std::filesy...s_block_file(3)

💖 NAME 💖

std::filesystem::directory_entry::is_block_file — std::filesystem::directory_entry::is_block_file. Iconic. This is the std::filesystem::directory_entry member that does the block-device vibe check, bestie.

Manual-name moment: std::filesy...s_block_file(3), straight outta the C++ Standard Libary.

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✨ Synopsis ✨

Two overloads, both since C++17, no cap:

bool is_block_file() const;                               (1) (since C++17)
bool is_block_file( std::error_code& ec ) const noexcept; (2) (since C++17)

Checks whether the pointed-to object is a block device. Effectively returns:

1. std::filesystem::is_block_file(status()).
2. std::filesystem::is_block_file(status(ec)).

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🎀 Parameters 🎀

• ec — out-parameter for error reporting in the non-throwing overload. She’s here to catch the tea if the OS call gets messy.

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💅 Return value 💅

true if the referred-to filesystem object is a block device, false otherwise. Periodt.

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🔥 Exceptions 🔥

Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails. Allocation said “not today,” basically.

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. 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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🦋 Example 🦋

Okay this demo is giving full main-character energy — it creates a whole little sandbox of file types and prints what each one is:

       // Run this code

        #include <cstdio>
        #include <cstring>
        #include <filesystem>
        #include <fstream>
        #include <functional>
        #include <iostream>
        #include <memory>
        #include <sys/socket.h>
        #include <sys/stat.h>
        #include <sys/un.h>
        #include <unistd.h>

        namespace fs = std::filesystem;

        void print_entry_type(const std::filesystem::directory_entry& entry)
        {
            std::cout << entry.path() << ": ";

            if (!entry.exists())
                std::cout << "does not exist ";
            if (entry.is_block_file())
                std::cout << "is a block device ";
            if (entry.is_character_file())
                std::cout << "is a character device ";
            if (entry.is_directory())
                std::cout << "is a directory ";
            if (entry.is_fifo())
                std::cout << "is a named IPC pipe ";
            if (entry.is_regular_file())
                std::cout << "is a regular file ";
            if (entry.is_socket())
                std::cout << "is a named IPC socket ";
            if (entry.is_symlink())
                std::cout << "(a symlink)";
            if (entry.is_other())
                std::cout << "(an `other` file)";

            std::cout << '\n';
        }

        template<typename Type, typename Fun>
        class scoped_cleanup
        {
            std::unique_ptr<Type, std::function<void(const Type*)>> u;

        public:
            scoped_cleanup(Type* ptr, Fun fun) : u{ptr, std::move(fun)} {}
        };

        int main()
        {
            // Create files of different kinds.
            std::filesystem::current_path(fs::temp_directory_path());
            const std::filesystem::path sandbox{"sandbox"};
            scoped_cleanup remove_all_at_exit{&sandbox, [](const fs::path* p)
            {
                std::cout << "cleanup: remove_all(" << *p << ")\n";
                fs::remove_all(*p);
            }};
            std::filesystem::create_directory(sandbox);
            std::ofstream{sandbox/"file"}; // Creates a regular file
            std::filesystem::create_directory(sandbox/"dir");

            mkfifo((sandbox/"pipe").string().data(), 0644);
            struct sockaddr_un addr; addr.sun_family = AF_UNIX;

            std::strcpy(addr.sun_path, (sandbox/"sock").string().data());
            int fd{socket(PF_UNIX, SOCK_STREAM, 0)};
            scoped_cleanup close_socket_at_exit{&fd, [](const int* f)
            {
                std::cout << "cleanup: close socket #" << *f << '\n';
                close(*f);
            }};
            bind(fd, reinterpret_cast<sockaddr*>(std::addressof(addr)), sizeof
       addr);

            fs::create_symlink("file", sandbox/"symlink");

            for (std::filesystem::directory_entry entry:
       fs::directory_iterator(sandbox))
                print_entry_type(entry);

            // Request file system objects status directly:
            for (const char* str : {"/dev/null", "/dev/cpu",
       "/usr/include/c++",
                                    "/usr/include/asm", "/usr/include/time.h"})
                print_entry_type(fs::directory_entry{str});

        } // Cleanup via scoped_cleanup objects

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🌈 Possible output 🌈

        "sandbox/symlink": is a regular file (a symlink)
        "sandbox/sock": is a named IPC socket (an `other` file)
        "sandbox/pipe": is a named IPC pipe (an `other` file)
        "sandbox/dir": is a directory
        "sandbox/file": is a regular file
        "/dev/null": is a character device (an `other` file)
        "/dev/cpu": does not exist
        "/usr/include/c++": is a directory
        "/usr/include/asm": is a directory (a symlink)
        "/usr/include/time.h": is a regular file
        cleanup: close socket #3
        cleanup: remove_all("sandbox")

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👑 See also 👑

• is_block_file checks whether the given path refers to block device (C++17) (function)

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Final cute sign-off: this page’s source/version is http://cppreference.com, dated 2024.06.10. Bye bestie, go check those block devices 💖