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ACCESS(2) System Calls Manual ACCESS(2)

access — check access permissions of a file or pathname

#include <unistd.h>

int
access(const char *path, int mode);

The () 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.

Even if a process 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.

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.

Access to the file is denied if:

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A component of the path prefix is not a directory.
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The pathname contains a character with the high-order bit set.
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A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
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The named file does not exist.
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Too many symbolic links were encountered in translating the pathname.
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Write access is requested for a file on a read-only file system.
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Write access is requested for a pure procedure (shared text) file presently being executed.
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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.
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Path points outside the process's allocated address space.
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An I/O error occurred while reading from or writing to the file system.

chmod(2), stat(2)

Access() conforms to IEEE Std 1003.1-1988 (“POSIX”).

() is a potential security hole and should never be used.

BSD 4 April 1, 1994 ACCESS(2)