Section: Linux Programmer's Manual (2)
access, faccessat, faccessat2 - check user's permissions for a file
int access(const char *pathname, int mode);
#include <fcntl.h> /* Definition of AT_* constants */
int faccessat(int dirfd, const char *pathname, int mode, int flags);
/* But see C library/kernel differences, below */
int faccessat2(int dirfd, const char *pathname, int mode, int flags);
Feature Test Macro Requirements for glibc (see
- Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
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.
Similarly, for the root user, the check uses the set of
permitted capabilities rather than the set of effective
capabilities; and for non-root users, the check uses an empty set
This allows set-user-ID programs and capability-endowed programs
to easily determine the invoking user's authority.
In other words,
does not answer the "can I read/write/execute this file?" question.
It answers a slightly different question:
"(assuming I'm a setuid binary) can
the user who invoked me
read/write/execute this file?",
which gives set-user-ID programs the possibility to
prevent malicious users from causing them to read files
which users shouldn't be able to read.
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.
operates in exactly the same way as
except for the differences described here.
If the pathname given in
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
(rather than relative to the current working directory of
the calling process, as is done by
for a relative pathname).
is relative and
is the special value
is interpreted relative to the current working
directory of the calling process (like
is absolute, then
is constructed by ORing together zero or more of the following values:
Perform access checks using the effective user and group IDs.
uses the real IDs (like
is a symbolic link, do not dereference it:
instead return information about the link itself.
for an explanation of the need for
The description of
given above corresponds to POSIX.1 and
to the implementation provided by glibc.
However, the glibc implementation was an imperfect emulation (see BUGS)
that papered over the fact that the raw Linux
system call does not have a
To allow for a proper implementation, Linux 5.8 added the
system call, which supports the
argument and allows a correct implementation of the
On success (all requested permissions granted, or
and the file exists), zero is returned.
On error (at least one bit in
asked for a permission that is denied, or
and the file does not exist, 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 filesystem.
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
The following additional errors can occur for
is not a valid file descriptor.
Invalid flag specified in
is relative and
is a file descriptor referring to a file other than a directory.
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
was added to Linux in version 5.8.
SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
Using these calls 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
(In the example just described,
a safer alternative would be to temporarily switch the process's
effective user ID to the real ID and then call
always dereferences symbolic links.
If you need to check the permissions on a symbolic link, use
with the flag
These calls return 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 an 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 accessible only 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 fails, 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 NFSv2 filesystems with UID mapping enabled,
because UID mapping is done on the server and hidden from the client,
which checks permissions. (NFS versions 3 and higher perform the check on
Similar problems can occur to FUSE mounts.
C library/kernel differences
system call takes only the first three arguments.
flags are actually implemented within the glibc wrapper function for
If either of these flags is specified, then the wrapper function employs
to determine access permissions, but see BUGS.
On older kernels where
is unavailable (and when the
flags are not specified),
the glibc wrapper function falls back to the use of
is a relative pathname,
glibc constructs a pathname based on the symbolic link in
that corresponds to the
Because the Linux kernel's
system call does not support a
argument, the glibc
wrapper function provided in glibc 2.32 and earlier
emulates the required functionality using
a combination of the
system call and
However, this emulation does not take ACLs into account.
Starting with glibc 2.33, the wrapper function avoids this bug
by making use of the
system call where it is provided by the underlying kernel.
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,
these calls ignored the effect of the
flag if it was used to
the underlying filesystem.
Since kernel 2.6.20, the
flag is honored.
This page is part of release 5.10 of the Linux
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