REALPATH(3) Linux Programmers Manual REALPATH(3)
realpath - return the canonicalized absolute pathname
char *realpath(const char *path, char *resolved_path);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
realpath(): _BSD_SOURCE || _XOPEN_SOURCE >= 500
realpath() expands all symbolic links and resolves references to /./,
/../ and extra '/' characters in the null terminated string named by
path and stores the canonicalized absolute pathname in the buffer of
size PATH_MAX named by resolved_path. The resulting path will have no
symbolic link, /./ or /../ components.
If there is no error, realpath() returns a pointer to the
Otherwise it returns a NULL pointer, and the contents of the array
resolved_path are undefined. The global variable errno is set to indi
cate the error.
EACCES Read or search permission was denied for a component of the path
EINVAL Either path or resolved_path is NULL. (In libc5 this would just
cause a segfault.) But, see NOTES below.
EIO An I/O error occurred while reading from the file system.
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.
On Linux this function appeared in libc 4.5.21.
In 4.4BSD and Solaris the limit on the pathname length is MAXPATHLEN
(found in ). SUSv2 prescribes PATH_MAX and NAME_MAX, as
found in 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 4.4BSD, Linux and SUSv2 versions always return an absolute path
name. Solaris may return a relative pathname when the path argument is
relative. The prototype of realpath() is given in in libc4
and libc5, but in everywhere else.
The glibc implementation of realpath() provides a non-standard exten
sion. If resolved_path is specified as NULL, then realpath() uses mal
loc(3) to allocate a buffer of up to PATH_MAX bytes to hold the
resolved pathname, and returns a pointer to this buffer. The caller
should deallocate this buffer using free(3).
Avoid using this function. It is broken by design since (unless using
the non-standard resolved_path == NULL feature) it is impossible to
determine a suitable size for the output buffer, resolved_path.
According to POSIX a buffer of size PATH_MAX suffices, but PATH_MAX
need not be a defined constant, and may have to be obtained using path
conf(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 unsuit
able for mallocing memory. And on the other hand pathconf(3) may
return -1 to signify that PATH_MAX is not bounded.
The libc4 and libc5 implementation contains a buffer overflow (fixed in
libc-5.4.13). Thus, set-user-ID programs like mount(8) need a private
readlink(2), canonicalize_file_name(3), getcwd(3), pathconf(3),
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