Endless loop in C/C++

Question

There are several possibilities to do an endless loop, here are a few I would choose:

• for(;;) {}
• while(1) {} / while(true) {}
• do {} while(1) / do {} while(true)

Is there a certain form which one should choose? And do modern compilers make a difference between the middle and the last statement or does it realize that it is an endless loop and skips the checking part entirely?

Edit: as it has been mentioned I forgot goto, but this was done out of the reason that I don't like it as a command at all.

Edit2: I made some grep on the latest versions taken from kernel.org. I does seems as nothing much changed over time (within the Kernel at least)

Answer 1

The problem with asking this question is that you'll get so many subjective answers that simply state "I prefer this...". Instead of making such pointless statements, I'll try to answer this question with facts and references, rather than personal opinions.

Through experience, we can probably start by excluding the do-while alternatives (and the goto), as they are not commonly used. I can't recall ever seeing them in live production code, written by professionals.

The while(1), while(true) and for(;;) are the 3 different versions commonly existing in real code. They are of course completely equivalent and results in the same machine code.

---

for(;;)

• This is the original, canonical example of an eternal loop. In the ancient C bible The C Programming Language by Kernighan and Ritchie, we can read that:

  K&R 2nd ed 3.5:

  for (;;) {
  ...
  }
  

  is an "infinite" loop, presumably to be broken by other means, such as a break or return. Whether to use while or for is largely a matter of personal preference.

  For a long while (but not forever), this book was regarded as canon and the very definition of the C language. Since K&R decided to show an example of for(;;), this would have been regarded as the most correct form at least up until the C standardization in 1990.

  However, K&R themselves already stated that it was a matter of preference.

  And today, K&R is a very questionable source to use as a canonical C reference. Not only is it outdated several times over (not addressing C99 nor C11), it also preaches programming practices that are often regarded as bad or blatantly dangerous in modern C programming.

  But despite K&R being a questionable source, this historical aspect seems to be the strongest argument in favour of the for(;;).

• The argument against the for(;;) loop is that it is somewhat obscure and unreadable. To understand what the code does, you must know the following rule from the standard:

  ISO 9899:2011 6.8.5.3:

  for ( clause-1 ; expression-2 ; expression-3 ) statement
  

  /--/

  Both clause-1 and expression-3 can be omitted. An omitted expression-2 is replaced by a nonzero constant.

  Based on this text from the standard, I think most will agree that it is not only obscure, it is subtle as well, since the 1st and 3rd part of the for loop are treated differently than the 2nd, when omitted.

---

while(1)

• This is supposedly a more readable form than for(;;). However, it relies on another obscure, although well-known rule, namely that C treats all non-zero expressions as boolean logical true. Every C programmer is aware of that, so it is not likely a big issue.

• There is one big, practical problem with this form, namely that compilers tend to give a warning for it: "condition is always true" or similar. That is a good warning, of a kind which you really don't want to disable, because it is useful for finding various bugs. For example a bug such as while(i = 1), when the programmer intended to write while(i == 1).

  Also, external static code analysers are likely to whine about "condition is always true".

---

while(true)

• To make while(1) even more readable, some use while(true) instead. The consensus among programmers seem to be that this is the most readable form.

• However, this form has the same problem as while(1), as described above: "condition is always true" warnings.

• When it comes to C, this form has another disadvantage, namely that it uses the macro true from stdbool.h. So in order to make this compile, we need to include a header file, which may or may not be inconvenient. In C++ this isn't an issue, since bool exists as a primitive data type and true is a language keyword.

• Yet another disadvantage of this form is that it uses the C99 bool type, which is only available on modern compilers and not backwards compatible. Again, this is only an issue in C and not in C++.

---

So which form to use? Neither seems perfect. It is, as K&R already said back in the dark ages, a matter of personal preference.

Personally, I always use for(;;) just to avoid the compiler/analyser warnings frequently generated by the other forms. But perhaps more importantly because of this:

If even a C beginner knows that for(;;) means an eternal loop, then who are you trying to make the code more readable for?

I guess that's what it all really boils down to. If you find yourself trying to make your source code readable for non-programmers, who don't even know the fundamental parts of the programming language, then you are only wasting time. They should not be reading your code.

And since everyone who should be reading your code already knows what for(;;) means, there is no point in making it further readable - it is already as readable as it gets.