Nigel Jones from Maryland says he uses the following questions to interview embedded systems C programmers.

What is the purpose of the preprocessor directive #error?
How do you code an infinite loop in C ?
What are the uses of the keyword static?
What does the following code output?
void foo(void)
{
unsigned int a=6;
int b=-20;
(a+b >6) ? puts(">6") : puts("<=6");
}

1. Either you know the answer to this, or you don't. This question is useful for differentiating between normal folks and the nerds. Only the nerds actually read the appendices of C textbooks to find out about such things. Of course, if you aren't looking for a nerd, the candidate better hope she does not know the answer.

2. My preferred solution is:
while(1)
{
}
Many programmers seem to prefer:
for(; ;)
{
}
This construct puzzles me because the syntax doesn't exactly spell out what's going on. Thus, if a candidate gives this as a solution, I'll use it as an opportunity to explore their rationale for doing so. If their answer is basically, "I was taught to do it this way and I haven't thought about it since," it tells me something (bad) about them.

A third solution is to use a goto:
Loop:
...
goto Loop;
Candidates who propose this are either assembly language programmers (which is probably good), or else they are closet BASIC/FORTRAN programmers looking to get into a new field.

3. This simple question is rarely answered completely. Static has three distinct uses in C:

A varaible declared static within the body of a function maintains its value between function invocations.
A varaible declared static within a module, (but outside the body of a function) is accessible by all functions within that module. It is not accessible from other modules. That is, it is a localized global.
Functions declared static within a module may only be called by other functions within that module. That is, the scope of the function is localized to the module within which it is declared.

Most candidates get the first part correct.A reasonable number get the second part correct, while a pitiful number understand the third answer. This is a serious weakness in a candidate, since he obviously doesn't understand the importance and benefits of localizing the scope of both data and code.

4. The question tests whether you understand the integer promotions rules in C - an area that I find is very poorly understood by many developers. Anyway, the answer is that this outputs ">6". The reason for this is that expressions involving signed and unsigned types have all operands promoted to unsigned types. Thus -20 becomes a very large positive integer and the expression evaluates to greater than 6.This is a very important point in embedded systems where unsigned data types should be used frequently. If you get this one wrong, you are perilously close to not getting the job.