The getcwd() function copies an absolute pathname of the current working directory to the array pointed to by buf, which is of length size.
If the current absolute path name would require a buffer longer than size elements, NULL is returned, and errno is set to ERANGE; an application should check for this error, and allocate a larger buffer if necessary.
If buf is NULL, the behaviour of getcwd() is undefined.
As an extension to the POSIX.1 standard, Linux (libc4, libc5, glibc) getcwd() allocates the buffer dynamically using malloc() if buf is NULL on call. In this case, the allocated buffer has the length size unless size is zero, when buf is allocated as big as necessary. It is possible (and, indeed, advisable) to free() the buffers if they have been obtained this way.
get_current_dir_name, which is only prototyped if _GNU_SOURCE is defined, will malloc(3) an array big enough to hold the current directory name. If the environment variable PWD is set, and its value is correct, then that value will be returned.