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std::filesystem::file_size(3) C++ Standard Libary std::filesystem::file_size(3)


std::filesystem::file_size - std::filesystem::file_size


Defined in header <filesystem>
std::uintmax_t file_size( const std::filesystem::path& p );

std::uintmax_t file_size( const std::filesystem::path& p, (1) (since C++17)

std::error_code& ec ) noexcept;

If p does not exist, reports an error.

For a regular file p, returns the size determined as if by reading the st_size
member of the structure obtained by POSIX stat (symlinks are followed).

The result of attempting to determine the size of a directory (as well as any other
file that is not a regular file or a symlink) is implementation-defined.

The non-throwing overload returns static_cast<std::uintmax_t>(-1) on errors.


p - path to examine
ec - out-parameter for error reporting in the non-throwing overload

Return value

The size of the file, in bytes.


The overload that does not take a std::error_code& parameter throws
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. 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.


// Run this code

#include <cmath>
#include <filesystem>
#include <fstream>
#include <iostream>
namespace fs = std::filesystem;

struct HumanReadable {
std::uintmax_t size{};
private: friend
std::ostream& operator<<(std::ostream& os, HumanReadable hr) {
int i{};
double mantissa = hr.size;
for (; mantissa >= 1024.; mantissa /= 1024., ++i) { }
mantissa = std::ceil(mantissa * 10.) / 10.;
os << mantissa << "BKMGTPE"[i];
return i == 0 ? os : os << "B (" << hr.size << ')';

int main(int, char const* argv[])
fs::path example = "example.bin";
fs::path p = fs::current_path() / example;
std::ofstream(p).put('a'); // create file of size 1
std::cout << example << " size = " << fs::file_size(p) << '\n';

p = argv[0];
std::cout << p << " size = " << HumanReadable{fs::file_size(p)} << '\n';

try {
std::cout << "Attempt to get size of a directory:\n";
} catch(fs::filesystem_error& e) {
std::cout << e.what() << '\n';

std::error_code ec;
for (fs::path bin: {"cat", "mouse"}) {
bin = "/bin"/bin;
std::uintmax_t size = fs::file_size(bin, ec);
if (ec) {
std::cout << bin << " : " << ec.message() << '\n';
} else {
std::cout << bin << " size = " << HumanReadable{size} << '\n';

Possible output:

"example.bin" size = 1
"./a.out" size = 22KB (22512)
Attempt to get size of a directory:
filesystem error: cannot get file size: Is a directory [/dev]
"/bin/cat" size = 50.9KB (52080)
"/bin/mouse" : No such file or directory

See also

resize_file changes the size of a regular file by truncation or zero-fill
(C++17) (function)
space determines available free space on the file system
(C++17) (function)
file_size returns the size of the file to which the directory entry refers
(public member function of std::filesystem::directory_entry)