realpath(3) -- Linux man page



realpath - return the canonicalized absolute pathname  


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

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


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, it 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.  


Read or search permission was denied for a component of the path prefix.
Either path or resolved_path is NULL. (In libc5 this would just cause a segfault.)
An I/O error occurred while reading from the file system.
Too many symbolic links were encountered in translating the pathname.
A component of a path name exceeded NAME_MAX characters, or an entire path name exceeded PATH_MAX characters.
The named file does not exist.
A component of the path prefix is not a directory.


Never use this function. It is broken by design since it is impossible to determine a suitable size for the output buffer. 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, suid programs like mount need a private version.  


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


In BSD 4.4 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;
  path_max = pathconf (path, _PC_PATH_MAX);
  if (path_max <= 0)
    path_max = 4096;
(But see the BUGS section.)

The BSD 4.4, Linux and SUSv2 versions always return an absolute path name. Solaris may return a relative path name 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.  


readlink(2), getcwd(3), pathconf(3), sysconf(3)