Comment by okanat

You can also design a "dual-pass" API. Basically you have an API call to estimate or calculate the memory usage needed and then another parameter / API call to actually do the operation.

For example the WideCharToMultiByte [1] function of Win32 handles UTF-16 to UTF-8 conversion. If you pass your destination pointer as NULL, then it purely calculates the number of bytes you'll need in your buffer. Then the main program actually allocates the memory and calls it again with the populated destination pointer.

[1]: https://learn.microsoft.com/en-us/windows/win32/api/stringap...