Multiple Returns and Error Checking

Summary: Whether or not to allow multiple returns from a function is a controversial matter, but I recommend having a single return statement AND avoiding use of goto.

Discussion:

If you've been programming robust embedded systems for a while, you've seen code that looks something like this:

int MyRoutine(...)
{ ...
  if(..something fails..) { ret = 0; }
  else 
  { .. do something ...
    if(..somethingelse fails..) {ret = 0;}
    else 
    { .. do something ..
      if(...yetanotherfail..) {ret = 0;}
      else 
      { .. do the computation ...
        ret = value; 
      }
    }
  }
  // perform default function
  return(ret);
}

(Note: the "ret = 0" and "ret = value" parts are usually more complex; I'm keeping this simple for the sake of the discussion.)

The general pattern to this code is doing a bunch of validity checks and then, finally, in the most deeply nested "else" doing the desired action. Generally the code in the most deeply nested "else" is actually the point of the entire routine -- that's the action that you really want to take if all the exception checks pass.  This type of code shows up a lot in a robust embedded system, but can be difficult to understand.

This problem has been dealt with a number of times and most likely every reasonable idea has been re-invented many times. But when I ran up against it in a recent discussion I did some digging and found out I mostly disagree with a lot of the common wisdom, so here is what I think.

A common way to simplify things is the following, which is the guard clause pattern described by Martin Fowler in his refactoring catalog (an interesting resource with accompanying book if you haven't run into it before.)

int MyRoutine(...)
{ ...
  if(..something fails..)     {return(0);}
  if(..somethingelse fails..) {return(0);}
  if(...yetanotherfail..)     {return(0);}
  // perform default function
  return(value);
}

Reasonable people can (and do!) disagree about how to handle this.   Steve McConnell devotes a few pages of his book Code Complete (Section 17.1, and 17.3 in the second edition) that covers this territory, with several alternate suggestions, including the guard clause pattern and a discussion that says maybe a goto is OK sometimes if used carefully.

I have a somewhat different take, possibly because I specialize in high-dependability systems where being absolutely sure you are not getting things wrong can be more important than subjective code elegance. (The brief point of view is: getting code just a little klunky but obviously right and easy to check for correctness is likely to be safe. Getting code to look nice, but with a residual small chance of missing a subtle bug might kill someone. More this line of thought can be found in a different posting.)

Here is a sketch of some of the approaches I've seen and my thoughts on them:

(1) Another Language. Use another language that does a better job at this.  Sure, but I'm going to assume that you're stuck with just C.

(2) Guard Clauses.  Use the guard class pattern with early returns as shown above . The downside of this is that it breaks the single return per function rule that is often imposed on safety critical code. While a very regular structure might work out OK in many cases, things get tricky if the returns are part of code that is more complex. Rather than deal with shades of gray about whether it is OK to have more than one return, I prefer a strict single-return-per-function rule. The reason is that it avoids having to waste a lot of time hashing out when multiple returns are OK and when they aren't -- and the accompanying risk of getting that judgment call wrong. In other words black-and-white rules take less interpretation.

If you are building a safety critical system, every subjective judgement call as to what is OK and what is not OK is a chance to get it wrong.  (Could you get airtight design rules in place? Perhaps. But you still need to check them accurately after every code modification. If you can't automate the checks, realistically the code quality will likely degrade over time, so it's a path I prefer not to go down.)

(3) Else If Chain.  Use an if/else chain and put the return at the end:

int MyRoutine(...)
{ ...
  if(..something fails..)          {ret=0;}
  else if(..somethingelse fails..) {ret=0;}
  else if(...yetanotherfail..)     {ret=0;}
  else { // perform default function
         ret = value;
        }
  return(ret);
}

Wait, isn't this the same code with flat indenting?  Not quite.  It uses "else if" rather than "else { if". That means that the if statements aren't really nested -- there is really only a single main path through the code (a bunch of checks and the main action in the final code segment), without the possibility of lots of complex conditions in the "if" sidetrack branches.

If your code is flat enough that this works then this is a reasonable way to go. Think of it as guard clauses without the multiple returns.  The regular "else if" structure makes it pretty clear that this is a sequence of alternatives, or often a set of "check that everything is OK and in the end take the action." However, there may be cases in which the code logic is difficult to flatten this way, and in those cases you need something else (keep reading for more ideas).  The trivial case is:

(3a) Simplify To An Else If Chain.  Require that all code be simplified enough that an "else if" chain works.  This is a nice goal, and my first choice of style approaches.  But sometimes you might need a more capable and flexible approach.  Which brings us to the rest of the techniques...

