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(5) 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(5).

SEE ALSO


Intro(2), chmod(2), stat(2), attributes(5), standards(5)


June 16, 2009 ACCESS(2)