How to get the file size in bytes with C++17

Question

Are there pitfalls for specific operating systems, I should know of?

There are many duplicates (1, 2, 3, 4, 5) of this question but they were answered decades ago. The very high voted answers in many of these questions are wrong today.

Methods from other (old QA's) on .sx

• stat.h (wrapper sprintstatf), uses syscall

• tellg(), returns per definition a position but not necessarily bytes. The return type is not int.

Answer 1

<filesystem> (added in C++17) makes this very straightforward.

#include <cstdint> #include <filesystem>  // ...  std::uintmax_t size = std::filesystem::file_size("c:\\foo\\bar.txt"); 

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As noted in comments, if you're planning to use this function to decide how many bytes to read from the file, keep in mind that...

...unless the file is exclusively opened by you, its size can be changed between the time you ask for it and the time you try to read data from it.
– Nicol Bolas