std::filesystem::resize_file (3) Linux Manual Page
std::filesystem::resize_file – std::filesystem::resize_file
Synopsis
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.
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 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.
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:
See also
file_size returns the size of a file
(C++17)
space determines available free space on the file system
(C++17)