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



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult 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 exe-
           cutable.

        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, Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                    ACCESS(3P)