std::filesystem::permissions(3)

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

Synopsis
   Defined in header <filesystem>
   void permissions( const std::filesystem::path& p,
                     std::filesystem::perms prms, (1) (since C++17)
                     std::filesystem::perm_options opts =
   perm_options::replace );
   void permissions( const std::filesystem::path& p,
                     std::filesystem::perms prms, (2) (since C++17)
                     std::error_code& ec ) noexcept;
   void permissions( const std::filesystem::path& p,
                     std::filesystem::perms prms, (3) (since C++17)
                     std::filesystem::perm_options opts,
                     std::error_code& ec );
   Changes access permissions of the file to which p resolves, as if by POSIX fchmodat.
   Symlinks are followed unless perm_options::nofollow is set in opts.
   The second signature behaves as if called with opts set to perm_options::replace.
   The effects depend on prms and opts as follows:
     * If opts is perm_options::replace, file permissions are set to exactly prms &
       std::filesystem::perms::mask (meaning, every valid bit of prms is applied).
     * If opts is perm_options::add, 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 opts is perm_options::remove, 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).
   opts is required to have only one of replace, add, or remove to be set.
   The non-throwing overload has no special action on error.

Parameters
   p - path to examine
   prms - permissions to set, add, or remove
   opts - options controlling the action taken by this function
   ec - out-parameter for error reporting in the non-throwing overload

Return value
   (none)

Exceptions
   Any overload not marked noexcept may throw std::bad_alloc if memory allocation
   fails.
   1) Throws std::filesystem::filesystem_error on underlying OS API errors, constructed
   with p as the first path argument and the OS error code as the error code argument.
   2,3) Sets a std::error_code& parameter to the OS API error code if an OS API call
   fails, and executes ec.clear() if no errors occur.

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 <filesystem>
 #include <fstream>
 #include <iostream>
 void demo_perms(std::filesystem::perms p)
 {
     using std::filesystem::perms;
     auto show = [=](char op, perms perm)
     {
         std::cout << (perms::none == (perm & p) ? '-' : op);
     };
     show('r', perms::owner_read);
     show('w', perms::owner_write);
     show('x', perms::owner_exec);
     show('r', perms::group_read);
     show('w', perms::group_write);
     show('x', perms::group_exec);
     show('r', perms::others_read);
     show('w', perms::others_write);
     show('x', perms::others_exec);
     std::cout << '\n';
 }
 int main()
 {
     std::ofstream("test.txt"); // create file
     std::cout << "Created file with permissions: ";
     demo_perms(std::filesystem::status("test.txt").permissions());
     std::filesystem::permissions(
         "test.txt",
         std::filesystem::perms::owner_all | std::filesystem::perms::group_all,
         std::filesystem::perm_options::add
     );
     std::cout << "After adding u+rwx and g+rwx: ";
     demo_perms(std::filesystem::status("test.txt").permissions());
     std::filesystem::remove("test.txt");
 }

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

See also
   perms identifies file system permissions
   (C++17) (enum)
   status determines file attributes
   symlink_status determines file attributes, checking the symlink target
   (C++17) (function)
   (C++17)