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access, faccessat - determine accessibility of a file

#include <unistd.h>
#include <sys/fcntl.h>

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

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

The access() function checks the file named by the pathname pointed to by the path argument for accessibility according to the bit pattern contained in amode, using the real user ID in place of the effective user ID and the real group ID in place of the effective group ID. This allows a setuid process to verify that the user running it would have had permission to access this file.

The value of amode is either the bitwise inclusive OR of the access permissions to be checked (R_OK, W_OK, X_OK) or the existence test, F_OK.

These constants are defined in <unistd.h> as follows:

R_OK

W_OK

X_OK

F_OK

See Intro(2) for additional information about "File Access Permission".

If any access permissions are to be checked, each will be checked individually, as described in Intro(2). If the process has appropriate privileges, an implementation may indicate success for X_OK even if none of the execute file permission bits are set.

The faccessat() function is equivalent to the access() function, 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 in the fd parameter the special value AT_FDCWD, defined in <fcntl.h>, 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

If the requested access is permitted, access() and faccessat()succeed and return 0. Otherwise, -1 is returned and errno is set to indicate the error.

The access() and faccessat() functions will fail if:

EACCES

EFAULT

EINTR

ELOOP

ENAMETOOLONG

ENOENT

ENOLINK

ENOTDIR

ENXIO

EROFS

The faccessat() function will fail if:

EBADF

The access() and faccessat() functions may fail if:

EINVAL

ENAMETOOLONG

ETXTBSY

The faccessat() function may fail if:

EINVAL

ENOTDIR

Additional values of amode other than the set defined in the description might be valid, for example, if a system has extended access controls.

The purpose of the faccessat() function is to enable the checking of the accessibility of files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to access(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the faccessat() function, it can be guaranteed that the file tested for accessibility is located relative to the desired directory.

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

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Committed
MT-Level	Async-Signal-Safe
Standard	See below.

For access(), see standards(5).

Intro(2), chmod(2), stat(2), attributes(5), standards(5)