Section: Linux Programmer's Manual (2)
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access - check real user's permissions for a file
int access(const char *pathname, int mode);
checks whether the calling process can access the file
is a symbolic link, it is dereferenced.
specifies the accessibility check(s) to be performed,
and is either the value
or a mask consisting of the bitwise OR of one or more of
R_OK, W_OK, and X_OK.
tests for the existence of the file.
R_OK, W_OK, and X_OK
test whether the file exists and grants read, write, and
execute permissions, respectively.
The check is done using the calling process's
UID and GID, rather than the effective IDs as is done when
actually attempting an operation (e.g.,
on the file.
This allows set-user-ID programs to
easily determine the invoking user's authority.
If the calling process is privileged (i.e., its real UID is zero),
check is successful for a regular file if execute permission
is enabled for any of the file owner, group, or other.
On success (all requested permissions granted), zero is returned.
On error (at least one bit in
asked for a permission that is denied, or some other error occurred),
-1 is returned, and
is set appropriately.
shall fail if:
The requested access would be denied to the file, or search permission
is denied for one of the directories in the path prefix of
Too many symbolic links were encountered in resolving
is too long.
A component of
does not exist or is a dangling symbolic link.
A component used as a directory in
is not, in fact, a directory.
Write permission was requested for a file on a read-only file system.
may fail if:
points outside your accessible address space.
was incorrectly specified.
An I/O error occurred.
Insufficient kernel memory was available.
Write access was requested to an executable which is being
SVr4, 4.3BSD, POSIX.1-2001.
to check if a user is authorized to, for example,
open a file before actually doing so using
creates a security hole, because the user might exploit the short time
interval between checking and opening the file to manipulate it.
For this reason, the use of this system call should be avoided.
returns an error if any of the access types in
is denied, even if some of the other access types in
If the calling process has appropriate privileges (i.e., is superuser),
POSIX.1-2001 permits implementation to indicate success for an
check even if none of the execute file permission bits are set.
Linux does not do this.
A file is only accessible if the permissions on each of the
directories in the path prefix of
grant search (i.e., execute) access.
If any directory is inaccessible, then the
call will fail, regardless of the permissions on the file itself.
Only access bits are checked, not the file type or contents.
Therefore, 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 "executable," but the
call will still fail.
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.
In kernel 2.4 (and earlier) there is some strangeness in the handling of
tests for superuser.
If all categories of execute permission are disabled
for a nondirectory file, then the only
test that returns -1 is when
is specified as just
is also specified in
returns 0 for such files.
Early 2.6 kernels (up to and including 2.6.3)
also behaved in the same way as kernel 2.4.
In kernels before 2.6.20,
ignored the effect of the
flag if it was used to
the underlying file system.
Since kernel 2.6.20,
honors this flag.
This page is part of release 3.27 of the Linux
A description of the project,
and information about reporting bugs,
can be found at
- RETURN VALUE
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- SEE ALSO
This document was created by
using the manual pages.