# std::filesystem::permissions

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< cpp‎ | filesystem

Defined in header `<filesystem>` |  |
void permissions( const std::filesystem::path& p, std::filesystem::perms prms, std::filesystem::perm_options opts = perm_options::replace ); void permissions( const std::filesystem::path& p, std::filesystem::perms prms, std::error_code& ec ) noexcept; void permissions( const std::filesystem::path& p, std::filesystem::perms prms, std::filesystem::perm_options opts, std::error_code& ec ); |  | (since C++17)

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

The overload that does not take a std::error_code& parameter throws `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. The overload taking a std::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. Any overload not marked `noexcept` may throw std::bad_alloc if memory allocation fails.

### 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 <iostream>
    #include <filesystem>

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

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