access, faccessat, faccessat2 - check user's permissions for a file
#include <unistd.h> int access(const char *pathname, int mode); #include <fcntl.h> /* Definition of AT_* constants */ #include <unistd.h> 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 feature_test_macros(7)):
- Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
access() checks whether the calling process can
access the file
pathname is a symbolic
link, it is dereferenced.
mode specifies the accessibility check(s) to be
performed, and 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. F_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,
The check is done using the calling process's
real UID and
GID, rather than the effective IDs as is done when actually attempting
an operation (e.g., open(2)) 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 of capabilities.
This allows set-user-ID programs and capability-endowed programs to
easily determine the invoking user's authority. In other words,
access() 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), then an X_OK check is successful for a regular file if execute permission is enabled for any of the file owner, group, or other.
If the pathname given in
pathname is relative, then it is
interpreted relative to the directory referred to by the file descriptor
dirfd (rather than relative to the current working directory of
the calling process, as is done by access() for a
pathname is relative and
dirfd is the special
value AT_FDCWD, then
pathname is interpreted
relative to the current working directory of the calling process (like
pathname is absolute, then
dirfd is ignored.
flags is constructed by ORing together zero or more of the
pathname is a symbolic link, do not dereference it:
instead return information about the link itself.
The description of faccessat() 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
faccessat() system call does not have a
argument. To allow for a proper implementation, Linux 5.8 added the
faccessat2() system call, which supports the
flags argument and allows a correct implementation of the
faccessat() wrapper function.
On success (all requested permissions granted, or
F_OK and the file exists), zero is returned. On error
(at least one bit in
mode asked for a permission that is
mode is F_OK and the file does not
exist, or some other error occurred), -1 is returned, and
is set appropriately.
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 is too long.
A component of
pathname does not exist or is a dangling
A component used as a directory in
pathname is not, in fact,
Write permission was requested for a file on a read-only filesystem.
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.
The following additional errors can occur for faccessat():
dirfd is not a valid file descriptor.
Invalid flag specified in
pathname is relative and
dirfd is a file descriptor
referring to a file other than a directory.
Warning: Using these calls to check if a user is authorized to, for example, open a file before actually doing so using open(2) 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 open(2).)
These calls return an error if any of the access types in
mode is denied, even if some of the other access types in
mode are permitted.
If the calling process has appropriate privileges (i.e., is superuser), POSIX.1-2001 permits an implementation to indicate success for an X_OK 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
pathname grant search (i.e.,
execute) access. If any directory is inaccessible, then the
access() 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 execve(2) call will still fail.
These calls 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 the server.) Similar problems can occur to FUSE mounts.
The raw faccessat() system call takes only the first three arguments. The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are actually implemented within the glibc wrapper function for faccessat(). If either of these flags is specified, then the wrapper function employs fstatat(2) to determine access permissions, but see BUGS.
On older kernels where faccessat() is unavailable
(and when the AT_EACCESS and
AT_SYMLINK_NOFOLLOW flags are not specified), the glibc
wrapper function falls back to the use of access().
pathname is a relative pathname, glibc constructs a
pathname based on the symbolic link in
corresponds to the
Because the Linux kernel's faccessat() system call
does not support a
flags argument, the glibc
faccessat() wrapper function provided in glibc 2.32 and
earlier emulates the required functionality using a combination of the
faccessat() system call and
fstatat(2). 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 faccessat2() system call where
it is provided by the underlying kernel.
In kernel 2.4 (and earlier) there is some strangeness in the handling
of X_OK tests for superuser. If all categories of
execute permission are disabled for a nondirectory file, then the only
access() test that returns -1 is when
specified as just X_OK; if R_OK or
W_OK is also specified in
access() returns 0 for such files. Early 2.6 kernels
(up to and including 2.6.3) also behaved in the same way as kernel
In kernels before 2.6.20, these calls ignored the effect of the MS_NOEXEC flag if it was used to mount(2) the underlying filesystem. Since kernel 2.6.20, the MS_NOEXEC flag is honored.
This page is part of release 5.10 of the Linux
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at