access

ACCESS(2)		  Linux Programmer’s Manual		    ACCESS(2)



NAME
       access - check user’s permissions for a file

SYNOPSIS
       #include <unistd.h>

       int access(const char *pathname, int mode);

DESCRIPTION
       access  checks  whether the process would be allowed to read, write or
       test for existence of the file (or other	 file  system  object)	whose
       name  is	 pathname.  If pathname is a symbolic link permissions of the
       file referred to by this symbolic link are tested.

       mode is a mask consisting of one or more of R_OK, W_OK, X_OK and F_OK.

       R_OK,  W_OK  and X_OK request checking whether the file exists and has
       read, write and execute permissions, respectively.  F_OK just requests
       checking for the existence of the file.

       The  tests  depend  on the permissions of the directories occurring in
       the path to the file, as given in pathname, and on the permissions  of
       directories and files referred to by symbolic links encountered on the
       way.

       The check is done with the process’s real uid  and  gid,	 rather	 than
       with  the  effective ids as is done when actually attempting an opera-
       tion.  This is to allow	set-UID	 programs  to  easily  determine  the
       invoking user’s authority.

       Only  access  bits are checked, not the file type or contents.  There-
       fore, if a directory is found to be "writable," it probably means that
       files  can be created in the directory, and not that the directory can
       be written as a file.  Similarly, a DOS file may be found to be	"exe-
       cutable," but the execve(2) call will still fail.

       If the process has appropriate privileges, an implementation may indi-
       cate success for X_OK even if none of the execute file permission bits
       are set.

RETURN VALUE
       On  success (all requested permissions granted), zero is returned.  On
       error (at least one bit in mode asked for a permission that is denied,
       or some other error occurred), -1 is returned, and errno is set appro-
       priately.

ERRORS
       access shall fail if:

       EACCES The requested access would be denied to the file or search per-
	      mission is denied to one of the directories in pathname.

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ENAMETOOLONG
	      pathname is too long.

       ENOENT A directory component in pathname would  have  been  accessible
	      but does not exist or was a dangling symbolic link.

       ENOTDIR
	      A	 component used as a directory in pathname is not, in fact, a
	      directory.

       EROFS  Write permission was  requested  for  a  file  on	 a  read-only
	      filesystem.

       access may fail if:

       EFAULT pathname points outside your accessible address space.

       EINVAL mode was incorrectly specified.

       EIO    An I/O error occurred.

       ENOMEM Insufficient kernel memory was available.

       ETXTBSY
	      Write access was requested to an executable which is being exe-
	      cuted.

RESTRICTIONS
       access returns an error if any of the access types  in  the  requested
       call fails, even if other types might be successful.

       access  may  not	 work  correctly on NFS file systems with UID mapping
       enabled, because UID mapping is done on the server and hidden from the
       client, which checks permissions.

       Using  access  to  check	 if  a user is authorized to e.g. open a file
       before actually doing  so  using	 open(2)  creates  a  security	hole,
       because	the user might exploit the short time interval between check-
       ing and opening the file to manipulate it.

CONFORMING TO
       SVID, AT&T, POSIX, X/OPEN, BSD 4.3

SEE ALSO
       stat(2), open(2), chmod(2), chown(2), setuid(2), setgid(2)



Linux				  2002-04-23			    ACCESS(2)