ACCESS(2) BSD System Calls Manual ACCESS(2)NAMEaccess — check access permissions of a file or pathname
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <unistd.h>
int
access(const char *path, int mode);
DESCRIPTION
The access() function checks the accessibility of the file named by path
for the access permissions indicated by mode. The value of mode is 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. All components of the pathname
path are checked for access permissions (including F_OK).
The real user ID is used in place of the effective user ID and the real
group access list (including the real group ID) are used in place of the
effective ID for verifying permission.
If a process has super-user privileges and indicates success for R_OK or
W_OK, the file may not actually have read or write permission bits set.
If a process has super-user privileges and indicates success for X_OK, at
least one of the user, group, or other execute bits is set. (However,
the file may still not be executable. See execve(2).)
RETURN VALUES
If path cannot be found or if any of the desired access modes would not
be granted, then a -1 value is returned; otherwise a 0 value is returned.
ERRORS
Access to the file is denied 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. The owner of a file has
permission checked with respect to the “owner” read,
write, and execute mode bits, members of the file's
group other than the owner have permission checked
with respect to the “group” mode bits, and all others
have permissions checked with respect to the “other”
mode bits.
[EFAULT] path points outside the process's allocated address
space.
[EIO] An I/O error occurred while reading from or writing to
the file system.
[ELOOP] Too many symbolic links were encountered in translat‐
ing the pathname.
[ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX} charac‐
ters, or an entire path name exceeded {PATH_MAX} char‐
acters.
[ENOENT] The named file does not exist.
[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.
[ETXTBSY] Write access is requested for a pure procedure (shared
text) file presently being executed.
SEE ALSOchmod(2), execve(2), stat(2), secure_path(3)STANDARDS
The access() function conforms to ISO/IEC 9945-1:1990 (“POSIX.1”).
SECURITY CONSIDERATIONS
The access() system call is a potential security hole due to race condi‐
tions. It should never be used. Set-user-ID and set-group-ID applica‐
tions 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 access() system call may however have some value in providing clues
to users as to whether certain operations make sense for a particular
filesystem object. Arguably it also allows a cheaper file existence test
than stat(2).
BSD May 3, 2010 BSD
