std::filesystem::resize_file
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< cpp | filesystem
Defined in header <filesystem> | ||
void resize_file(conststd::filesystem::path& p, std::uintmax_t new_size ); | (1) | (since C++17) |
void resize_file(conststd::filesystem::path& p, std::uintmax_t new_size, | (2) | (since C++17) |
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
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 KBstd::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
[edit]See also
(C++17) | returns the size of a file (function) |
(C++17) | determines available free space on the file system (function) |
