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15. Using GDB with Different Languages

Although programming languages generally have common aspects, they are rarely expressed in the same manner. For instance, in ANSI C, dereferencing a pointer p is accomplished by *p, but in Modula-2, it is accomplished by p^. Values can also be represented (and displayed) differently. Hex numbers in C appear as `0x1ae', while in Modula-2 they appear as `1AEH'.

Language-specific information is built into GDB for some languages, allowing you to express operations like the above in your program's native language, and allowing GDB to output values in a manner consistent with the syntax of your program's native language. The language you use to build expressions is called the working language.


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15.1 Switching Between Source Languages

There are two ways to control the working language--either have GDB set it automatically, or select it manually yourself. You can use the set language command for either purpose. On startup, GDB defaults to setting the language automatically. The working language is used to determine how expressions you type are interpreted, how values are printed, etc.

In addition to the working language, every source file that GDB knows about has its own working language. For some object file formats, the compiler might indicate which language a particular source file is in. However, most of the time GDB infers the language from the name of the file. The language of a source file controls whether C++ names are demangled--this way backtrace can show each frame appropriately for its own language. There is no way to set the language of a source file from within GDB, but you can set the language associated with a filename extension. See section Displaying the Language.

This is most commonly a problem when you use a program, such as cfront or f2c, that generates C but is written in another language. In that case, make the program use #line directives in its C output; that way GDB will know the correct language of the source code of the original program, and will display that source code, not the generated C code.


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15.1.1 List of Filename Extensions and Languages

If a source file name ends in one of the following extensions, then GDB infers that its language is the one indicated.

`.ada'
`.ads'
`.adb'
`.a'

Ada source file.

`.c'

C source file

`.C'
`.cc'
`.cp'
`.cpp'
`.cxx'
`.c++'

C++ source file

`.m'

Objective-C source file

`.f'
`.F'

Fortran source file

`.mod'

Modula-2 source file

`.s'
`.S'

Assembler source file. This actually behaves almost like C, but GDB does not skip over function prologues when stepping.

In addition, you may set the language associated with a filename extension. See section Displaying the Language.


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15.1.2 Setting the Working Language

If you allow GDB to set the language automatically, expressions are interpreted the same way in your debugging session and your program.

If you wish, you may set the language manually. To do this, issue the command `set language lang', where lang is the name of a language, such as c or modula-2. For a list of the supported languages, type `set language'.

Setting the language manually prevents GDB from updating the working language automatically. This can lead to confusion if you try to debug a program when the working language is not the same as the source language, when an expression is acceptable to both languages--but means different things. For instance, if the current source file were written in C, and GDB was parsing Modula-2, a command such as:

 
print a = b + c

might not have the effect you intended. In C, this means to add b and c and place the result in a. The result printed would be the value of a. In Modula-2, this means to compare a to the result of b+c, yielding a BOOLEAN value.


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15.1.3 Having GDB Infer the Source Language

To have GDB set the working language automatically, use `set language local' or `set language auto'. GDB then infers the working language. That is, when your program stops in a frame (usually by encountering a breakpoint), GDB sets the working language to the language recorded for the function in that frame. If the language for a frame is unknown (that is, if the function or block corresponding to the frame was defined in a source file that does not have a recognized extension), the current working language is not changed, and GDB issues a warning.

This may not seem necessary for most programs, which are written entirely in one source language. However, program modules and libraries written in one source language can be used by a main program written in a different source language. Using `set language auto' in this case frees you from having to set the working language manually.


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15.2 Displaying the Language

The following commands help you find out which language is the working language, and also what language source files were written in.

show language

Display the current working language. This is the language you can use with commands such as print to build and compute expressions that may involve variables in your program.

info frame

Display the source language for this frame. This language becomes the working language if you use an identifier from this frame. See section Information about a Frame, to identify the other information listed here.

info source

Display the source language of this source file. See section Examining the Symbol Table, to identify the other information listed here.

In unusual circumstances, you may have source files with extensions not in the standard list. You can then set the extension associated with a language explicitly:

set extension-language ext language

Tell GDB that source files with extension ext are to be assumed as written in the source language language.

info extensions

List all the filename extensions and the associated languages.


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15.3 Type and Range Checking

Warning: In this release, the GDB commands for type and range checking are included, but they do not yet have any effect. This section documents the intended facilities.

Some languages are designed to guard you against making seemingly common errors through a series of compile- and run-time checks. These include checking the type of arguments to functions and operators, and making sure mathematical overflows are caught at run time. Checks such as these help to ensure a program's correctness once it has been compiled by eliminating type mismatches, and providing active checks for range errors when your program is running.

GDB can check for conditions like the above if you wish. Although GDB does not check the statements in your program, it can check expressions entered directly into GDB for evaluation via the print command, for example. As with the working language, GDB can also decide whether or not to check automatically based on your program's source language. See section Supported Languages, for the default settings of supported languages.


