NAME¶

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

Synopsis¶

Defined in header <filesystem>
void resize_file( const std::filesystem::path& p, (1) (since C++17)
std::uintmax_t new_size );
void resize_file( const std::filesystem::path& p,

std::uintmax_t new_size, (2) (since C++17)

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¶

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¶

Any overload not marked noexcept may throw std::bad_alloc if memory allocation
fails.

1) Throws std::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.
2) Sets a std::error_code& parameter to the OS API error code if an OS API call
fails, and executes ec.clear() if no errors occur.

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 creating a sparse file has on the free space.

// Run this code

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

Possible output:¶

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

See also¶

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