std::filesystem::resize_file

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

 

Filesystem library

Classes
filesystem::path filesystem::filesystem_error filesystem::directory_entry filesystem::directory_iterator filesystem::recursive_directory_iterator filesystem::file_status	filesystem::space_info filesystem::file_type filesystem::perms filesystem::copy_options filesystem::directory_options filesystem::file_time_type
Functions
filesystem::absolute filesystem::system_complete filesystem::canonicalfilesystem::weakly_canonical filesystem::relativefilesystem::proximate filesystem::copy filesystem::copy_file filesystem::copy_symlink filesystem::create_directory filesystem::create_directories filesystem::create_hard_link filesystem::create_symlink filesystem::create_directory_symlink filesystem::current_path	filesystem::exists filesystem::equivalent filesystem::file_size filesystem::hard_link_count filesystem::last_write_time filesystem::permissions filesystem::read_symlink filesystem::remove filesystem::remove_all filesystem::rename filesystem::resize_file filesystem::space filesystem::status filesystem::symlink_status filesystem::temp_directory_path
File types
filesystem::is_block_file filesystem::is_character_file filesystem::is_directory filesystem::is_empty filesystem::status_known	filesystem::is_fifo filesystem::is_other filesystem::is_regular_file filesystem::is_socket filesystem::is_symlink

 

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);		(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 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 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. 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 <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)