access — check user's permissions for a file
#include <unistd.h>
int
access( |
const char * | pathname, |
int | mode) ; |
access
() checks whether the
calling 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 either the
value F_OK
or a mask consisting
of the bitwise OR of one or more of R_OK
, W_OK
and X_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 calling process's real
UID and GID, rather than
with the effective IDs as is done when actually attempting an
operation. This is to allow set-user-ID programs to easily
determine the invoking user's authority.
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 execve(2) call will still fail.
If the calling process has appropriate privileges, an
implementation may indicate success for X_OK
even if none of the execute file
permission bits are set.
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
appropriately.
access
() 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 pathname
. (See also
path_resolution(7).)
Too many symbolic links were encountered in
resolving pathname
.
pathname
is
too long.
A component of pathname
does not exist
or is a dangling symbolic link.
A component used as a directory in pathname
is not, in fact,
a directory.
Write permission was requested for a file on a read-only filesystem.
access
() may fail if:
pathname
points outside your accessible address space.
mode
was
incorrectly specified.
An I/O error occurred.
Insufficient kernel memory was available.
Write access was requested to an executable which is being executed.
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.
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Warning |
---|---|
Using |
In kernels before 2.6.20, access
() ignored the effect of the
MS_NOEXEC
flag if it was used
to mount(2) the underlying
file system. Since kernel 2.6.20, access
() honours this flag.
chmod(2), chown(2), faccessat(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3), path_resolution(7)
|