char *realpath(const char *path, char *resolved_path);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
realpath() expands all symbolic links and resolves references to /./,
/../ and extra '/' characters in the null-terminated string named by
path to produce a canonicalized absolute pathname. The resulting path-
name is stored as a null-terminated string, up to a maximum of PATH_MAX
bytes, in the buffer pointed to by resolved_path. The resulting path
will have no symbolic link, /./ or /../ components.
If resolved_path is specified as NULL, then realpath() uses malloc(3)
to allocate a buffer of up to PATH_MAX bytes to hold the resolved path-
name, and returns a pointer to this buffer. The caller should deallo-
cate this buffer using free(3).
If there is no error, realpath() returns a pointer to the
Otherwise, it returns a NULL pointer, the contents of the array
resolved_path are undefined, and errno is set to indicate the error.
EACCES Read or search permission was denied for a component of the path
EINVAL path is NULL. (In glibc versions before 2.3, this error is also
returned if resolved_path is NULL.)
EIO An I/O error occurred while reading from the filesystem.
ELOOP Too many symbolic links were encountered in translating the
A component of a pathname exceeded NAME_MAX characters, or an
entire pathname exceeded PATH_MAX characters.
ENOENT The named file does not exist.
A component of the path prefix is not a directory.
(found in <sys/param.h>). SUSv2 prescribes PATH_MAX and NAME_MAX, as
found in <limits.h> or provided by the pathconf(3) function. A typical
source fragment would be
path_max = PATH_MAX;
path_max = pathconf(path, _PC_PATH_MAX);
if (path_max <= 0)
path_max = 4096;
(But see the BUGS section.)
The prototype of realpath() is given in <unistd.h> in libc4 and libc5,
but in <stdlib.h> everywhere else.
If the call fails with either EACCES or ENOENT and resolved_path is not
NULL, then the prefix of path that is not readable or does not exist is
returned in resolved_path.
The POSIX.1-2001 standard version of this function is broken by design,
since it is impossible to determine a suitable size for the output buf-
fer, resolved_path. According to POSIX.1-2001 a buffer of size
PATH_MAX suffices, but PATH_MAX need not be a defined constant, and may
have to be obtained using pathconf(3). And asking pathconf(3) does not
really help, since, on the one hand POSIX warns that the result of
pathconf(3) may be huge and unsuitable for mallocing memory, and on the
other hand pathconf(3) may return -1 to signify that PATH_MAX is not
bounded. The resolved_path == NULL feature, not standardized in
POSIX.1-2001, but standardized in POSIX.1-2008, allows this design
problem to be avoided.
The libc4 and libc5 implementation contained a buffer overflow (fixed
in libc-5.4.13). Thus, set-user-ID programs like mount(8) needed a
readlink(2), canonicalize_file_name(3), getcwd(3), pathconf(3),
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