REALPATH(3)

NAME
realpath - return the canonicalized absolute pathname

SYNOPSIS
#include <limits.h>
#include <stdlib.h>

char *realpath(const char *path, char *resolved_path);

DESCRIPTION
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.

RETURN VALUE
If there is no error, realpath() returns a pointer to the resolved_path.

Otherwise it returns a NULL pointer, and the contents of the array resolved_path are undefined. The global variable errno is set to indicate the error.

ERRORS

EACCES
    Read or search permission was denied for a component of the path prefix.

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 pathname.

ENAMETOOLONG
    A component of a pathname exceeded NAME_MAX characters, or an entire pathname exceeded PATH_MAX characters.

ENOENT
    The named file does not exist.

ENOTDIR
    A component of the path prefix is not a directory.

NOTES
The glibc implementation of realpath() provides a non-standard extension. 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 pathname, and returns a pointer to this buffer. The caller should deallocate this buffer using free(3).

BUGS
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 pathconf(). And asking pathconf() does not really help, since on the one hand POSIX warns that the result of pathconf() may be huge and unsuitable for mallocing memory. And on the other hand pathconf() 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 need a private version.

HISTORY
The realpath() function first appeared in 4.4BSD, contributed by Jan-Simon Pendry. In Linux this function appears in libc 4.5.21.

CONFORMING TO
In 4.4BSD and Solaris the limit on the pathname length is MAXPATHLEN (found in <sys/param.h>). The SUSv2 prescribes PATH_MAX and NAME_MAX, as found in <limits.h> or provided by the pathconf() function. A typical source fragment would be

    #ifdef PATH_MAX
      path_max = PATH_MAX;
    #else
      path_max = pathconf (path, _PC_PATH_MAX);
      if (path_max <= 0)
        path_max = 4096;
    #endif
(But see the BUGS section.)

The 4.4BSD, Linux and SUSv2 versions always return an absolute pathname. Solaris may return a relative pathname when the path argument is relative. The prototype of realpath() is given in <unistd.h> in libc4 and libc5, but in <stdlib.h> everywhere else.

SEE ALSO
readlink(2), canonicalize_file_name(3), getcwd(3), pathconf(3), sysconf(3)