Error handling in C code

Question

What do you consider "best practice" when it comes to error handling errors in a consistent way in a C library.

There are two ways I've been thinking of:

Always return error code. A typical function would look like this:

MYAPI_ERROR getObjectSize(MYAPIHandle h, int* returnedSize);

The always provide an error pointer approach:

int getObjectSize(MYAPIHandle h, MYAPI_ERROR* returnedError);

When using the first approach it's possible to write code like this where the error handling check is directly placed on the function call:

int size;
if(getObjectSize(h, &size) != MYAPI_SUCCESS) {
  // Error handling
}

Which looks better than the error handling code here.

MYAPIError error;
int size;
size = getObjectSize(h, &error);
if(error != MYAPI_SUCCESS) {
    // Error handling
}

However, I think using the return value for returning data makes the code more readable, It's obvious that something was written to the size variable in the second example.

Do you have any ideas on why I should prefer any of those approaches or perhaps mix them or use something else? I'm not a fan of global error states since it tends to make multi threaded use of the library way more painful.

EDIT: C++ specific ideas on this would also be interesting to hear about as long as they are not involving exceptions since it's not an option for me at the moment...

Answer 1

Second approach lets the compiler produce more optimized code, because when address of a variable is passed to a function, the compiler cannot keep its value in register(s) during subsequent calls to other functions. The completion code usually is used only once, just after the call, whereas "real" data returned from the call may be used more often

Answer 2

Use setjmp.

http://en.wikipedia.org/wiki/Setjmp.h

http://aszt.inf.elte.hu/~gsd/halado_cpp/ch02s03.html

http://www.di.unipi.it/~nids/docs/longjump_try_trow_catch.html

#include <setjmp.h>
#include <stdio.h>

jmp_buf x;

void f()
{
    longjmp(x,5); // throw 5;
}

int main()
{
    // output of this program is 5.

    int i = 0;

    if ( (i = setjmp(x)) == 0 )// try{
    {
        f();
    } // } --> end of try{
    else // catch(i){
    {
        switch( i )
        {
        case  1:
        case  2:
        default: fprintf( stdout, "error code = %d\n", i); break;
        }
    } // } --> end of catch(i){
    return 0;
}

---

#include <stdio.h>
#include <setjmp.h>

#define TRY do{ jmp_buf ex_buf__; if( !setjmp(ex_buf__) ){
#define CATCH } else {
#define ETRY } }while(0)
#define THROW longjmp(ex_buf__, 1)

int
main(int argc, char** argv)
{
   TRY
   {
      printf("In Try Statement\n");
      THROW;
      printf("I do not appear\n");
   }
   CATCH
   {
      printf("Got Exception!\n");
   }
   ETRY;

   return 0;
}

Answer 3

I personally prefer the former approach (returning an error indicator).

Where necessary the return result should just indicate that an error occurred, with another function being used to find out the exact error.

In your getSize() example I'd consider that sizes must always be zero or positive, so returning a negative result can indicate an error, much like UNIX system calls do.

I can't think of any library that I've used that goes for the latter approach with an error object passed in as a pointer. stdio, etc all go with a return value.

Answer 4

I have done a lot of C programming in the past. And I really apreciated the error code return value. But is has several possible pitfalls:

• Duplicate error numbers, this can be solved with a global errors.h file.
• Forgetting to check the error code, this should be solved with a cluebat and long debugging hours. But in the end you will learn (or you will know that someone else will do the debugging).

Answer 5

In addition the other great answers, I suggest that you try to separate the error flag and the error code in order to save one line on each call, i.e.:

if( !doit(a, b, c, &errcode) )
{   (* handle *)
    (* thine  *)
    (* error  *)
}

When you have lots of error-checking, this little simplification really helps.

Answer 6

The UNIX approach is most similar to your second suggestion. Return either the result or a single "it went wrong" value. For instance, open will return the file descriptor on success or -1 on failure. On failure it also sets errno, an external global integer to indicate which failure occurred.

For what it's worth, Cocoa has also been adopting a similar approach. A number of methods return BOOL, and take an NSError ** parameter, so that on failure they set the error and return NO. Then the error handling looks like:

NSError *error = nil;
if ([myThing doThingError: &error] == NO)
{
  // error handling
}

which is somewhere between your two options :-).