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.Dd May 21, 2024 .Dt ACCESS 2 .Os .Sh NAME .Nm access , .Nm eaccess , .Nm faccessat .Nd check accessibility of a file .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn access "const char *path" "int mode" .Ft int .Fn eaccess "const char *path" "int mode" .Ft int .Fn faccessat "int fd" "const char *path" "int mode" "int flag" .Sh DESCRIPTION The .Fn access , .Fn eaccess and .Fn faccessat system calls report whether an attempt to access the file designated by their .Fa path in the manner described by their .Fa mode argument is likely to succeed. The value of .Fa mode is either the bitwise-inclusive OR of the desired permissions
o .Dv R_OK for read permission, .Dv W_OK for write permission, and .Dv X_OK for execute / search permission
c or .Dv F_OK to simply check whether the file exists.
p For a number of reasons, these system calls cannot be relied upon to give a correct and definitive answer. They can at best provide an early indication of the expected outcome, to be confirmed by actually attempting the operation. For existence checks, either .Xr stat 2 or .Xr lstat 2 should be used instead. See also .Sx SECURITY CONSIDERATIONS below.
p The .Fn eaccess system call uses the effective user ID and the group access list to authorize the request; the .Fn 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.
p See the .Sx DEFINITIONS section of .Xr intro 2 for additional information on file access permissions and real vs. effective user and group IDs.
p The .Fn faccessat system call is equivalent to .Fn access except in the case where .Fa 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 .Fa fd instead of the current working directory. If .Fn faccessat is passed the special value .Dv AT_FDCWD in the .Fa fd parameter, the current working directory is used and the behavior is identical to a call to .Fn access . Values for .Fa flag are constructed by a bitwise-inclusive OR of flags from the following list, defined in n fcntl.h : l -tag -width indent t Dv AT_EACCESS The checks are performed using the effective user and group IDs, like .Fn eaccess , instead of the real user and group ID, like .Fn access . t Dv AT_RESOLVE_BENEATH Only walk paths below the directory specified by the .Ar fd descriptor. See the description of the .Dv O_RESOLVE_BENEATH flag in the .Xr open 2 manual page. t Dv AT_EMPTY_PATH If the .Fa path argument is an empty string, operate on the file or directory referenced by the descriptor .Fa fd . If .Fa fd is equal to .Dv AT_FDCWD , operate on the current working directory. .El
p Even if a process's real or effective user has appropriate privileges and indicates success for .Dv X_OK , the file may not actually have execute permission bits set. Likewise for .Dv R_OK and .Dv W_OK . .Sh RETURN VALUES .Rv -std .Sh ERRORS The .Fn access , .Fn eaccess , and .Fn faccessat system calls may fail if: l -tag -width Er t Bq Er EINVAL The value of the .Fa mode argument is invalid. t Bq Er ENOTDIR A component of the path prefix is not a directory. t Bq Er ENAMETOOLONG A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters. t Bq Er ENOENT The named file does not exist. t Bq Er ELOOP Too many symbolic links were encountered in translating the pathname. t Bq Er EROFS Write access is requested for a file on a read-only file system. t Bq Er ETXTBSY Write access is requested for a pure procedure (shared text) file presently being executed. t Bq Er 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. t Bq Er EFAULT The .Fa path argument points outside the process's allocated address space. t Bq Er EIO An I/O error occurred while reading from or writing to the file system. t Bq Er EINTEGRITY Corrupted data was detected while reading from the file system. .El
p Also, the .Fn faccessat system call may fail if: l -tag -width Er t Bq Er EBADF The .Fa path argument does not specify an absolute path and the .Fa fd argument is neither .Dv AT_FDCWD nor a valid file descriptor. t Bq Er EINVAL The value of the .Fa flag argument is not valid. t Bq Er ENOTDIR The .Fa path argument is not an absolute path and .Fa fd is neither .Dv AT_FDCWD nor a file descriptor associated with a directory. t Bq Er ENOTCAPABLE .Fa path is an absolute path, or contained a ".." component leading to a directory outside of the directory hierarchy specified by .Fa fd , and the process is in capability mode. .El .Sh SEE ALSO .Xr chmod 2 , .Xr intro 2 , .Xr stat 2 .Sh STANDARDS The .Fn access system call is expected to conform to .St -p1003.1-90 . The .Fn faccessat system call follows The Open Group Extended API Set 2 specification. .Sh HISTORY The .Fn access function appeared in .At v7 . The .Fn faccessat system call appeared in .Fx 8.0 . .Sh SECURITY CONSIDERATIONS The .Fn access , .Fn eaccess , and .Fn faccessat system calls are subject to time-of-check-to-time-of-use races and should not be relied upon for file permission enforcement purposes. Instead, applications should perform the desired action using the requesting user's credentials.