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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.

       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  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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