_add function adds the string str to the end of the argz vector *argz, and updates *argz and *argz_len accordingly.

Function: error_t argz_add_sep (char **argz, size_t *argz_len, const char *str, int delim)

The argz_add_sep function is similar to argz_add, but str is split into separate elements in the result at occurrences of the character delim. This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of ':' for delim.

Function: error_t argz_append (char **argz, size_t *argz_len, const char *buf, size_t buf_len)

The argz_append function appends buf_len bytes starting at buf to the argz vector *argz, reallocating *argz to accommodate it, and adding buf_len to *argz_len.

Function: void argz_delete (char **argz, size_t *argz_len, char *entry)

If entry points to the beginning of one of the elements in the argz vector *argz, the argz_delete function will remove this entry and reallocate *argz, modifying *argz and *argz_len accordingly. Note that as destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as entry will then become invalid.

Function: error_t argz_insert (char **argz, size_t *argz_len, char *before, const char *entry)

The argz_insert function inserts the string entry into the argz vector *argz at a point just before the existing element pointed to by before, reallocating *argz and updating *argz and *argz_len. If before is 0, entry is added to the end instead (as if by argz_add). Since the first element is in fact the same as *argz, passing in *argz as the value of before will result in entry being inserted at the beginning.

Function: char * argz_next (char *argz, size_t argz_len, const char *entry)

The argz_next function provides a convenient way of iterating over the elements in the argz vector argz. It returns a pointer to the next element in argz after the element entry, or 0 if there are no elements following entry. If entry is 0, the first element of argz is returned.

This behavior suggests two styles of iteration:

 
    char *entry = 0;
    while ((entry = argz_next (argz, argz_len, entry)))
      action;

(the double parentheses are necessary to make some C compilers shut up about what they consider a questionable while-test) and:

 
    char *entry;
    for (entry = argz;
         entry;
         entry = argz_next (argz, argz_len, entry))
      action;

Note that the latter depends on argz having a value of 0 if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here.

Function: error_t argz_replace (char **argz, size_t *argz_len, const char *str, const char *with, unsigned *replace_count)

Replace any occurrences of the string str in argz with with, reallocating argz as necessary. If replace_count is non-zero, *replace_count will be incremented by number of replacements performed.


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5.12.2 Envz Functions

Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.

Each element in an envz vector is a name-value pair, separated by a '=' character; if multiple '=' characters are present in an element, those after the first are considered part of the value, and treated like all other non-'\0' characters.

If no '=' characters are present in an element, that element is considered the name of a "null" entry, as distinct from an entry with an empty value: envz_get will return 0 if given the name of null entry, whereas an entry with an empty value would result in a value of ""; envz_entry will still find such entries, however. Null entries can be removed with envz_strip function.

As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of error_t, and return either 0 or ENOMEM.

These functions are declared in the standard include file `envz.h'.

Function: char * envz_entry (const char *envz, size_t envz_len, const char *name)

The envz_entry function finds the entry in envz with the name name, and returns a pointer to the whole entry--that is, the argz element which begins with name followed by a '=' character. If there is no entry with that name, 0 is returned.

Function: char * envz_get (const char *envz, size_t envz_len, const char *name)

The envz_get function finds the entry in envz with the name name (like envz_entry), and returns a pointer to the value portion of that entry (following the '='). If there is no entry with that name (or only a null entry), 0 is returned.

Function: error_t envz_add (char **envz, size_t *envz_len, const char *name, const char *value)

The envz_add function adds an entry to *envz (updating *envz and *envz_len) with the name name, and value value. If an entry with the same name already exists in envz, it is removed first. If value is 0, then the new entry will the special null type of entry (mentioned above).

Function: error_t envz_merge (char **envz, size_t *envz_len, const char *envz2, size_t envz2_len, int override)

The envz_merge function adds each entry in envz2 to envz, as if with envz_add, updating *envz and *envz_len. If override is true, then values in envz2 will supersede those with the same name in envz, otherwise not.

Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.

Function: void envz_strip (char **envz, size_t *envz_len)

The envz_strip function removes any null entries from envz, updating *envz and *envz_len.


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