std::experimental::filesystem::permissions(3)

NAME
std::experimental::filesystem::permissions - std::experimental::filesystem::permissions

Synopsis
   Defined in header <experimental/filesystem>
   void permissions(const path& p, perms prms); (filesystem TS)
   void permissions(const path& p, perms prms, error_code& ec);
   Changes access permissions of the file to which p resolves, as if by POSIX fchmodat.
   Symlinks are followed if prms::resolve_symlinks is set.
   The effects depend on prms as follows:
     * If neither perms::add_perms nor perms::remove_perms is set, file permissions are
       set to exactly prms & fs::perms::mask (meaning, every valid bit of prms is
       applied)
     * If perms::add_perms, the file permissions are set to exactly
       status(p).permissions() | (prms & perms::mask) (meaning, any valid bit that is
       set in prms, but not in the file's current permissions is added to the file's
       permissions)
     * If perms::remove_perms is set, the file permissions are set to exactly
       status(p).permissions() & ~(prms & perms::mask) (meaning, any valid bit that is
       clear in prms, but set in the file's current permissions is cleared in the
       file's permissions)
     * If both perms::add_perms and perms::remove_perms are set, error occurs
   The non-throwing overload has no special action on error.

Parameters
   p - path to examine
   prms - permissions to set, add, or remove
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   (none)

Exceptions
   The overload that does not take a error_code& parameter throws filesystem_error on
   underlying OS API errors, constructed with p as the first argument and the OS error
   code as the error code argument. std::bad_alloc may be thrown if memory allocation
   fails. The overload taking a error_code& parameter sets it to the OS API error code
   if an OS API call fails, and executes ec.clear() if no errors occur. This overload
   has
   noexcept specification:
   noexcept

Notes
   Permissions may not necessarily be implemented as bits, but they are treated that
   way conceptually.
   Some permission bits may be ignored on some systems, and changing some bits may
   automatically change others (e.g. on platforms without owner/group/all distinction,
   setting any of the three write bits set all three)

Example
// Run this code
 #include <fstream>
 #include <bitset>
 #include <iostream>
 #include <experimental/filesystem>
 namespace fs = std::experimental::filesystem;
 void demo_perms(fs::perms p)
 {
      std::cout << ((p & fs::perms::owner_read) != fs::perms::none ? "r" : "-")
               << ((p & fs::perms::owner_write) != fs::perms::none ? "w" : "-")
               << ((p & fs::perms::owner_exec) != fs::perms::none ? "x" : "-")
               << ((p & fs::perms::group_read) != fs::perms::none ? "r" : "-")
               << ((p & fs::perms::group_write) != fs::perms::none ? "w" : "-")
               << ((p & fs::perms::group_exec) != fs::perms::none ? "x" : "-")
               << ((p & fs::perms::others_read) != fs::perms::none ? "r" : "-")
               << ((p & fs::perms::others_write) != fs::perms::none ? "w" : "-")
               << ((p & fs::perms::others_exec) != fs::perms::none ? "x" : "-")
               << '\n';
 }
 int main()
 {
     std::ofstream("test.txt"); // create file
     std::cout << "Created file with permissions: ";
     demo_perms(fs::status("test.txt").permissions());
     fs::permissions("test.txt", fs::perms::add_perms |
                        fs::perms::owner_all | fs::perms::group_all);
     std::cout << "After adding o+rwx and g+rwx: ";
     demo_perms(fs::status("test.txt").permissions());
     fs::remove("test.txt");
 }

Possible output:
 Created file with permissions: rw-r--r--
 After adding o+rwx and g+rwx: rwxrwxr--

See also
   perms identifies file system permissions
                  (enum)
   status determines file attributes
   symlink_status determines file attributes, checking the symlink target
                  (function)