Zeuthen: Writing a C library, part 1 [LWN.net]

Zeuthen: Writing a C library, part 1

Posted Jun 29, 2011 7:16 UTC (Wed) by gowen (guest, #23914)
In reply to: Zeuthen: Writing a C library, part 1 by cmccabe
Parent article: Zeuthen: Writing a C library, part 1

Returning error codes is a great convention because you can flag them with __attribute__((warn_unused)). Then the programmer will get a warning from the compiler unless he checks the return code.

Returning error codes is fine in certain circumstances (particularly for functions where the side-effects are the point). Sometimes, though, you want your functions to be functions in the lambda-calculus sense - you want to return *results*.

In general, you can't return both results *and* error codes. (As I said, for pointers you can return NULL, and for functions whose domain of valid results is limited in some sense [abs()], you can, but if you're returning anything other than a pointer, int-type or floating-point-type, you're basically hosed.

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Zeuthen: Writing a C library, part 1

Posted Jul 7, 2011 0:22 UTC (Thu) by cmccabe (guest, #60281) [Link]

> In general, you can't return both results *and* error codes. (As I said,
> for pointers you can return NULL, and for functions whose domain of valid
> results is limited in some sense [abs()], you can, but if you're returning
> anything other than a pointer, int-type or floating-point-type, you're
> basically hosed.
>

If we're still talking about C/C++, then you can only ever return:
* a primitive (int, float, etc.)
* a pointer
* a struct

All of those have a natural 'none of the above' value. Integer types have 0 or a negative, floats and doubles have NaN, and pointers have NULL.

If you're returning a struct by value, then you're probably using C++, since C programmers rarely return an entire structure by value. The obvious thing to do is to either return an integer error code and take a reference to the thing to be modified, or use C++ exceptions. Either way, problem solved.

Being able to return multiple values at once is nice, but it's hardly the biggest challenge when using C or C++.

Zeuthen: Writing a C library, part 1

Posted Jul 7, 2011 12:08 UTC (Thu) by jwakely (subscriber, #60262) [Link]

> In general, you can't return both results *and* error codes.

std::future<double> squerrt(double x)
{
  std::promise<double> p;
  if (x < 0)
    p.set_exception(copy_exception(std::domain_error("negative")));
  else
    p.set_value(sqrt(x));
  return p.get_future();
}

int main()
{
  double i = squerrt(-1).get();  // boom
}