std::experimental::filesystem::resize_file

 

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Filesystem library (filesystem TS)

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

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 an 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 an 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 <experimental/filesystem>
#include <fstream>
#include <iostream>
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

See also

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