std::filesy...resize_file(3)

💅 std::filesystem::resize_file 💅

okay so bestie, this is the std::filesy...resize_file(3) page from the C++ Standard Libary manual — yes, the header is literally truncated with dots, and yes, we are respecting the lore 💀. It covers std::filesystem::resize_file, and it’s all about changing the size of a regular file. let’s go. ✨

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🌸 NAME 🌸

std::filesystem::resize_file - std::filesystem::resize_file.

that’s the whole name section. it ate and left no crumbs. 💅

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✨ Synopsis ✨

Defined in header <filesystem>:

void resize_file( const std::filesystem::path& p, (1) (since C++17)
                  std::uintmax_t new_size );
void resize_file( const std::filesystem::path& p,

                  std::uintmax_t new_size,        (2) (since C++17)

                  std::error_code& ec ) noexcept;

so what does it do? It changes the size of the regular file named by p as if by POSIX truncate: if the file size was previously larger than new_size, the remainder of the file is discarded. If the file was previously smaller than new_size, the file size is increased and the new area appears as if zero-filled. that’s the tea 🫖.

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🎀 Parameters 🎀

• p — path to resize.
• new_size — size that the file will now have.
• ec — out-parameter for error reporting in the non-throwing overload.

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💖 Return value 💖

(none) — literally no return value, bestie. periodt. 💖

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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. 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 🦋

On systems that support sparse files, increasing the file size does not increase the space it occupies on the file system: space allocation takes place only when non-zero bytes are written to the file.

like, the file can be bigger without eating more space until you actually write non-zero bytes into it — sparse-file magic, iykyk. 🦋

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🌈 Example 🌈

Demonstrates the effect creating a sparse file has on the free space.

the page also has this line before the code: // Run this code. preserving it for the culture:

        #include <filesystem>
        #include <fstream>
        #include <iostream>
        #include <locale>

        int main()
        {
            auto p = std::filesystem::temp_directory_path() / "example.bin";
            std::ofstream{p}.put('a');
            std::cout.imbue(std::locale{"en_US.UTF8"});
            std::cout << "File size:  " << std::filesystem::file_size(p) <<
       '\n'
                      << "Free space: " << std::filesystem::space(p).free <<
       '\n';
            std::filesystem::resize_file(p, 64*1024); // resize to 64 KB
            std::cout << "File size:  " << std::filesystem::file_size(p) <<
       '\n'
                      << "Free space: " << std::filesystem::space(p).free <<
       '\n';
            std::filesystem::remove(p);
        }

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⭐ Possible output ⭐

and the possible output is literally this, commas and all:

        File size:  1
        Free space: 42,954,108,928
        File size:  65,536
        Free space: 42,954,108,928

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👑 See also 👑

• file_size — returns the size of a file (C++17) (function).
• space — determines available free space on the file system (C++17) (function).

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mwah 💋 sign-off: this page came from http://cppreference.com, version/date 2024.06.10, and the footer name is std::filesy...resize_file(3). go resize something, bestie. 🎀