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 - determine accessibility of a file


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


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, 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 IEEE Std 1003.1-2001, Chapter 3, Definitions. 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.


If the requested access is permitted, access() succeeds and shall return 0; otherwise, -1 shall be returned and errno shall be set to indicate the error.


The access() function shall fail if:
Permission bits of the file mode do not permit the requested access, or search permission is denied on a component of the path prefix.
A loop exists in symbolic links encountered during resolution of the path argument.
The length of the path argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.
A component of path does not name an existing file or path is an empty string.
A component of the path prefix is not a directory.
Write access is requested for a file on a read-only file system.

The access() function may fail if:

The value of the amode argument is invalid.
More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument.
As a result of encountering a symbolic link in resolution of the path argument, the length of the substituted pathname string exceeded {PATH_MAX}.
Write access is requested for a pure procedure (shared text) file that is being executed.

The following sections are informative.


Testing for the Existence of a File

The following example tests whether a file named myfile exists in the /tmp directory.

#include <unistd.h>
int result;
const char *filename = "/tmp/myfile";


result = access (filename, F_OK);


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


In early proposals, some inadequacies in the access() function led to the creation of an eaccess() function because:
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.
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 IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001.

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.




chmod(), stat(), the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>


Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .
2003 IEEE/The Open Group