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)