#include <unistd.h> char *getcwd(char *buf, size_t size); char *getwd(char *buf); char *get_current_dir_name(void);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || (_XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) && !(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
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 length of the absolute pathname of the current working directory, including the terminating null byte, exceeds size bytes, NULL is returned, and errno is set to ERANGE; an application should check for this error, and allocate a larger buffer if necessary.
As an extension to the POSIX.1-2001 standard, glibc's getcwd() allocates the buffer dynamically using malloc(3) if buf is NULL. In this case, the allocated buffer has the length size unless size is zero, when buf is allocated as big as necessary. The caller should free(3) the returned buffer.
get_current_dir_name() will malloc(3) an array big enough to hold the absolute pathname of the current working directory. If the environment variable PWD is set, and its value is correct, then that value will be returned. The caller should free(3) the returned buffer.
getwd() does not malloc(3) any memory. The buf argument should be a pointer to an array at least PATH_MAX bytes long. If the length of the absolute pathname of the current working directory, including the terminating null byte, exceeds PATH_MAX bytes, NULL is returned, and errno is set to ENAMETOOLONG. (Note that on some systems, PATH_MAX may not be a compile-time constant; furthermore, its value may depend on the filesystem, see pathconf(3).) For portability and security reasons, use of getwd() is deprecated.
getwd() is present in POSIX.1-2001, but marked LEGACY. POSIX.1-2008 removes the specification of getwd(). Use getcwd() instead. POSIX.1-2001 does not define any errors for getwd().
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.