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std::experimental::filesystem::resize_file

From cppreference.com
< cpp‎ | experimental‎ | fs
 
 
Experimental
Technical Specification
Filesystem library (filesystem TS)
Library fundamentals (library fundamentals TS)
Library fundamentals 2 (library fundamentals TS v2)
Library fundamentals 3 (library fundamentals TS v3)
Extensions for parallelism (parallelism TS)
Extensions for parallelism 2 (parallelism TS v2)
Extensions for concurrency (concurrency TS)
Extensions for concurrency 2 (concurrency TS v2)
Concepts (concepts TS)
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Reflection (reflection TS)
Mathematical special functions (special functions TR)
Experimental Non-TS
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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.

Contents

[edit] 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

[edit] Return value

(none)

[edit] 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
  

[edit] 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.

[edit] Example

Demonstrates the effect of creating a sparse file on the free space.

#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

[edit] See also

returns the size of a file
(function) [edit]
determines available free space on the file system
(function) [edit]