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15.3.1 An Overview of Type Checking

Some languages, such as Modula-2, are strongly typed, meaning that the arguments to operators and functions have to be of the correct type, otherwise an error occurs. These checks prevent type mismatch errors from ever causing any run-time problems. For example,

 
1 + 2 ⇒ 3
but
error--> 1 + 2.3

The second example fails because the CARDINAL 1 is not type-compatible with the REAL 2.3.

For the expressions you use in GDB commands, you can tell the GDB type checker to skip checking; to treat any mismatches as errors and abandon the expression; or to only issue warnings when type mismatches occur, but evaluate the expression anyway. When you choose the last of these, GDB evaluates expressions like the second example above, but also issues a warning.

Even if you turn type checking off, there may be other reasons related to type that prevent GDB from evaluating an expression. For instance, GDB does not know how to add an int and a struct foo. These particular type errors have nothing to do with the language in use, and usually arise from expressions, such as the one described above, which make little sense to evaluate anyway.

Each language defines to what degree it is strict about type. For instance, both Modula-2 and C require the arguments to arithmetical operators to be numbers. In C, enumerated types and pointers can be represented as numbers, so that they are valid arguments to mathematical operators. See section Supported Languages, for further details on specific languages.

GDB provides some additional commands for controlling the type checker:

set check type auto

Set type checking on or off based on the current working language. See section Supported Languages, for the default settings for each language.

set check type on
set check type off

Set type checking on or off, overriding the default setting for the current working language. Issue a warning if the setting does not match the language default. If any type mismatches occur in evaluating an expression while type checking is on, GDB prints a message and aborts evaluation of the expression.

set check type warn

Cause the type checker to issue warnings, but to always attempt to evaluate the expression. Evaluating the expression may still be impossible for other reasons. For example, GDB cannot add numbers and structures.

show type

Show the current setting of the type checker, and whether or not GDB is setting it automatically.


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15.3.2 An Overview of Range Checking

In some languages (such as Modula-2), it is an error to exceed the bounds of a type; this is enforced with run-time checks. Such range checking is meant to ensure program correctness by making sure computations do not overflow, or indices on an array element access do not exceed the bounds of the array.

For expressions you use in GDB commands, you can tell GDB to treat range errors in one of three ways: ignore them, always treat them as errors and abandon the expression, or issue warnings but evaluate the expression anyway.

A range error can result from numerical overflow, from exceeding an array index bound, or when you type a constant that is not a member of any type. Some languages, however, do not treat overflows as an error. In many implementations of C, mathematical overflow causes the result to "wrap around" to lower values--for example, if m is the largest integer value, and s is the smallest, then

 
m + 1 ⇒ s

This, too, is specific to individual languages, and in some cases specific to individual compilers or machines. See section Supported Languages, for further details on specific languages.

GDB provides some additional commands for controlling the range checker:

set check range auto

Set range checking on or off based on the current working language. See section Supported Languages, for the default settings for each language.

set check range on
set check range off

Set range checking on or off, overriding the default setting for the current working language. A warning is issued if the setting does not match the language default. If a range error occurs and range checking is on, then a message is printed and evaluation of the expression is aborted.

set check range warn

Output messages when the GDB range checker detects a range error, but attempt to evaluate the expression anyway. Evaluating the expression may still be impossible for other reasons, such as accessing memory that the process does not own (a typical example from many Unix systems).

show range

Show the current setting of the range checker, and whether or not it is being set automatically by GDB.


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15.4 Supported Languages

GDB supports C, C++, Objective-C, Fortran, Java, Pascal, assembly, Modula-2, and Ada. Some GDB features may be used in expressions regardless of the language you use: the GDB @ and :: operators, and the `{type}addr' construct (see section Expressions) can be used with the constructs of any supported language.

The following sections detail to what degree each source language is supported by GDB. These sections are not meant to be language tutorials or references, but serve only as a reference guide to what the GDB expression parser accepts, and what input and output formats should look like for different languages. There are many good books written on each of these languages; please look to these for a language reference or tutorial.


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15.4.1 C and C++

Since C and C++ are so closely related, many features of GDB apply to both languages. Whenever this is the case, we discuss those languages together.

The C++ debugging facilities are jointly implemented by the C++ compiler and GDB. Therefore, to debug your C++ code effectively, you must compile your C++ programs with a supported C++ compiler, such as GNU g++, or the HP ANSI C++ compiler (aCC).

For best results when using GNU C++, use the DWARF 2 debugging format; if it doesn't work on your system, try the stabs+ debugging format. You can select those formats explicitly with the g++ command-line options `-gdwarf-2' and `-gstabs+'. See (gcc.info)Debugging Options section `Options for Debugging Your Program or GCC' in Using the GNU Compiler Collection (GCC).


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15.4.1.1 C and C++ Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on structures. Operators are often defined on groups of types.

For the purposes of C and C++, the following definitions hold:

The following operators are supported. They are listed here in order of increasing precedence:

,

The comma or sequencing operator. Expressions in a comma-separated list are evaluated from left to right, with the result of the entire expression being the last expression evaluated.

