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std::filesystem::resize_file(3) C++ Standard Libary std::filesystem::resize_file(3)


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


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

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.


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::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

// Run this code

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

Possible output:

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

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)