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.

If the requested access is permitted, access() succeeds and returns 0. Otherwise, -1 is returned and errno is set to indicate the error.

The access() function 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

path points to an illegal address.

EINTR

A signal was caught during the access() function.

ELOOP

Too many symbolic links were encountered in resolving path.

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

path points to a remote machine and the link to that machine is no longer active.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.

EROFS

Write access is requested for a file on a read-only file system.

The access() function 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.

Additional values of amode other than the set defined in the description may be valid, for example, if a system has extended access controls.

See attributes(5) for descriptions of the following attributes:

intro(2), chmod(2), stat(2), attributes(5)