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Showing content from https://en.cppreference.com/w/cpp/language/../error/error_code/../../filesystem/permissions.html below:

std::filesystem::permissions - cppreference.com

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.

(none)

Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.

Throws

on underlying OS API errors, constructed with

as the first path argument and the OS error code as the error code argument.

Sets a

parameter to the OS API error code if an OS API call fails, and executes

if no errors occur.

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).

#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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