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       access - check real user's permissions for a file


       #include <unistd.h>

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


       access()  checks  whether  the  calling  process  can  access  the file
       pathname.  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
       easily 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


       access() shall fail if:

       EACCES The requested access would be denied  to  the  file,  or  search
              permission  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:

       EFAULT pathname points outside your accessible address space.

       EINVAL mode was incorrectly specified.

       EIO    An I/O error occurred.

       ENOMEM Insufficient kernel memory was available.

              Write  access  was  requested  to  an  executable which is being


       SVr4, 4.3BSD, POSIX.1-2001.


       Warning: Using access() to check  if  a  user  is  authorized  to,  for
       example,  open  a file before actually doing so using open(2) creates a
       security hole, because the user might exploit the short  time  interval
       between  checking  and  opening  the  file  to manipulate it.  For this
       reason, 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

       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.
       Therefore, 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
       "executable," 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
       kernel 2.6.20, access() honors this flag.


       chmod(2),   chown(2),   faccessat(2),  open(2),  setgid(2),  setuid(2),
       stat(2), euidaccess(3), credentials(7), path_resolution(7)


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