=

Assignment. The value of an assignment expression is the value assigned. Defined on scalar types.

op=

Used in an expression of the form a op= b, and translated to a = a op b. op= and = have the same precedence. op is any one of the operators |, ^, &, <<, >>, +, -, *, /, %.

?:

The ternary operator. a ? b : c can be thought of as: if a then b else c. a should be of an integral type.

||

Logical OR. Defined on integral types.

&&

Logical AND. Defined on integral types.

|

Bitwise OR. Defined on integral types.

^

Bitwise exclusive-OR. Defined on integral types.

&

Bitwise AND. Defined on integral types.

==, !=

Equality and inequality. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<, >, <=, >=

Less than, greater than, less than or equal, greater than or equal. Defined on scalar types. The value of these expressions is 0 for false and non-zero for true.

<<, >>

left shift, and right shift. Defined on integral types.

@

The GDB "artificial array" operator (see section Expressions).

+, -

Addition and subtraction. Defined on integral types, floating-point types and pointer types.

*, /, %

Multiplication, division, and modulus. Multiplication and division are defined on integral and floating-point types. Modulus is defined on integral types.

++, --

Increment and decrement. When appearing before a variable, the operation is performed before the variable is used in an expression; when appearing after it, the variable's value is used before the operation takes place.

*

Pointer dereferencing. Defined on pointer types. Same precedence as ++.

&

Address operator. Defined on variables. Same precedence as ++.

For debugging C++, GDB implements a use of `&' beyond what is allowed in the C++ language itself: you can use `&(&ref)' to examine the address where a C++ reference variable (declared with `&ref') is stored.

-

Negative. Defined on integral and floating-point types. Same precedence as ++.

!

Logical negation. Defined on integral types. Same precedence as ++.

~

Bitwise complement operator. Defined on integral types. Same precedence as ++.

., ->

Structure member, and pointer-to-structure member. For convenience, GDB regards the two as equivalent, choosing whether to dereference a pointer based on the stored type information. Defined on struct and union data.

.*, ->*

Dereferences of pointers to members.

[]

Array indexing. a[i] is defined as *(a+i). Same precedence as ->.

()

Function parameter list. Same precedence as ->.

::

C++ scope resolution operator. Defined on struct, union, and class types.

::

Doubled colons also represent the GDB scope operator (see section Expressions). Same precedence as ::, above.

If an operator is redefined in the user code, GDB usually attempts to invoke the redefined version instead of using the operator's predefined meaning.


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15.4.1.2 C and C++ Constants

GDB allows you to express the constants of C and C++ in the following ways:


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15.4.1.3 C++ Expressions

GDB expression handling can interpret most C++ expressions.

Warning: GDB can only debug C++ code if you use the proper compiler and the proper debug format. Currently, GDB works best when debugging C++ code that is compiled with GCC 2.95.3 or with GCC 3.1 or newer, using the options `-gdwarf-2' or `-gstabs+'. DWARF 2 is preferred over stabs+. Most configurations of GCC emit either DWARF 2 or stabs+ as their default debug format, so you usually don't need to specify a debug format explicitly. Other compilers and/or debug formats are likely to work badly or not at all when using GDB to debug C++ code.

  1. Member function calls are allowed; you can use expressions like
     
    count = aml->GetOriginal(x, y)
    
  2. While a member function is active (in the selected stack frame), your expressions have the same namespace available as the member function; that is, GDB allows implicit references to the class instance pointer this following the same rules as C++.
  3. You can call overloaded functions; GDB resolves the function call to the right definition, with some restrictions. GDB does not perform overload resolution involving user-defined type conversions, calls to constructors, or instantiations of templates that do not exist in the program. It also cannot handle ellipsis argument lists or default arguments.

    It does perform integral conversions and promotions, floating-point promotions, arithmetic conversions, pointer conversions, conversions of class objects to base classes, and standard conversions such as those of functions or arrays to pointers; it requires an exact match on the number of function arguments.

    Overload resolution is always performed, unless you have specified set overload-resolution off. See section GDB Features for C++.

    You must specify set overload-resolution off in order to use an explicit function signature to call an overloaded function, as in

     
    p 'foo(char,int)'('x', 13)
    

    The GDB command-completion facility can simplify this; see Command Completion.

  4. GDB understands variables declared as C++ references; you can use them in expressions just as you do in C++ source--they are automatically dereferenced.

    In the parameter list shown when GDB displays a frame, the values of reference variables are not displayed (unlike other variables); this avoids clutter, since references are often used for large structures. The address of a reference variable is always shown, unless you have specified `set print address off'.

  5. GDB supports the C++ name resolution operator ::--your expressions can use it just as expressions in your program do. Since one scope may be defined in another, you can use :: repeatedly if necessary, for example in an expression like `scope1::scope2::name'. GDB also allows resolving name scope by reference to source files, in both C and C++ debugging (see section Program Variables).

