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ACCESS
Section: POSIX Programmer's Manual (3P)
Updated: 2003
Index
Return to Main Contents
PROLOG
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.
NAME
access - determine accessibility of a file
SYNOPSIS
#include <unistd.h>
int access(const char * path, int amode);
DESCRIPTION
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.
RETURN VALUE
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.
ERRORS
The access() function 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 the path argument exceeds {PATH_MAX} or a pathname
component 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 is not a directory.
- EROFS
-
Write access is requested for a file on a read-only file system.
The access() function 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
-
As a result of encountering a symbolic link in resolution of the path
argument, the length of the substituted pathname
string exceeded {PATH_MAX}.
- ETXTBSY
-
Write access is requested for a pure procedure (shared text) file
that is being executed.
The following sections are informative.
EXAMPLES
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);
APPLICATION USAGE
Additional values of amode other than the set defined in the
description may be valid; for example, if a system has
extended access controls.
RATIONALE
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 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.
FUTURE DIRECTIONS
None.
SEE ALSO
chmod(), stat(), the Base Definitions volume of
IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
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 .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Testing for the Existence of a File
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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