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ACCESS(2) System Calls Manual ACCESS(2)

access, faccessat —
check access permissions of a file or pathname

#include <unistd.h>
int
access(const char *path, int amode);
#include <fcntl.h>
#include <unistd.h>
int
faccessat(int fd, const char *path, int amode, int flag);

The access() function checks the accessibility of the file named by path for the access permissions indicated by amode. The amode argument is either the bitwise OR of one or more 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) is used in place of the effective ID for verifying permission.
If the invoking process has superuser privileges, access() will always indicate success for R_OK and W_OK, regardless of the actual file permission bits. Likewise, for X_OK, if the file has any of the execute bits set and path is not a directory, access() will indicate success.
The faccessat() function is equivalent to access() except that where path specifies a relative path, the file whose accessibility is checked is determined relative to the directory associated with file descriptor fd instead of the current working directory.
If faccessat() is passed the special value AT_FDCWD (defined in <fcntl.h>) in the fd parameter, the current working directory is used. If flag is also zero, the behavior is identical to a call to access().
The flag argument is the bitwise OR of zero or more of the following values:
The checks for accessibility are performed using the effective user and group IDs instead of the real user and group IDs.

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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A component of a pathname exceeded NAME_MAX characters, or an entire pathname (including the terminating NUL) exceeded PATH_MAX bytes.
 
 
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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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Write access has been requested and the named file has its immutable flag set (see chflags(2)).
 
 
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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.
 
 
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An invalid value was specified for amode.
Additionally, faccessat() will fail if:
 
 
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The value of the flag argument was neither zero nor AT_EACCESS.
 
 
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The path argument specifies a relative path and the fd argument is neither AT_FDCWD nor a valid file descriptor.
 
 
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The path argument specifies a relative path and the fd argument is a valid file descriptor but it does not reference a directory.
 
 
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The path argument specifies a relative path but search permission is denied for the directory which the fd file descriptor references.

chmod(2), stat(2)

The access() and faccessat() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”).

access() first appeared as an internal kernel function in Version 1 AT&T UNIX and was reimplemented in C before the release of Version 4 AT&T UNIX. It was first promoted to a system call in the Programmer's Workbench (PWB/UNIX), which was later ported to Version 7 AT&T UNIX and 2BSD.
The faccessat() function appeared in OpenBSD 5.0.

Ken Thompson first implemented the access() kernel function in C.

access() and faccessat() should never be used for actual access control. Doing so can result in a time of check vs. time of use security hole.
January 19, 2015 OpenBSD-current