Thursday, October 13, 2011

access system call

access system call linux
access – check access permissions of a file or pathname

#include <unistd.h>
int access(const char *path, int mode);


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 an existence test.
F_OK  All components of the pathname path are checked for access
      permissions (including F_OK).

The real user ID is used instead of the effective user ID, and the real group access list (including the real group ID) are used instead of the effective ID for checking permissions.

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

If path cannot be found or if any of the desired access modes would not be granted, a value of -1 is returned; otherwise a 0 value is returned.

Access to the file is denied if:

[ENOTDIR]       A component of the path prefix is not a directory.
[EINVAL]        The pathname contains a character with the
                high-order bit set.
[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 currently 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. The owner
                of a file has permissions checked with respect
                to the “owner” read, write, and execute mode
                bits. Members of the file’s group other than the
                owner have permissions 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.

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

Interface Stability

No comments:

Post a Comment