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           #include <unistd.h>
           int access(const char *pathname, int mode);
           #include <fcntl.h>           /* Definition of AT_* constants */
           #include <unistd.h>
           int faccessat(int dirfd, const char *pathname, int mode, int flags);
       Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
               Since glibc 2.10:
                   _XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
               Before glibc 2.10:


           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.
       faccessat ()
           The  faccessat()  system  call  operates  in  exactly  the  same way as
           access(), except for the differences described here.
           If the pathname given in pathname is relative, then it  is  interpreted
           relative  to  the  directory  referred  to by the file descriptor dirfd
           (rather than relative to the current working directory of  the  calling
           process, as is done by access() for a relative pathname).
           If  pathname  is relative and dirfd is the special value AT_FDCWD, then
           pathname is interpreted relative to the current  working  directory  of
           the calling process (like access()).
           If pathname is absolute, then dirfd is ignored.
           On success (all requested permissions granted, or mode is F_OK and  the
           file  exists),  zero  is  returned.  On error (at least one bit in mode
           asked for a permission that is denied, or mode is  F_OK  and  the  file
           does  not  exist,  or  some  other error occurred), -1 is returned, and
           errno is set appropriately.


           access() and faccessat() 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
           access() and faccessat() 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  exe-
           The following additional errors can occur for faccessat():
           EBADF  dirfd is not a valid file descriptor.
           EINVAL Invalid flag specified in flags.
                  pathname is relative and dirfd is a file descriptor referring to
                  a file other than a directory.
           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-
           These  calls  return  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 accessible only 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.
           These calls may not work correctly on NFSv2 filesystems with  UID  map-
           ping enabled, because UID mapping is done on the server and hidden from
           the client, which checks permissions.  (NFS versions 3 and higher  per-
           form  the  check  on  the  server.)  Similar problems can occur to FUSE
       faccessat ()
           The raw faccessat() system call takes only the first  three  arguments.
           The  AT_EACCESS  and AT_SYMLINK_NOFOLLOW flags are actually implemented
           within the glibc wrapper function for faccessat().  If either of  these
           flags  is  specified,  then  the wrapper function employs fstatat(2) to
           determine access 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,  these  calls  ignored  the  effect  of  the
           MS_NOEXEC  flag  if  it was used to mount(2) the underlying filesystem.
           Since kernel 2.6.20, the MS_NOEXEC is honored

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