access, faccessat - 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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
- Since glibc 2.10:
- _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
() checks whether the calling process can access the file
. If pathname
is a symbolic link, it is dereferenced.
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
, and X_OK
tests for the
existence of the file. R_OK
, and X_OK
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 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 of capabilities.
This allows set-user-ID programs and capability-endowed programs to easily
determine the invoking user's authority. In other words, access
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
check is successful for a regular file if execute permission is
enabled for any of the file owner, group, or other.
() system call operates in exactly the same way as
(), 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 descriptor dirfd
(rather than relative to the current working directory of the calling process,
as is done by access
() for a relative pathname).
is relative and dirfd
is the special value
, then pathname
is interpreted relative to the current
working directory of the calling process (like access
is absolute, then dirfd
is constructed by ORing together zero or more of the following
- Perform access checks using the effective user and group
IDs. By default, faccessat() uses the real IDs (like
- If pathname is a symbolic link, do not dereference
it: instead return information about the link itself.
for an explanation of the need for faccessat
On success (all requested permissions granted, or mode
the file exists), zero is returned. On error (at least one bit in mode
asked for a permission that is denied, or mode
file does not exist, or some other error occurred), -1 is returned, and
is set appropriately.
() and faccessat
() 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 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
() and faccessat
() may fail if:
- pathname points outside your accessible address
- mode 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 faccessat
- dirfd is not a valid file descriptor.
- Invalid flag specified in flags.
- pathname is relative and dirfd 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.
(): 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 open(2)
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)
() always dereferences symbolic links. If you need to check the
permissions on a symbolic link, use faccessat
() with the flag
These calls return an error if any of the access types in mode
even if some of the other access types in mode
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
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 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.
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
flags are actually
implemented within the glibc wrapper function for faccessat
either of these flags is specified, then the wrapper function employs
to determine access permissions.
On older kernels where faccessat
() is unavailable (and when the
flags are not specified), the
glibc wrapper function falls back to the use of access
is a relative pathname, glibc constructs a pathname based on
the symbolic link in /proc/self/fd
that corresponds to the dirfd
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 access
() test that
returns -1 is when mode
is specified as just X_OK
is also specified in mode
() 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 MS_NOEXEC
flag if it was used to mount(2)
the underlying filesystem. Since kernel
2.6.20, the MS_NOEXEC
flag is honored.
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