Ever heard of an ifdowhile-Loop?

July 15, 2014

471 words, a 3 minutes read

I hope you didn’t…

OpenSSL-cleanup project LibreSSL has a commit fixing an ifdowhile loop into a simple while loop. Wait, what?

This piece of code has made it into one of the most used crypto libraries:

if (*p == '\0')
do {
	if (RAND_bytes(p, 1) <= 0)
	return 0;
} while (*p == '\0');

Programming in C: Lesson 1

This makes no sense at all. According to K&R:

while (expression)
The expression is evaluated. If it is non-zero, statement is executed and expression is reevaluated. This cycle continues until expression becomes zero, at which point execution resumes after statement.

… which basically means “it’ll do what you told it, as long as expression is true”. The while statement is a pre-test-loop which means that it tests first, then runs the code. If the test fails on the first run, the code won’t run.

Then there’s the do-while loop. Let’s get back to praised K&R:

while (expression);
The statement is executed, then expression is evaluated. If it is true, statement is evaluated again, and so on. When the expression becomes false, the loop terminates.

… which means about the same, except this is a post-test-loop. This means the code will run at least once, even if the test will fail afterwards; it stops then. (There are for loops to consider as well, but they’re more for counting or iterating).

I think someone got these two concepts a bit wrong and ended up mixing them together. This is like saying:
If the washing machine is running
then wait another minute until checking again
repeat only if the washing machine is running

Which any sane person would shorten to:
While the washing machine is running
then wait another minute until checking again

Conclusion & Fix

I am not sure if this is some low-level thing that makes sense in any way (like using goto in the Linux kernel for readability reasons), but I don’t get it. How can a programmer that has not just started learning C come up with this kind of code?

Update next day: after thinking about it some time (remember taking a step back) I’ve come to the conclusion that the developer simply did not know or forgot about the pre-test while. He knew that there is a way to repeat something as long as there is a condition given, but did not know how to make the entry point, so he combined if and do-while.

Does anyone know if this was the right way to do some time ago? So that a while loop is essentially syntactic sugar, like the for loop can be broken down into a while loop and a set of switch cases into if-elseif?

Fortunately, LibreSSL corrected this to:

while (*p == '\0') {
	if (RAND_bytes(p, 1) <= 0)
	return 0;