ACCESS(2) System Calls Manual ACCESS(2)

access, faccessat — check accessibility of a file

#include <unistd.h>

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

int
faccessat(int fd, const char *path, int mode, int flag);

The () system call checks 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 () 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 () 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>:

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().
If path names a symbolic link, the status of the symbolic link is returned.
If path names a symbolic link, the status of the symbolic link is returned and if the path has any other symbolic links, an error is returned.

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.

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.

access() or faccessat() will fail if:

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The value of the mode argument is invalid.
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A component of the path prefix is not a directory.
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A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} 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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AT_SYMLINK_NOFOLLOW_ANY was passed and a symbolic link was 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.
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The path argument 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.

Also, the faccessat() system call may fail if:

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The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor.
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The value of the flag argument is not valid.
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The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory.

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

The access() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”). The faccessat() system call is expected to conform to POSIX.1-2008 .

The access() function appeared in Version 7 AT&T UNIX.

The result of access() should not be used to make an actual access control decision, since its response, even if correct at the moment it is formed, may be outdated at the time you act on it. access() results should only be used to pre-flight, such as when configuring user interface elements or for optimization purposes. The actual access control decision should be made by attempting to execute the relevant system call while holding the applicable credentials, and properly handling any resulting errors; and this must be done even though access() may have predicted success.

Additionally, 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.

September 15, 2014 Mac OS X 12