#include <unistd.h> char *getcwd(char *buf, size_t size); char *get_current_dir_name(void); char *getwd(char *buf);
If the current absolute pathname 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.1-2001 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.
getwd(), which is only prototyped if _BSD_SOURCE or _XOPEN_SOURCE_EXTENDED is defined, will not malloc(3) any memory. The buf argument should be a pointer to an array at least PATH_MAX bytes long. getwd() does only return the first PATH_MAX bytes of the actual pathname. Note that PATH_MAX need not be a compile-time constant; it may depend on the filesystem and may even be unlimited. For portability and security reasons, use of getwd() is deprecated.
These functions are often used to save the location of the current working directory for the purpose of returning to it later. Opening the current directory (".") and calling fchdir(2) to return is usually a faster and more reliable alternative when sufficiently many file descriptors are available, especially on platforms other than Linux.