(4) GOTO. Use a "goto" to break out of the flow and jump to the end of the routine.  I have seen many discussion postings saying this is the right thing to do because the code is cleaner than lots of messy if/else structures. At the risk of being flamed for this, I respectfully disagree. The usual argument is that a good programmer exhibiting discipline will do just fine.  But that entirely misses what I consider to be the bigger picture.  I don't care how smart the programmer who wrote the code is (or thinks he is).  I care a lot about whoever has to check and maintain the code. Sure, a good programmer on a good day can use a "goto" and basically emulate a try/throw/catch structure.  But not all programmers are top 10 percentile, and a lot of code is written by newbies who simply don't have enough experience to have acquired mature judgment on such matters. Beyond that, nobody has all good days.

The big issue isn't whether a programmer is likely to get it right. The issue is how hard (and error-prone) it is for a code reviewer and static analysis tools to make sure the programmer got it right (not almost right, or subtly wrong). Using a goto is like pointing a loaded gun at your foot. If you are careful it won't go off. But even a single goto shoves you onto a heavily greased slippery slope. Better not to go there in the first place. Better to find a technique that might seem a little more klunky, but that gets the job done with minimum fuss, low overhead, and minimal chance to make a mistake. Again, see my posting on not getting things wrong.

(Note: this is with respect to unrestricted "goto" commands used by human programmers in C. Generated code might be a different matter, as might be a language where "goto" is restricted.)

(5) Longjmp.  Use setjmp/longjmp to set a target and then jump to that target if the list of "if" error checks wants to return early. In the final analysis this is really the moral equivalent of a "goto," although it is a bit better controlled. Moreover, it uses a pointer to code (the setjmp/longjmp variable), so it is an indirect goto, and that in general can be hazardous. I've used setjmp/longjmp and it can be made to work (or at least seem to work), but dealing with pointers to code in your source code is always a dicey proposition. Jumping to a corrupted or uninitialized pointer can easily crash your system (or sometimes worse).  I'd say avoid using this approach.

I've seen discussion forum posts that wrap longjmp-based approaches up in macros to approximate Try/Throw/Catch. I can certainly appreciate the appeal of this approach, and I could see it being made to work. But I worry about things such as whether it will work if the macros get nested, whether reviewers will be aware of any implementation assumptions, and what will happen with static analysis tools on those structures. In high-dependability code if you can't be sure it will work, you shouldn't do it.

(6) Do..While..Break. Out of the classical C approaches I've seen, the one I like the most (beyond else..if chains) is using a "do..while..break" structure:

int MyRoutine(...)
{ ...
  do { //  start error handling code sequence
    if(..something fails..)     {ret=0; break;}
    if(..somethingelse fails..) {ret=0; break;}
    if(...yetanotherfail..)     {ret=0; break;}
    // perform default function
    ret = value;
  } while (0); // end error handling code sequence
  return(ret);
}

This code is a hybrid of the guard pattern and the "else if" block pattern. It uses a "break" to skip the rest of the code in the code block (jumping to the while(0), which always exits the loop). The while(0) converts this structure from a loop into just a structured block of code with the ability to branch to the end of the code block using the "break" keyword. This code ought to compile to an executable that is has essentially identical efficiency to code using goto or code using an else..if chain. But, it puts an important restriction on the goto-like capability -- all the jumps have to point to the end of the do..while without exception.

What this means in practice is that when reviewing the code (or a change to the code) there is no question as to whether a goto is well behaved or not. There are no gotos, so you can't make an unstructured goto mistake with this approach.

While this toy example looks pretty much the same as the "else if" structure, an important point is that the "break" can be placed anywhere -- even deeply within a nested if statement -- without raising questions as to what happens when the "break" is hit. If in doubt or there is some reason why this technique won't work for you, I'd suggest falling back on restructuring the code so "else if" or this technique works if the code gets too complex to handle. The main problem to keep in mind is that if you nest do..while structures the break will un-nest only one level.

I recognize that this area falls a little bit into a matter of taste and context. My taste is for code that is easy to review and unlikely to have bugs. If that is at odds with subjective notions of elegance, so be it. In part my preference is to outlaw the routine use of techniques that require manual analysis to determine of a potentially unsafe structure is being used the "right" way. Every such judgement call is a chance to get it wrong, and a distraction of human reviewer attention away from more important things. And I dislike arguments of the form that a "good" and experienced programmer won't make a mistake. It is just too easy to miss a subtle bug, especially when you're modifying code in a hurry.

If you've run into another way to handle this problem let me know.

Update 11/2022:  Olayiwola Ayinde has a similar take, pointing out that multiple exits make it too easy to forget to de-allocate resources that might have been allocated at the start of a procedure:

https://olayiwolaayinde.medium.com/c-programming-why-single-exit-point-6b2f8c43054f