Sets the file position indicator for the given file pointer to the given offset byte into the file stream. Equivalent to calling (in C) gzseek(zp, offset, SEEK_SET).
If the file is opened for reading, this function is emulated but can be extremely slow. If the file is opened for writing, only forward seeks are supported; gzseek() then compresses a sequence of zeroes up to the new starting position.
The gz-file pointer. It must be valid, and must point to a file successfully opened by gzopen().
The seeked offset.
Upon success, returns 0; otherwise, returns -1. Note that seeking past EOF is not considered an error.