std::filesystem::resize_file

void resize_file( const std::filesystem::path& p,
                  std::uintmax_t new_size );
void resize_file( const std::filesystem::path& p,
                  std::uintmax_t new_size,
                  std::error_code& ec ) noexcept;

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.

Parameters

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

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.

Example

#include <iostream>
#include <filesystem>
#include <fstream>
#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);
}

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

See also