trusty (3) access.3posix.gz

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 .