access(2) System Calls Manual access(2)
NAME
access, faccessat, faccessat2 - check user's permissions for a file
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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 */
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_faccessat2,
int dirfd, const char *pathname, int mode, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
faccessat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
access() checks whether the calling process can access the file pathname. If pathname is a symbolic link, it is derefer‐
enced.
The 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, respectively.
The check is done using the calling process's real UID and GID, rather than the effective IDs as is done when actually at‐
tempting an operation (e.g., open(2)) on the file. Similarly, for the root user, the check uses the set of permitted capa‐
bilities 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.
faccessat()
faccessat() operates in exactly the same way as access(), except for the differences described here.
If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file de‐
scriptor dirfd (rather than relative to the current working directory of the calling process, as is done by access() for a
relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current work‐
ing directory of the calling process (like access()).
If pathname is absolute, then dirfd is ignored.
flags is constructed by ORing together zero or more of the following values:
AT_EACCESS
Perform access checks using the effective user and group IDs. By default, faccessat() uses the real IDs (like ac‐
cess()).
AT_SYMLINK_NOFOLLOW
If pathname is a symbolic link, do not dereference it: instead return information about the link itself.
See openat(2) for an explanation of the need for faccessat().
faccessat2()
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 flags 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.
RETURN VALUE
On success (all requested permissions granted, or mode is F_OK and the file exists), zero is returned. On error (at least
one bit in mode asked for a permission that is denied, or mode is F_OK and the file does not exist, or some other error oc‐
curred), -1 is returned, and errno is set to indicate the error.
ERRORS
EACCES 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).)
EBADF (faccessat()) pathname is relative but dirfd is neither AT_FDCWD (faccessat()) nor a valid file descriptor.
EFAULT pathname points outside your accessible address space.
EINVAL mode was incorrectly specified.
EINVAL (faccessat()) Invalid flag specified in flags.
EIO An I/O error occurred.
ELOOP Too many symbolic links were encountered in resolving pathname.
ENAMETOOLONG
pathname is too long.
ENOENT A component of pathname does not exist or is a dangling symbolic link.
ENOMEM Insufficient kernel memory was available.
ENOTDIR
A component used as a directory in pathname is not, in fact, a directory.
ENOTDIR
(faccessat()) pathname is relative and dirfd is a file descriptor referring to a file other than a directory.
EPERM Write permission was requested to a file that has the immutable flag set. See also ioctl_iflags(2).
EROFS Write permission was requested for a file on a read-only filesystem.
ETXTBSY
Write access was requested to an executable which is being executed.
VERSIONS
faccessat() was added in Linux 2.6.16; library support was added in glibc 2.4.
faccessat2() was added in Linux 5.8.
STANDARDS
access(): SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
faccessat(): POSIX.1-2008.
faccessat2(): Linux-specific.
NOTES
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).)
access() always dereferences symbolic links. If you need to check the permissions on a symbolic link, use faccessat() with
the flag AT_SYMLINK_NOFOLLOW.
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 reported as 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.
C library/kernel differences
The raw faccessat() system call takes only the first three arguments. The AT_EACCESS and AT_SYMLINK_NOFOLLOW flags are ac‐
tually implemented within the glibc wrapper function for faccessat(). If either of these flags is specified, then the wrap‐
per function employs fstatat(2) to determine access permissions, but see BUGS.
glibc notes
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(). When pathname is a relative pathname, glibc constructs a
pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
BUGS
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 Linux 2.4 (and earlier) there is some strangeness in the handling of X_OK tests for superuser. If all categories of exe‐
cute permission are disabled for a nondirectory file, then the only access() test that returns -1 is when mode is specified
as just X_OK; if R_OK or W_OK is also specified in mode, then access() returns 0 for such files. Early Linux 2.6 (up to and
including Linux 2.6.3) also behaved in the same way as Linux 2.4.
Before Linux 2.6.20, these calls ignored the effect of the MS_NOEXEC flag if it was used to mount(2) the underlying filesys‐
tem. Since Linux 2.6.20, the MS_NOEXEC flag is honored.
SEE ALSO
chmod(2), chown(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3), credentials(7), path_resolution(7), symlink(7)
Linux man-pages 6.03 2023-02-05 access(2)