access(2) [netbsd man page]
ACCESS(2) BSD System Calls Manual ACCESS(2) NAME
access -- check access permissions of a file or pathname LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> int access(const char *path, int mode); DESCRIPTION
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 exe- cute/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. If a process has super-user privileges and indicates success for R_OK or W_OK, the file may not actually have read or write permission bits set. If a process has super-user privileges and indicates success for X_OK, at least one of the user, group, or other execute bits is set. (However, the file may still not be executable. See execve(2).) RETURN VALUES
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. ERRORS
Access to the file is denied if: [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 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. [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. [ELOOP] Too many symbolic links were encountered in translating the pathname. [ENAMETOOLONG] A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters. [ENOENT] The named file does not exist. [ENOTDIR] A component of the path prefix is not a directory. [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 presently being executed. SEE ALSO
chmod(2), execve(2), stat(2), secure_path(3) STANDARDS
The access() function conforms to ISO/IEC 9945-1:1990 (``POSIX.1''). SECURITY CONSIDERATIONS
The access() system call is a potential security hole due to race conditions. It should never be used. Set-user-ID and set-group-ID appli- cations 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. The access() system call may however have some value in providing clues to users as to whether certain operations make sense for a particular filesystem object. Arguably it also allows a cheaper file existence test than stat(2). BSD
May 3, 2010 BSD
Check Out this Related Man Page
ACCESS(2) BSD System Calls Manual ACCESS(2) NAME
access, eaccess, faccessat -- check accessibility of a file LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <unistd.h> int access(const char *path, int mode); int eaccess(const char *path, int mode); int faccessat(int fd, const char *path, int mode, int flag); DESCRIPTION
The access() and eaccess() system calls check 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 permis- sion, 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 eaccess() system call uses the effective user ID and the group access list to authorize the request; the access() 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 faccessat() 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>: AT_EACCESS 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(). Even if a process's real or effective user 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. RETURN VALUES
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. ERRORS
access(), eaccess(), or faccessat() will fail if: [EINVAL] The value of the mode argument is invalid. [ENOTDIR] A component of the path prefix is not a directory. [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 presently 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. [EFAULT] The path argument points outside the process's allocated address space. [EIO] An I/O error occurred while reading from or writing to the file system. Also, the faccessat() system call may fail if: [EBADF] The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor. [EINVAL] The value of the flag argument is not valid. [ENOTDIR] The path argument is not an absolute path and fd is neither AT_FDCWD nor a file descriptor associated with a directory. SEE ALSO
chmod(2), intro(2), stat(2) STANDARDS
The access() system call is expected to conform to ISO/IEC 9945-1:1990 (``POSIX.1''). The faccessat() system call follows The Open Group Extended API Set 2 specification. HISTORY
The access() function appeared in Version 7 AT&T UNIX. The faccessat() system call appeared in FreeBSD 8.0. SECURITY CONSIDERATIONS
The access() system call is a potential security hole due to race conditions and should never be used. Set-user-ID and set-group-ID applica- tions 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. The eaccess() system call likewise may be subject to races if used inappropriately. access() remains useful for providing clues to users as to whether operations make sense for particular filesystem objects (e.g. 'delete' menu item only highlighted in a writable folder ... avoiding interpretation of the st_mode bits that the application might not understand -- e.g. in the case of AFS). It also allows a cheaper file existence test than stat(2). BSD
September 15, 2014 BSD