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access() - Unix, Linux System Call


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NAME

access - check user’s permissions for a file

SYNOPSIS

#include <unistd.h> 

int access(const char *pathname, int mode);

DESCRIPTION

access() checks whether the process would be allowed to read, write or test for existence of the file (or other file system object) whose name is pathname. If pathname is a symbolic link permissions of the file referred to by this symbolic link are tested.

mode is a mask consisting of one or more of R_OK, W_OK, X_OK and F_OK.

R_OK, W_OK and X_OK request checking whether the file exists and has read, write and execute permissions, respectively. F_OK just requests checking for the existence of the file.

The tests depend on the permissions of the directories occurring in the path to the file, as given in pathname, and on the permissions of directories and files referred to by symbolic links encountered on the way.

The check is done with the process’s real UID and GID, rather than with the effective IDs as is done when actually attempting an operation. This is to allow set-user-ID programs to easily determine the invoking user’s authority.

Only access bits are checked, not the file type or contents. Therefore, 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 "executable," but the execve(2) call will still fail.

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.

RETURN VALUE

On success (all requested permissions granted), zero is returned. On error (at least one bit in mode asked for a permission that is denied, or some other error occurred), -1 is returned, and errno is set appropriately.

ERRORS

access() shall fail if:

TagDescription
EACCES The requested access would be denied to the file or search permission is denied for one of the directories in the path prefix of pathname. (See also path_resolution(2).)
ELOOP Too many symbolic links were encountered in resolving pathname.
ENAMETOOLONG pathname is too long.
ENOENT A directory component in pathname would have been accessible but does not exist or was 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 filesystem.

access() may fail if:

TagDescription
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 executed.

RESTRICTIONS

access() returns an error if any of the access types in the requested call fails, even if other types might be successful.

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.

Using access() to check if a user is authorized to e.g. open a file before actually doing so using open(2) creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it.

CONFORMING TO

SVr4, POSIX.1-2001, 4.3BSD

SEE ALSO



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