std::filesystem::resize_file

Defined in header `<filesystem>`
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;		(since C++17)

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

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

Return value

(none)

Exceptions

The overload that does not take a std::error_code& parameter throws 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

demonstrates the effect of creating a sparse file on the free space

Run this code

#include <iostream>
#include <iomanip>
#include <fstream>
#include <filesystem>
namespace fs = std::filesystem;
int main()
{
    fs::path p = fs::current_path() / "example.bin";
    std::ofstream(p).put('a');
    std::cout << "File size: " << std::setw(10) << fs::file_size(p)
              << " Free space: " << fs::space(p).free << '\n';
    fs::resize_file(p, 1024*1024*1024); // resize to 1 G
    std::cout << "File size: " << fs::file_size(p)
              << " Free space: " << fs::space(p).free << '\n';
    fs::remove(p);
}

Possible output:

File size:          1 Free space: 3724541952
File size: 1073741824 Free space: 3724476416

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