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

std::uintmax_t file_size( const std::filesystem::path& p,

                          std::error_code& ec ) noexcept;
(1) (since C++17)

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 -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_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.


#include <iostream>
#include <fstream>
#include <filesystem>
namespace fs = std::filesystem;
int main()
    fs::path p = fs::current_path() / "example.bin";
    std::ofstream(p).put('a'); // create file of size 1
    std::cout << "File size = " << fs::file_size(p) << '\n';
    try {
        fs::file_size("/dev"); // attempt to get size of a directory
    } catch(fs::filesystem_error& e) {
        std::cout << e.what() << '\n';

Possible output:

File size = 1
filesystem error: cannot get file size: Is a directory [/dev]

See also

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