In addition, when used with HP's C++ compiler, GDB supports calling virtual functions correctly, printing out virtual bases of objects, calling functions in a base subobject, casting objects, and invoking user-defined operators.


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15.4.1.4 C and C++ Defaults

If you allow GDB to set type and range checking automatically, they both default to off whenever the working language changes to C or C++. This happens regardless of whether you or GDB selects the working language.

If you allow GDB to set the language automatically, it recognizes source files whose names end with `.c', `.C', or `.cc', etc, and when GDB enters code compiled from one of these files, it sets the working language to C or C++. See section Having GDB Infer the Source Language, for further details.


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15.4.1.5 C and C++ Type and Range Checks

By default, when GDB parses C or C++ expressions, type checking is not used. However, if you turn type checking on, GDB considers two variables type equivalent if:

Range checking, if turned on, is done on mathematical operations. Array indices are not checked, since they are often used to index a pointer that is not itself an array.


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15.4.1.6 GDB and C

The set print union and show print union commands apply to the union type. When set to `on', any union that is inside a struct or class is also printed. Otherwise, it appears as `{...}'.

The @ operator aids in the debugging of dynamic arrays, formed with pointers and a memory allocation function. See section Expressions.


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15.4.1.7 GDB Features for C++

Some GDB commands are particularly useful with C++, and some are designed specifically for use with C++. Here is a summary:

breakpoint menus

When you want a breakpoint in a function whose name is overloaded, GDB has the capability to display a menu of possible breakpoint locations to help you specify which function definition you want. See section Ambiguous Expressions.

rbreak regex

Setting breakpoints using regular expressions is helpful for setting breakpoints on overloaded functions that are not members of any special classes. See section Setting Breakpoints.

catch throw
catch catch

Debug C++ exception handling using these commands. See section Setting Catchpoints.

ptype typename

Print inheritance relationships as well as other information for type typename. See section Examining the Symbol Table.

set print demangle
show print demangle
set print asm-demangle
show print asm-demangle

Control whether C++ symbols display in their source form, both when displaying code as C++ source and when displaying disassemblies. See section Print Settings.

set print object
show print object

Choose whether to print derived (actual) or declared types of objects. See section Print Settings.

set print vtbl
show print vtbl

Control the format for printing virtual function tables. See section Print Settings. (The vtbl commands do not work on programs compiled with the HP ANSI C++ compiler (aCC).)

set overload-resolution on

Enable overload resolution for C++ expression evaluation. The default is on. For overloaded functions, GDB evaluates the arguments and searches for a function whose signature matches the argument types, using the standard C++ conversion rules (see C++ Expressions, for details). If it cannot find a match, it emits a message.

set overload-resolution off

Disable overload resolution for C++ expression evaluation. For overloaded functions that are not class member functions, GDB chooses the first function of the specified name that it finds in the symbol table, whether or not its arguments are of the correct type. For overloaded functions that are class member functions, GDB searches for a function whose signature exactly matches the argument types.

show overload-resolution

Show the current setting of overload resolution.

Overloaded symbol names

You can specify a particular definition of an overloaded symbol, using the same notation that is used to declare such symbols in C++: type symbol(types) rather than just symbol. You can also use the GDB command-line word completion facilities to list the available choices, or to finish the type list for you. See section Command Completion, for details on how to do this.


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15.4.1.8 Decimal Floating Point format

GDB can examine, set and perform computations with numbers in decimal floating point format, which in the C language correspond to the _Decimal32, _Decimal64 and _Decimal128 types as specified by the extension to support decimal floating-point arithmetic.

There are two encodings in use, depending on the architecture: BID (Binary Integer Decimal) for x86 and x86-64, and DPD (Densely Packed Decimal) for PowerPC. GDB will use the appropriate encoding for the configured target.

Because of a limitation in `libdecnumber', the library used by GDB to manipulate decimal floating point numbers, it is not possible to convert (using a cast, for example) integers wider than 32-bit to decimal float.

In addition, in order to imitate GDB's behaviour with binary floating point computations, error checking in decimal float operations ignores underflow, overflow and divide by zero exceptions.

In the PowerPC architecture, GDB provides a set of pseudo-registers to inspect _Decimal128 values stored in floating point registers. See PowerPC for more details.


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15.4.2 Objective-C

This section provides information about some commands and command options that are useful for debugging Objective-C code. See also info classes, and info selectors, for a few more commands specific to Objective-C support.


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15.4.2.1 Method Names in Commands

The following commands have been extended to accept Objective-C method names as line specifications:

A fully qualified Objective-C method name is specified as

 
-[Class methodName]

where the minus sign is used to indicate an instance method and a plus sign (not shown) is used to indicate a class method. The class name Class and method name methodName are enclosed in brackets, similar to the way messages are specified in Objective-C source code. For example, to set a breakpoint at the create instance method of class Fruit in the program currently being debugged, enter:

 
break -[Fruit create]

To list ten program lines around the initialize class method, enter:

 
list +[NSText initialize]

In the current version of GDB, the plus or minus sign is required. In future versions of GDB, the plus or minus sign will be optional, but you can use it to narrow the search. It is also possible to specify just a method name:

 
break create

You must specify the complete method name, including any colons. If your program's source files contain more than one create method, you'll be presented with a numbered list of classes that implement that method. Indicate your choice by number, or type `0' to exit if none apply.

