std::filesystem::resize_file
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Filesystem library
filesystem::resize_file
Defined in header
<filesystem>
void resize_file( const std::filesystem::path & p,
std::uintmax_t new_size );
(1)
(since C++17)
std::uintmax_t new_size );
void resize_file( const std::filesystem::path & p,
(2)
(since C++17)
std::uintmax_t new_size,
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.
[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
Any overload not marked noexcept may throw std::bad_alloc if memory allocation fails.
1) Throws std::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.
2) Sets a std::error_code & parameter to the OS API error code if an OS API call fails, and executes ec.clear () if no errors occur.
[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 creating a sparse file has on the free space.
Run this code
#include <filesystem> #include <fstream> #include <iostream> #include <locale> int main() { auto p = std::filesystem::temp_directory_path () / "example.bin"; std::ofstream {p}.put('a'); std::cout.imbue(std::locale {"en_US.UTF8"}); std::cout << "File size: " << std::filesystem::file_size (p) << '\n' << "Free space: " << std::filesystem::space (p).free << '\n'; std::filesystem::resize_file(p, 64*1024); // resize to 64 KB std::cout << "File size: " << std::filesystem::file_size (p) << '\n' << "Free space: " << std::filesystem::space (p).free << '\n'; std::filesystem::remove (p); }
Possible output:
File size: 1 Free space: 42,954,108,928 File size: 65,536 Free space: 42,954,108,928