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.


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



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.


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.


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

#include <iostream>
#include <iomanip>
#include <fstream>
#include <filesystem>
namespace fs = std::filesystem;
int main()
    fs::path p = fs::current_path() / "example.bin";
    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';

Possible output:

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

See also

returns the size of a file
determines available free space on the file system