As another example, to clear a breakpoint established at the makeKeyAndOrderFront: method of the NSWindow class, enter:

 
clear -[NSWindow makeKeyAndOrderFront:]

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15.4.2.2 The Print Command With Objective-C

The print command has also been extended to accept methods. For example:

 
print -[object hash]

will tell GDB to send the hash message to object and print the result. Also, an additional command has been added, print-object or po for short, which is meant to print the description of an object. However, this command may only work with certain Objective-C libraries that have a particular hook function, _NSPrintForDebugger, defined.


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15.4.3 Fortran

GDB can be used to debug programs written in Fortran, but it currently supports only the features of Fortran 77 language.

Some Fortran compilers (GNU Fortran 77 and Fortran 95 compilers among them) append an underscore to the names of variables and functions. When you debug programs compiled by those compilers, you will need to refer to variables and functions with a trailing underscore.


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15.4.3.1 Fortran Operators and Expressions

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on characters or other non- arithmetic types. Operators are often defined on groups of types.

**

The exponentiation operator. It raises the first operand to the power of the second one.

:

The range operator. Normally used in the form of array(low:high) to represent a section of array.

%

The access component operator. Normally used to access elements in derived types. Also suitable for unions. As unions aren't part of regular Fortran, this can only happen when accessing a register that uses a gdbarch-defined union type.


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15.4.3.2 Fortran Defaults

Fortran symbols are usually case-insensitive, so GDB by default uses case-insensitive matches for Fortran symbols. You can change that with the `set case-insensitive' command, see Examining the Symbol Table, for the details.


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15.4.3.3 Special Fortran Commands

GDB has some commands to support Fortran-specific features, such as displaying common blocks.

info common [common-name]

This command prints the values contained in the Fortran COMMON block whose name is common-name. With no argument, the names of all COMMON blocks visible at the current program location are printed.


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15.4.4 Pascal

Debugging Pascal programs which use sets, subranges, file variables, or nested functions does not currently work. GDB does not support entering expressions, printing values, or similar features using Pascal syntax.

The Pascal-specific command set print pascal_static-members controls whether static members of Pascal objects are displayed. See section pascal_static-members.


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15.4.5 Modula-2

The extensions made to GDB to support Modula-2 only support output from the GNU Modula-2 compiler (which is currently being developed). Other Modula-2 compilers are not currently supported, and attempting to debug executables produced by them is most likely to give an error as GDB reads in the executable's symbol table.


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15.4.5.1 Operators

Operators must be defined on values of specific types. For instance, + is defined on numbers, but not on structures. Operators are often defined on groups of types. For the purposes of Modula-2, the following definitions hold:

The following operators are supported, and appear in order of increasing precedence:

,

Function argument or array index separator.

:=

Assignment. The value of var := value is value.

<, >

Less than, greater than on integral, floating-point, or enumerated types.

<=, >=

Less than or equal to, greater than or equal to on integral, floating-point and enumerated types, or set inclusion on set types. Same precedence as <.

=, <>, #

Equality and two ways of expressing inequality, valid on scalar types. Same precedence as <. In GDB scripts, only <> is available for inequality, since # conflicts with the script comment character.

IN

Set membership. Defined on set types and the types of their members. Same precedence as <.

OR

Boolean disjunction. Defined on boolean types.

AND, &

Boolean conjunction. Defined on boolean types.

@

The GDB "artificial array" operator (see section Expressions).

+, -

Addition and subtraction on integral and floating-point types, or union and difference on set types.

*

Multiplication on integral and floating-point types, or set intersection on set types.

/

Division on floating-point types, or symmetric set difference on set types. Same precedence as *.

DIV, MOD

Integer division and remainder. Defined on integral types. Same precedence as *.

-

Negative. Defined on INTEGER and REAL data.

^

Pointer dereferencing. Defined on pointer types.

NOT

Boolean negation. Defined on boolean types. Same precedence as ^.

.

RECORD field selector. Defined on RECORD data. Same precedence as ^.

[]

Array indexing. Defined on ARRAY data. Same precedence as ^.

()

Procedure argument list. Defined on PROCEDURE objects. Same precedence as ^.

::, .

GDB and Modula-2 scope operators.

Warning: Set expressions and their operations are not yet supported, so GDB treats the use of the operator IN, or the use of operators +, -, *, /, =, , <>, #, <=, and >= on sets as an error.


