NAME
access - check real user's permissions for a file SYNOPSIS
#include
int access(const char *pathname, int mode); DESCRIPTION access() checks whether the calling process can access the file path‐ name. If pathname is a symbolic link, it is dereferenced. The mode specifies the accessibility check(s) to be performed, and is either the value FOK, or a mask consisting of the bitwise OR of one or more of ROK, WOK, and XOK. FOK tests for the existence of the file. ROK, WOK, and XOK test whether the file exists and grants read, write, and execute permissions, respectively. The check is done using the calling process's real UID and GID, rather than the effective IDs as is done when actually attempting an operation (e.g., open(2)) on the file. This allows set-user-ID programs to eas‐ ily determine the invoking user's authority. If the calling process is privileged (i.e., its real UID is zero), then an XOK check is successful for a regular file if execute permission is enabled for any of the file owner, group, or other. RETURN VALUE On success (all requested permissions granted, or mode is FOK and the file exists), zero is returned. On error (at least one bit in mode asked for a permission that is denied, or mode is FOK and the file
does not exist, or some other error occurred), -1 is returned, and errno is set appropriately. ERRORS access() shall fail if: EACCES The requested access would be denied to the file, or search per‐ mission is denied for one of the directories in the path prefix of pathname. (See also pathresolution(7).) ELOOP Too many symbolic links were encountered in resolving pathname. ENAMETOOLONG pathname is too long. ENOENT A component of pathname does not exist or is a dangling symbolic link. ENOTDIR A component used as a directory in pathname is not, in fact, a directory.
EROFS Write permission was requested for a file on a read-only file system. access() may fail if: EFAULT pathname points outside your accessible address space. EINVAL mode was incorrectly specified. EIO An I/O error occurred. ENOMEM Insufficient kernel memory was available. ETXTBSY Write access was requested to an executable which is being exe‐ cuted. CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. NOTES Warning: Using access() to check if a user is authorized to, for exam‐ ple, open a file before actually doing so using open(2) creates a secu‐ rity hole, because the user might exploit the short time interval between checking and opening the file to manipulate it. For this rea‐ son, the use of this system call should be avoided. (In the example just described, a safer alternative would be to temporarily switch the process's effective user ID to the real ID and then call open(2).) access() always dereferences symbolic links. If you need to check the permissions on a symbolic link, use faccessat(2) with the flag ATSYM‐ LINKNOFOLLOW. access() returns an error if any of the access types in mode is denied, even if some of the other access types in mode are permitted. If the calling process has appropriate privileges (i.e., is superuser),
POSIX.1-2001 permits an implementation to indicate success for an XOK check even if none of the execute file permission bits are set. Linux does not do this. A file is accessible only if the permissions on each of the directories in the path prefix of pathname grant search (i.e., execute) access. If any directory is inaccessible, then the access() call will fail, regardless of the permissions on the file itself. Only access bits are checked, not the file type or contents. There‐ fore, if a directory is found to be writable, it probably means that files can be created in the directory, and not that the directory can be written as a file. Similarly, a DOS file may be found to be "exe‐ cutable," but the execve(2) call will still fail. access() may not work correctly on NFS file systems with UID mapping enabled, because UID mapping is done on the server and hidden from the client, which checks permissions. Similar problems can occur to FUSE mounts. BUGS In kernel 2.4 (and earlier) there is some strangeness in the handling of XOK tests for superuser. If all categories of execute permission are disabled for a nondirectory file, then the only access() test that
returns -1 is when mode is specified as just XOK; if ROK or WOK is also specified in mode, then access() returns 0 for such files. Early 2.6 kernels (up to and including 2.6.3) also behaved in the same way as kernel 2.4. In kernels before 2.6.20, access() ignored the effect of the MSNOEXEC flag if it was used to mount(2) the underlying file system. Since ker‐ nel 2.6.20, access() honors this flag. SEE ALSO chmod(2), chown(2), faccessat(2), open(2), setgid(2), setuid(2), stat(2), euidaccess(3), credentials(7), pathresolution(7) COLOPHON
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Linux 2013-04-16 ACCESS(2)