#include <unistd.h>

       int access(const char *pathname, int mode);

       access()  checks  whether the calling process can access the file path-
       name.  If pathname is a symbolic link, it is dereferenced.

       The mode specifies the accessibility check(s) to be performed,  and  is
       either the value F_OK, or a mask consisting of the bitwise OR of one or
       more of R_OK, W_OK, and X_OK.  F_OK tests  for  the  existence  of  the
       file.   R_OK,  W_OK,  and  X_OK test whether the file exists and grants
       read, write, and execute permissions, respectively.

       The check is done using the calling process's real UID and GID,  rather
       than the effective IDs as is done when actually attempting an operation
       (e.g., open(2)) on the file.  This allows set-user-ID programs to  eas-
       ily determine the invoking user's authority.

       If the calling process is privileged (i.e., its real UID is zero), then
       an X_OK check is successful for a regular file if execute permission is
       enabled for any of the file owner, group, or other.

       On  success  (all requested permissions granted), zero is returned.  On
       error (at least one bit in mode asked for a permission that is  denied,
       or  some other error occurred), -1 is returned, and errno is set appro-

       access() shall fail if:

       EACCES The requested access would be denied to the file, or search per-
              mission  is denied for one of the directories in the path prefix
              of pathname.  (See also path_resolution(7).)

       ELOOP  Too many symbolic links were encountered in resolving pathname.

              pathname is too long.

       ENOENT A component of pathname does not exist or is a dangling symbolic

              A  component  used as a directory in pathname is not, in fact, a

       EROFS  Write permission was requested for a file on  a  read-only  file

       access() may fail if:

       SVr4, 4.3BSD, POSIX.1-2001.

       Warning: Using access() to check if a user is authorized to, for  exam-
       ple, open a file before actually doing so using open(2) creates a secu-
       rity hole, because the user  might  exploit  the  short  time  interval
       between  checking and opening the file to manipulate it.  For this rea-
       son, the use of this system call should be avoided.   (In  the  example
       just  described, a safer alternative would be to temporarily switch the
       process's effective user ID to the real ID and then call open(2).)

       access() always dereferences symbolic links.  If you need to check  the
       permissions  on a symbolic link, use faccessat(2) with the flag AT_SYM-

       access() returns an error if any of the access types in mode is denied,
       even if some of the other access types in mode are permitted.

       If the calling process has appropriate privileges (i.e., is superuser),
       POSIX.1-2001 permits an implementation to indicate success for an  X_OK
       check  even if none of the execute file permission bits are set.  Linux
       does not do this.

       A file is only accessible if the permissions on each of the directories
       in the path prefix of pathname grant search (i.e., execute) access.  If
       any directory is  inaccessible,  then  the  access()  call  will  fail,
       regardless of the permissions on the file itself.

       Only  access  bits  are checked, not the file type or contents.  There-
       fore, if a directory is found to be writable, it  probably  means  that
       files  can  be created in the directory, and not that the directory can
       be written as a file.  Similarly, a DOS file may be found to  be  "exe-
       cutable," but the execve(2) call will still fail.

       access()  may  not  work correctly on NFS file systems with UID mapping
       enabled, because UID mapping is done on the server and hidden from  the
       client, which checks permissions.

       In  kernel  2.4 (and earlier) there is some strangeness in the handling
       of X_OK tests for superuser.  If all categories of  execute  permission
       are  disabled for a nondirectory file, then the only access() test that
       returns -1 is when mode is specified as just X_OK; if R_OK or  W_OK  is
       also  specified in mode, then access() returns 0 for such files.  Early
       2.6 kernels (up to and including 2.6.3) also behaved in the same way as
       kernel 2.4.

       In  kernels before 2.6.20, access() ignored the effect of the MS_NOEXEC
       flag if it was used to mount(2) the underlying file system.  Since ker-
       nel 2.6.20, access() honors this flag.

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