When should we use asserts in C?

Question

I am writing a function in C. As a matter of style, when is it good to use assert compared to returning an error code. Lets say the function is dividing two numbers. Should

Answer 1

First, assert from the header can be disabled (e.g. by compiling with gcc -DNDEBUG), and sometimes is disabled for the "production" version of a binary.

Second, as stated by Linux man page,

   The  purpose  of  this macro is to help the programmer find bugs in his
   program.   The  message  "assertion  failed  in  file  foo.c,  function
   do_bar(), line 1287" is of no help at all to a user.

So assert should fail only in buggy situations. In exceptional or error situations, you should do something else.

Some tools (or even compilers) might use assert-ions to e.g. optimize your code.

In your example of a quotient function, you'll use assert if, inside your entire program, you are sure that the divisor should be non-zero (but then it could make sense to name the function differently, perhaps quotient_by_non_zero). If you consider that it could happen, make it a fatal message, an exception (i.e. longjmp in C), an error code, etc.