ACCESS(2)

NAME
access, eaccess, faccessat - check accessibility of a file

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
access(const char *path, int mode);
int
eaccess(const char *path, int mode);
int
faccessat(int fd, const char *path, int mode, int flag);

DESCRIPTION
The access(), eaccess() and faccessat() system calls report whether an attempt to access the file designated by their path in the manner described by their mode argument is likely to succeed. The value of mode is either the bitwise-inclusive OR of the desired permissions ( R_OK for read permission, W_OK for write permission, and X_OK for execute / search permission) or F_OK to simply check whether the file exists.

For a number of reasons, these system calls cannot be relied upon to give a correct and definitive answer. They can at best provide an early indication of the expected outcome, to be confirmed by actually attempting the operation. For existence checks, either stat(2) or lstat(2) should be used instead. See also SECURITY CONSIDERATIONS below.

The eaccess() system call uses the effective user ID and the group access list to authorize the request; the access() system call uses the real user ID in place of the effective user ID, the real group ID in place of the effective group ID, and the rest of the group access list.

See the DEFINITIONS section of intro(2) for additional information on file access permissions and real vs. effective user and group IDs.

The faccessat() system call is equivalent to access() 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 the special value AT_FDCWD in the fd parameter, 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 are performed using the effective user and group IDs, like eaccess(), instead of the real user and group ID, like access().

AT_RESOLVE_BENEATH
    Only walk paths below the directory specified by the fd descriptor. See the description of the O_RESOLVE_BENEATH flag in the open(2) manual page.

AT_EMPTY_PATH
    If the path argument is an empty string, operate on the file or directory referenced by the descriptor fd. If fd is equal to AT_FDCWD, operate on the current working directory.

AT_SYMLINK_NOFOLLOW
    If path names a symbolic link, access of the symbolic link is evaluated.

Even if a process's real or effective user has appropriate privileges and indicates success for X_OK, the file may not actually have execute permission bits set. Likewise for R_OK and W_OK.

RETURN VALUES

ERRORS
The access(), eaccess(), and faccessat() system calls may fail if:

[EINVAL]
    The value of the mode argument is invalid.

[ENOTDIR]
    A component of the path prefix is not a directory.

[ENAMETOOLONG]
    A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

[ENOENT]
    The named file does not exist.

[ELOOP]
    Too many symbolic links were encountered in translating the pathname.

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

[ETXTBSY]
    Write access is requested for a pure procedure (shared text) file presently being executed.

[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 outside the process's allocated address space.

[EIO]
    An I/O error occurred while reading from or writing to the file system.

[EINTEGRITY]
    Corrupted data was detected while reading from the file system.

Also, the faccessat() system call may 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.

[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.

[ENOTCAPABLE]
    path is an absolute path, or contained a ".." component leading to a directory outside of the directory hierarchy specified by fd, and the process is in capability mode.

SEE ALSO
chmod(2), intro(2), stat(2)

STANDARDS
The access() system call is expected to conform to p1003.1-90. The faccessat() system call follows The Open Group Extended API Set 2 specification.

HISTORY
The access() function appeared in AT&T UNIX. The faccessat() system call appeared in FreeBSD 8.0.

SECURITY CONSIDERATIONS
The access(), eaccess(), and faccessat() system calls are subject to time-of-check-to-time-of-use races and should not be relied upon for file permission enforcement purposes. Instead, applications should perform the desired action using the requesting user's credentials.