oracular (2) access.2freebsd.gz

NAME

     access, eaccess, faccessat — check accessibility of a file

LIBRARY

     Standard C Library (libc, -lc)

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() and eaccess() system calls check the accessibility of the file named by the path argument for
     the access permissions indicated by the mode argument.  The value of mode is either the bitwise-inclusive
     OR of the access permissions to be checked (R_OK for read permission, W_OK for write permission, and X_OK
     for execute/search permission), or the existence test (F_OK).

     For additional information, see the File Access Permission section of intro(2).

     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.

     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 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().

     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

     Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global
     variable errno is set to indicate the error.

ERRORS

     access(), eaccess(), or faccessat() will 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.

SEE ALSO

     chmod(2), intro(2), stat(2)

STANDARDS

     The access() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”).  The faccessat() system
     call follows The Open Group Extended API Set 2 specification.

HISTORY

     The access() function appeared in Version 7 AT&T UNIX.  The faccessat() system call appeared in
     FreeBSD 8.0.

SECURITY CONSIDERATIONS

     The access() system call is a potential security hole due to race conditions and should never be used.
     Set-user-ID and set-group-ID applications should restore the effective user or group ID, and perform
     actions directly rather than use access() to simulate access checks for the real user or group ID.  The
     eaccess() system call likewise may be subject to races if used inappropriately.

     access() remains useful for providing clues to users as to whether operations make sense for particular
     filesystem objects (e.g. 'delete' menu item only highlighted in a writable folder ... avoiding
     interpretation of the st_mode bits that the application might not understand -- e.g. in the case of AFS).
     It also allows a cheaper file existence test than stat(2).