NAME

std::experimental::filesystem::resize_file - std::experimental::filesystem::resize_file

Synopsis

Defined in header <experimental/filesystem>
void resize_file(const path& p, std::uintmax_t new_size);
void resize_file(const path& p, std::uintmax_t new_size, (filesystem TS)
error_code& ec);

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 error_code& parameter throws filesystem_error on
underlying OS API errors, constructed with p as the first argument and the OS error
code as the error code argument. std::bad_alloc may be thrown if memory allocation
fails. The overload taking a 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. This overload
has
noexcept specification:
noexcept

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 <fstream>
#include <experimental/filesystem>
namespace fs = std::experimental::filesystem;
int main()
{
fs::path p = fs::temp_directory_path() / "example.bin";
std::ofstream(p).put('a');
std::cout << "File size: " << fs::file_size(p) << '\n'
<< "Free space: " << fs::space(p).free << '\n';
fs::resize_file(p, 64*1024); // resize to 64 KB
std::cout << "File size: " << fs::file_size(p) << '\n'
<< "Free space: " << fs::space(p).free << '\n';
fs::remove(p);
}

Possible output:

File size: 1
Free space: 31805444096
File size: 65536
Free space: 31805444096