# std::filesystem::resize_file

 Defined in header  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.

## Contents

### 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

(none)

### Exceptions

The overload that does not take a 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 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

#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

### See also

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