Poster of Linux kernelThe best gift for a Linux geek
ACCESS

ACCESS

Section: POSIX Programmer's Manual (P) Updated: 2003
Local index Up
 

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

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
EXAMPLES
Testing for the Existence of a File
APPLICATION USAGE
RATIONALE
FUTURE DIRECTIONS
SEE ALSO
COPYRIGHT

This document was created by man2html, using the manual pages.
Time: 21:41:37 GMT, April 16, 2011