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ACCESS
Section: System calls (2)
Updated: January 13, 1998
Index NAME
access - check user's permissions for a file
SYNOPSIS
#include <unistd.h>int access(const char *pathname, int mode);
DESCRIPTION
accesschecks whether the process would be allowed to read,write or test for existence of the file (or other file systemobject) whose name is
pathname.If
pathnameis a symbolic link permissions of the file referred to by thissymbolic link are tested.
modeis a mask consisting of one or more ofR_OK, W_OK, X_OK and F_OK.
R_OK, W_OK and X_OKrequest checking whether the file exists and has read, write andexecute permissions, respectively.F_OKjust requests checking for the existence of the file.
The tests depend on the permissions of the directoriesoccurring in the path to the file, as given inpathname,and on the permissions of directories and files referred to by symboliclinks encountered on the way.
The check is done with the process'srealuid and gid, rather than with the effective ids as is done whenactually attempting an operation. This is to allow set-UID programs toeasily determine the invoking user's authority.
Only access bits are checked, not the file type or contents. Therefore, ifa directory is found to be "writable," it probably means that files can becreated in the directory, and not that the directory can be written as afile. Similarly, a DOS file may be found to be "executable," but theexecve(2)call will still fail.
RETURN VALUE
On success (all requested permissions granted), zero is returned.On error (at least one bit in
modeasked for a permission that is denied, or some other error occurred),-1 is returned, and
errnois set appropriately.
ERRORS
- EACCES
- The requested access would be denied to the file or search permissionis denied to one of the directories inpathname.
- EROFS
- Write permission was requested for a file on a read-only filesystem.
- EFAULT
- pathnamepoints outside your accessible address space.
- EINVAL
- modewas incorrectly specified.
- ENAMETOOLONG
- pathnameis too long.
- ENOENT
- A directory component inpathnamewould have been accessible but does not exist or was a danglingsymbolic link.
- ENOTDIR
- A component used as a directory inpathnameis not, in fact, a directory.
- ENOMEM
- Insufficient kernel memory was available.
- ELOOP
- Too many symbolic links were encountered in resolvingpathname.
- EIO
- An I/O error occurred.
RESTRICTIONS
accessreturns an error if any of the access types in the requested callfails, even if other types might be successful.
accessmay 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.
Usingaccessto check if a user is authorized to e.g. open a file before actuallydoing so usingopen(2)creates a security hole, because the user might exploit the short timeinterval between checking 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).
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- RETURN VALUE
- ERRORS
- RESTRICTIONS
- CONFORMING TO
- SEE ALSO
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