std::filesy...permissions(3)

✨ NAME ✨

std::filesystem::permissions — okay bestie, the function is literally named std::filesystem::permissions, and this is its page from the C++ Standard Libary manual. The page tag is std::filesy...permissions(3). It’s giving permissions main character energy. 💅

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🌸 Synopsis 🌸

Defined in header <filesystem>:

(1) (since C++17)
void permissions( const std::filesystem::path& p,
                  std::filesystem::perms prms,
                  std::filesystem::perm_options opts =
                  perm_options::replace );

(2) (since C++17)
void permissions( const std::filesystem::path& p,
                  std::filesystem::perms prms,
                  std::error_code& ec ) noexcept;

(3) (since C++17)
void permissions( const std::filesystem::path& p,
                  std::filesystem::perms prms,
                  std::filesystem::perm_options opts,
                  std::error_code& ec );

Okay so this function changes the access permissions of the file that p resolves to, 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. No mixing and matching, bestie — that’s the rule 🫶

The non-throwing overload has no special action on error.

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💖 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

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🎀 Return value 🎀

(none) — literally no return value, not even a little one. 🫶

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🚨 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.

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🧠 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). Lowkey chaotic, but that’s the deal 💀

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💻 Example 💻

Run this code, bestie:

// 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--

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👀 See also 👀

These are the related refs, with their exact badge groups because the original badge cluster is a little scrambled:

• perms — identifies file system permissions
• status — determines file attributes
• symlink_status — determines file attributes, checking the symlink target

Exact badge groups from the page: (C++17) (enum); (C++17) (function); (C++17).

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source/version/date: http://cppreference.com, 2024.06.10, manual tag std::filesy...permissions(3) — kiss kiss, you ate this page, periodt 🫶✨💜