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15.4.5.2 Built-in Functions and Procedures

Modula-2 also makes available several built-in procedures and functions. In describing these, the following metavariables are used:

a

represents an ARRAY variable.

c

represents a CHAR constant or variable.

i

represents a variable or constant of integral type.

m

represents an identifier that belongs to a set. Generally used in the same function with the metavariable s. The type of s should be SET OF mtype (where mtype is the type of m).

n

represents a variable or constant of integral or floating-point type.

r

represents a variable or constant of floating-point type.

t

represents a type.

v

represents a variable.

x

represents a variable or constant of one of many types. See the explanation of the function for details.

All Modula-2 built-in procedures also return a result, described below.

ABS(n)

Returns the absolute value of n.

CAP(c)

If c is a lower case letter, it returns its upper case equivalent, otherwise it returns its argument.

CHR(i)

Returns the character whose ordinal value is i.

DEC(v)

Decrements the value in the variable v by one. Returns the new value.

DEC(v,i)

Decrements the value in the variable v by i. Returns the new value.

EXCL(m,s)

Removes the element m from the set s. Returns the new set.

FLOAT(i)

Returns the floating point equivalent of the integer i.

HIGH(a)

Returns the index of the last member of a.

INC(v)

Increments the value in the variable v by one. Returns the new value.

INC(v,i)

Increments the value in the variable v by i. Returns the new value.

INCL(m,s)

Adds the element m to the set s if it is not already there. Returns the new set.

MAX(t)

Returns the maximum value of the type t.

MIN(t)

Returns the minimum value of the type t.

ODD(i)

Returns boolean TRUE if i is an odd number.

ORD(x)

Returns the ordinal value of its argument. For example, the ordinal value of a character is its ASCII value (on machines supporting the ASCII character set). x must be of an ordered type, which include integral, character and enumerated types.

SIZE(x)

Returns the size of its argument. x can be a variable or a type.

TRUNC(r)

Returns the integral part of r.

TSIZE(x)

Returns the size of its argument. x can be a variable or a type.

VAL(t,i)

Returns the member of the type t whose ordinal value is i.

Warning: Sets and their operations are not yet supported, so GDB treats the use of procedures INCL and EXCL as an error.


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15.4.5.3 Constants

GDB allows you to express the constants of Modula-2 in the following ways:


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15.4.5.4 Modula-2 Types

Currently GDB can print the following data types in Modula-2 syntax: array types, record types, set types, pointer types, procedure types, enumerated types, subrange types and base types. You can also print the contents of variables declared using these type. This section gives a number of simple source code examples together with sample GDB sessions.

The first example contains the following section of code:

 
VAR
   s: SET OF CHAR ;
   r: [20..40] ;

and you can request GDB to interrogate the type and value of r and s.

 
(gdb) print s
{'A'..'C', 'Z'}
(gdb) ptype s
SET OF CHAR
(gdb) print r
21
(gdb) ptype r
[20..40]

Likewise if your source code declares s as:

 
VAR
   s: SET ['A'..'Z'] ;

then you may query the type of s by:

 
(gdb) ptype s
type = SET ['A'..'Z']

Note that at present you cannot interactively manipulate set expressions using the debugger.

The following example shows how you might declare an array in Modula-2 and how you can interact with GDB to print its type and contents:

 
VAR
   s: ARRAY [-10..10] OF CHAR ;
 
(gdb) ptype s
ARRAY [-10..10] OF CHAR

Note that the array handling is not yet complete and although the type is printed correctly, expression handling still assumes that all arrays have a lower bound of zero and not -10 as in the example above.

Here are some more type related Modula-2 examples:

 
TYPE
   colour = (blue, red, yellow, green) ;
   t = [blue..yellow] ;
VAR
   s: t ;
BEGIN
   s := blue ;

The GDB interaction shows how you can query the data type and value of a variable.

 
(gdb) print s
$1 = blue
(gdb) ptype t
type = [blue..yellow]

In this example a Modula-2 array is declared and its contents displayed. Observe that the contents are written in the same way as their C counterparts.

 
VAR
   s: ARRAY [1..5] OF CARDINAL ;
BEGIN
   s[1] := 1 ;
 
(gdb) print s
$1 = {1, 0, 0, 0, 0}
(gdb) ptype s
type = ARRAY [1..5] OF CARDINAL

The Modula-2 language interface to GDB also understands pointer types as shown in this example:

 
VAR
   s: POINTER TO ARRAY [1..5] OF CARDINAL ;
BEGIN
   NEW(s) ;
   s^[1] := 1 ;

and you can request that GDB describes the type of s.

 
(gdb) ptype s
type = POINTER TO ARRAY [1..5] OF CARDINAL

GDB handles compound types as we can see in this example. Here we combine array types, record types, pointer types and subrange types:

 
TYPE
   foo = RECORD
            f1: CARDINAL ;
            f2: CHAR ;
            f3: myarray ;
         END ;

   myarray = ARRAY myrange OF CARDINAL ;
   myrange = [-2..2] ;
VAR
   s: POINTER TO ARRAY myrange OF foo ;

and you can ask GDB to describe the type of s as shown below.

