ACCESS(2) System Calls ACCESS(2)
NAME
access, faccessat - determine accessibility of a file
SYNOPSIS
#include <unistd.h>
#include <sys/fcntl.h>
int access(
const char *path,
int amode);
int faccessat(
int fd,
const char *path,
int amode,
int flag);
DESCRIPTION
The
access() function checks the file named by the pathname pointed
to by the
path argument for accessibility according to the bit
pattern contained in
amode, using the real user
ID in place of the
effective user
ID and the real group
ID in place of the effective
group ID. This allows a setuid process to verify that the user
running it would have had permission to access this file.
The value of
amode is either the bitwise inclusive
OR of the access
permissions to be checked (
R_OK,
W_OK,
X_OK) or the existence test,
F_OK.
These constants are defined in <
unistd.h> as follows:
R_OK Test for read permission.
W_OK Test for write permission.
X_OK Test for execute or search permission.
F_OK Check existence of file
See
Intro(2) for additional information about "File Access
Permission".
If any access permissions are to be checked, each will be checked
individually, as described in
Intro(2). If the process has
appropriate privileges, an implementation may indicate success for
X_OK even if none of the execute file permission bits are set.
The
faccessat() function is equivalent to the
access() function,
except in the case where
path specifies a relative path. In this case
the file whose accessibility is to be determined is located relative
to the directory associated with the file descriptor
fd instead of
the current working directory.
If
faccessat() is passed in the
fd parameter the special value
AT_FDCWD, defined in
<fcntl.h>, the current working directory is used
and the behavior is identical to a call to
access().
Values for
flag are constructed by a bitwise-inclusive OR of flags
from the following list, defined in
<fcntl.h>:
AT_EACCESS The checks for accessibility are performed using the
effective user and group IDs instead of the real user
and group ID as required in a call to
access().
RETURN VALUES
If the requested access is permitted,
access() and
faccessat()succeed
and return
0. Otherwise,
-1 is returned and
errno is set to indicate
the error.
ERRORS
The
access() and
faccessat() functions will fail if:
EACCES Permission bits of the file mode do not permit the
requested access, or search permission is denied on a
component of the path prefix.
EFAULT The
path argument points to an illegal address.
EINTR A signal was caught during the
access() function.
ELOOP Too many symbolic links were encountered in resolving
path, or loop exists in symbolic links encountered
during resolution of the
path argument.
ENAMETOOLONG The length of the
path argument exceeds {
PATH_MAX},
or a pathname component is longer than {
NAME_MAX}
while
_POSIX_NO_TRUNC is in effect.
ENOENT A component of
path does not name an existing file or
path is an empty string.
ENOLINK The
path argument points to a remote machine and the
link to that machine is no longer active.
ENOTDIR A component of the path prefix is not a directory.
ENXIO The
path argument points to a character or block
device special file and the corresponding device has
been retired by the fault management framework.
EROFS Write access is requested for a file on a read-only
file system.
The
faccessat() function will fail if:
EBADF The
path argument does not specify an absolute path and the
fd argument is neither
AT_FDCWD nor a valid file descriptor
open for reading or searching.
The
access() and
faccessat() functions may fail if:
EINVAL The value of the
amode argument is invalid.
ENAMETOOLONG Pathname resolution of a symbolic link produced an
intermediate result whose length exceeds {
PATH_MAX}.
ETXTBSY Write access is requested for a pure procedure
(shared text) file that is being executed.
The
faccessat() function may fail if:
EINVAL The value of the
flag argument is not valid.
ENOTDIR The
path argument is not an absolute path and
fd is
neither
AT_FDCWD nor a file descriptor associated with a
directory.
USAGE
Additional values of
amode other than the set defined in the
description might be valid, for example, if a system has extended
access controls.
The purpose of the
faccessat() function is to enable the checking of
the accessibility of files in directories other than the current
working directory without exposure to race conditions. Any part of
the path of a file could be changed in parallel to a call to
access(), resulting in unspecified behavior. By opening a file
descriptor for the target directory and using the
faccessat() function, it can be guaranteed that the file tested for accessibility
is located relative to the desired directory.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See below. |
+--------------------+-------------------+
For
access(), see
standards(7).
SEE ALSO
Intro(2),
chmod(2),
stat(2),
attributes(7),
standards(7) June 16, 2009 ACCESS(2)
