ACCESS(3P) POSIX Programmer's Manual ACCESS(3P)

This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.

access, faccessat — determine accessibility of a file descriptor

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

The access() function shall check the file named by the pathname pointed to by the path argument for accessibility according to the bit pattern contained in amode. The checks for accessibility (including directory permissions checked during pathname resolution) shall be performed using the real user ID in place of the effective user ID and the real group ID in place of the effective group ID.

The value of amode is either the bitwise-inclusive OR of the access permissions to be checked (R_OK, W_OK, X_OK) or the existence test (F_OK).

If any access permissions are checked, each shall be checked individually, as described in the Base Definitions volume of POSIX.1‐2017, Section 4.5, File Access Permissions, except that where that description refers to execute permission for a process with appropriate privileges, an implementation may indicate success for X_OK even if execute permission is not granted to any user.

The faccessat() function, when called with a flag value of zero, shall be equivalent to the access() function, except in the case where path specifies a relative path. In this case the file whose accessibility is to be determined shall be located relative to the directory associated with the file descriptor fd instead of the current working directory. If the access mode of the open file description associated with the file descriptor is not O_SEARCH, the function shall check whether directory searches are permitted using the current permissions of the directory underlying the file descriptor. If the access mode is O_SEARCH, the function shall not perform the check.

If faccessat() is passed the special value AT_FDCWD in the fd parameter, the current working directory shall be used and, if flag is zero, the behavior shall be 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 (including directory permissions checked during pathname resolution) shall be performed using the effective user ID and group ID instead of the real user ID and group ID as required in a call to access().

Upon successful completion, these functions shall return 0. Otherwise, these functions shall return -1 and set errno to indicate the error.

These functions shall fail 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.
ELOOP
A loop exists in symbolic links encountered during resolution of the path argument.
ENAMETOOLONG

The length of a component of a pathname is longer than {NAME_MAX}.
ENOENT
A component of path does not name an existing file or path is an empty string.
ENOTDIR
A component of the path prefix names an existing file that is neither a directory nor a symbolic link to a directory, or the path argument contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.
EROFS
Write access is requested for a file on a read-only file system.

The faccessat() function shall fail if:

EACCES
The access mode of the open file description associated with fd is not O_SEARCH and the permissions of the directory underlying fd do not permit directory searches.
EBADF
The path argument does not specify an absolute path and the fd argument is neither AT_FDCWD nor a valid file descriptor open for reading or searching.
ENOTDIR
The path argument is not an absolute path and fd is a file descriptor associated with a non-directory file.

These functions may fail if:

EINVAL
The value of the amode argument is invalid.
ELOOP
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
ENAMETOOLONG

The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.
ETXTBSY
Write access is requested for a pure procedure (shared text) file that is being executed.

The faccessat() function may fail if:

EINVAL
The value of the flag argument is not valid.

The following sections are informative.

The following example tests whether a file named myfile exists in the /tmp directory.
#include <unistd.h>
...
int result;
const char *pathname = "/tmp/myfile";
result = access (pathname, F_OK);

Use of these functions is discouraged since by the time the returned information is acted upon, it is out-of-date. (That is, acting upon the information always leads to a time-of-check-to-time-of-use race condition.) An application should instead attempt the action itself and handle the [EACCES] error that occurs if the file is not accessible (with a change of effective user and group IDs beforehand, and perhaps a change back afterwards, in the case where access() or faccessat() without AT_EACCES would have been used.)

Historically, one of the uses of access() was in set-user-ID root programs to check whether the user running the program had access to a file. This relied on ``super-user'' privileges which were granted based on the effective user ID being zero, so that when access() used the real user ID to check accessibility those privileges were not taken into account. On newer systems where privileges can be assigned which have no association with user or group IDs, if a program with such privileges calls access(), the change of IDs has no effect on the privileges and therefore they are taken into account in the accessibility checks. Thus, access() (and faccessat() with flag zero) cannot be used for this historical purpose in such programs. Likewise, if a system provides any additional or alternate file access control mechanisms that are not user ID-based, they will still be taken into account.

If a relative pathname is used, no account is taken of whether the current directory (or the directory associated with the file descriptor fd) is accessible via any absolute pathname. Applications using access(), or faccessat() without AT_EACCES, may consequently act as if the file would be accessible to a user with the real user ID and group ID of the process when such a user would not in practice be able to access the file because access would be denied at some point above the current directory (or the directory associated with the file descriptor fd) in the file hierarchy.

If access() or faccessat() is used with W_OK to check for write access to a directory which has the S_ISVTX bit set, a return value indicating the directory is writable can be misleading since some operations on files in the directory would not be permitted based on the ownership of those files (see the Base Definitions volume of POSIX.1‐2017, Section 4.3, Directory Protection).

Additional values of amode other than the set defined in the description may be valid; for example, if a system has extended access controls.

The use of the AT_EACCESS value for flag enables functionality not available in access().

In early proposals, some inadequacies in the access() function led to the creation of an eaccess() function because:
1.
Historical implementations of access() do not test file access correctly when the process' real user ID is superuser. In particular, they always return zero when testing execute permissions without regard to whether the file is executable.
2.
The superuser has complete access to all files on a system. As a consequence, programs started by the superuser and switched to the effective user ID with lesser privileges cannot use access() to test their file access permissions.

However, the historical model of eaccess() does not resolve problem (1), so this volume of POSIX.1‐2017 now allows access() to behave in the desired way because several implementations have corrected the problem. It was also argued that problem (2) is more easily solved by using open(), chdir(), or one of the exec functions as appropriate and responding to the error, rather than creating a new function that would not be as reliable. Therefore, eaccess() is not included in this volume of POSIX.1‐2017.

The sentence concerning appropriate privileges and execute permission bits reflects the two possibilities implemented by historical implementations when checking superuser access for X_OK.

New implementations are discouraged from returning X_OK unless at least one execution permission bit is set.

The purpose of the faccessat() function is to enable the checking of the accessibility of files in directories other than the current working directory without exposure to race conditions. Any part of the path of a file could be changed in parallel to a call to access(), resulting in unspecified behavior. By opening a file descriptor for the target directory and using the faccessat() function it can be guaranteed that the file tested for accessibility is located relative to the desired directory.

These functions may be formally deprecated (for example, by shading them OB) in a future version of this standard.

chmod(), fstatat()

The Base Definitions volume of POSIX.1‐2017, Section 4.5, File Access Permissions, <fcntl.h>, <unistd.h>

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

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2017 IEEE/The Open Group