 
(gdb) ptype s
type = POINTER TO ARRAY [-2..2] OF foo = RECORD
    f1 : CARDINAL;
    f2 : CHAR;
    f3 : ARRAY [-2..2] OF CARDINAL;
END 

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15.4.5.5 Modula-2 Defaults

If type and range checking are set automatically by GDB, they both default to on whenever the working language changes to Modula-2. This happens regardless of whether you or GDB selected the working language.

If you allow GDB to set the language automatically, then entering code compiled from a file whose name ends with `.mod' sets the working language to Modula-2. See section Having GDB Infer the Source Language, for further details.


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15.4.5.6 Deviations from Standard Modula-2

A few changes have been made to make Modula-2 programs easier to debug. This is done primarily via loosening its type strictness:


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15.4.5.7 Modula-2 Type and Range Checks

Warning: in this release, GDB does not yet perform type or range checking.

GDB considers two Modula-2 variables type equivalent if:

As long as type checking is enabled, any attempt to combine variables whose types are not equivalent is an error.

Range checking is done on all mathematical operations, assignment, array index bounds, and all built-in functions and procedures.


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15.4.5.8 The Scope Operators :: and .

There are a few subtle differences between the Modula-2 scope operator (.) and the GDB scope operator (::). The two have similar syntax:

 
module . id
scope :: id

where scope is the name of a module or a procedure, module the name of a module, and id is any declared identifier within your program, except another module.

Using the :: operator makes GDB search the scope specified by scope for the identifier id. If it is not found in the specified scope, then GDB searches all scopes enclosing the one specified by scope.

Using the . operator makes GDB search the current scope for the identifier specified by id that was imported from the definition module specified by module. With this operator, it is an error if the identifier id was not imported from definition module module, or if id is not an identifier in module.


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15.4.5.9 GDB and Modula-2

Some GDB commands have little use when debugging Modula-2 programs. Five subcommands of set print and show print apply specifically to C and C++: `vtbl', `demangle', `asm-demangle', `object', and `union'. The first four apply to C++, and the last to the C union type, which has no direct analogue in Modula-2.

The @ operator (see section Expressions), while available with any language, is not useful with Modula-2. Its intent is to aid the debugging of dynamic arrays, which cannot be created in Modula-2 as they can in C or C++. However, because an address can be specified by an integral constant, the construct `{type}adrexp' is still useful.

In GDB scripts, the Modula-2 inequality operator # is interpreted as the beginning of a comment. Use <> instead.


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15.4.6 Ada

The extensions made to GDB for Ada only support output from the GNU Ada (GNAT) compiler. Other Ada compilers are not currently supported, and attempting to debug executables produced by them is most likely to be difficult.


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15.4.6.1 Introduction

The Ada mode of GDB supports a fairly large subset of Ada expression syntax, with some extensions. The philosophy behind the design of this subset is

Thus, for brevity, the debugger acts as if all names declared in user-written packages are directly visible, even if they are not visible according to Ada rules, thus making it unnecessary to fully qualify most names with their packages, regardless of context. Where this causes ambiguity, GDB asks the user's intent.

The debugger will start in Ada mode if it detects an Ada main program. As for other languages, it will enter Ada mode when stopped in a program that was translated from an Ada source file.

While in Ada mode, you may use `-' for comments. This is useful mostly for documenting command files. The standard GDB comment (`#') still works at the beginning of a line in Ada mode, but not in the middle (to allow based literals).

The debugger supports limited overloading. Given a subprogram call in which the function symbol has multiple definitions, it will use the number of actual parameters and some information about their types to attempt to narrow the set of definitions. It also makes very limited use of context, preferring procedures to functions in the context of the call command, and functions to procedures elsewhere.


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15.4.6.2 Omissions from Ada

Here are the notable omissions from the subset:


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15.4.6.3 Additions to Ada

As it does for other languages, GDB makes certain generic extensions to Ada (see section Expressions):

In addition, GDB provides a few other shortcuts and outright additions specific to Ada:


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15.4.6.4 Stopping at the Very Beginning

It is sometimes necessary to debug the program during elaboration, and before reaching the main procedure. As defined in the Ada Reference Manual, the elaboration code is invoked from a procedure called adainit. To run your program up to the beginning of elaboration, simply use the following two commands: tbreak adainit and run.


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15.4.6.5 Extensions for Ada Tasks

Support for Ada tasks is analogous to that for threads (see section Debugging Programs with Multiple Threads). GDB provides the following task-related commands:

info tasks

This command shows a list of current Ada tasks, as in the following example:

 
(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1   8088000   0   15 Child Activation Wait main_task
   2   80a4000   1   15 Accept Statement      b
   3   809a800   1   15 Child Activation Wait a
*  4   80ae800   3   15 Runnable              c

In this listing, the asterisk before the last task indicates it to be the task currently being inspected.

ID

Represents GDB's internal task number.

TID

The Ada task ID.

P-ID

The parent's task ID (GDB's internal task number).

Pri

The base priority of the task.

State

Current state of the task.

Unactivated

The task has been created but has not been activated. It cannot be executing.

Runnable

The task is not blocked for any reason known to Ada. (It may be waiting for a mutex, though.) It is conceptually "executing" in normal mode.

Terminated

The task is terminated, in the sense of ARM 9.3 (5). Any dependents that were waiting on terminate alternatives have been awakened and have terminated themselves.

Child Activation Wait

The task is waiting for created tasks to complete activation.

Accept Statement

The task is waiting on an accept or selective wait statement.

Waiting on entry call

The task is waiting on an entry call.

Async Select Wait

The task is waiting to start the abortable part of an asynchronous select statement.

Delay Sleep

The task is waiting on a select statement with only a delay alternative open.

Child Termination Wait

The task is sleeping having completed a master within itself, and is waiting for the tasks dependent on that master to become terminated or waiting on a terminate Phase.

Wait Child in Term Alt

The task is sleeping waiting for tasks on terminate alternatives to finish terminating.

Accepting RV with taskno

The task is accepting a rendez-vous with the task taskno.

Name

Name of the task in the program.

info task taskno

This command shows detailled informations on the specified task, as in the following example:

 
(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077880    0  15 Child Activation Wait  main_task
*  2   807c468    1  15 Runnable               task_1
(gdb) info task 2
Ada Task: 0x807c468
Name: task_1
Thread: 0x807f378
Parent: 1 (main_task)
Base Priority: 15
State: Runnable
task

This command prints the ID of the current task.

 
(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077870    0  15 Child Activation Wait  main_task
*  2   807c458    1  15 Runnable               t
(gdb) task
[Current task is 2]
task taskno

This command is like the thread threadno command (see section Debugging Programs with Multiple Threads). It switches the context of debugging from the current task to the given task.

 
(gdb) info tasks
  ID       TID P-ID Pri State                  Name
   1   8077870    0  15 Child Activation Wait  main_task
*  2   807c458    1  15 Runnable               t
(gdb) task 1
[Switching to task 1]
#0  0x8067726 in pthread_cond_wait ()
(gdb) bt
#0  0x8067726 in pthread_cond_wait ()
#1  0x8056714 in system.os_interface.pthread_cond_wait ()
#2  0x805cb63 in system.task_primitives.operations.sleep ()
#3  0x806153e in system.tasking.stages.activate_tasks ()
#4  0x804aacc in un () at un.adb:5
break linespec task taskno
break linespec task taskno if …

These commands are like the break … thread … command (see section Stopping and Starting Multi-thread Programs). linespec specifies source lines, as described in Specifying a Location.

Use the qualifier `task taskno' with a breakpoint command to specify that you only want GDB to stop the program when a particular Ada task reaches this breakpoint. taskno is one of the numeric task identifiers assigned by GDB, shown in the first column of the `info tasks' display.

If you do not specify `task taskno' when you set a breakpoint, the breakpoint applies to all tasks of your program.

You can use the task qualifier on conditional breakpoints as well; in this case, place `task taskno' before the breakpoint condition (before the if).

For example,

 
(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1 140022020   0   15 Child Activation Wait main_task
   2 140045060   1   15 Accept/Select Wait    t2
   3 140044840   1   15 Runnable              t1
*  4 140056040   1   15 Runnable              t3
(gdb) b 15 task 2
Breakpoint 5 at 0x120044cb0: file test_task_debug.adb, line 15.
(gdb) cont
Continuing.
task # 1 running
task # 2 running

Breakpoint 5, test_task_debug () at test_task_debug.adb:15
15               flush;
(gdb) info tasks
  ID       TID P-ID Pri State                 Name
   1 140022020   0   15 Child Activation Wait main_task
*  2 140045060   1   15 Runnable              t2
   3 140044840   1   15 Runnable              t1
   4 140056040   1   15 Delay Sleep           t3

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15.4.6.6 Tasking Support when Debugging Core Files

When inspecting a core file, as opposed to debugging a live program, tasking support may be limited or even unavailable, depending on the platform being used. For instance, on x86-linux, the list of tasks is available, but task switching is not supported. On Tru64, however, task switching will work as usual.

On certain platforms, including Tru64, the debugger needs to perform some memory writes in order to provide Ada tasking support. When inspecting a core file, this means that the core file must be opened with read-write privileges, using the command `"set write on"' (see section Patching Programs). Under these circumstances, you should make a backup copy of the core file before inspecting it with GDB.


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15.4.6.7 Known Peculiarities of Ada Mode

Besides the omissions listed previously (see section Omissions from Ada), we know of several problems with and limitations of Ada mode in GDB, some of which will be fixed with planned future releases of the debugger and the GNU Ada compiler.


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15.5 Unsupported Languages

In addition to the other fully-supported programming languages, GDB also provides a pseudo-language, called minimal. It does not represent a real programming language, but provides a set of capabilities close to what the C or assembly languages provide. This should allow most simple operations to be performed while debugging an application that uses a language currently not supported by GDB.

If the language is set to auto, GDB will automatically select this language if the current frame corresponds to an unsupported language.


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