Question

I tried to call a self-written function without handling its return value. gcc told me at the line of the function call that this would be a statement with no effect.
To check whether my function gets called at all, I added some printf statement, but didn’t get any output from the program.

Is it possible that gcc simply ignores the function call? As far as I know, I never had any problems with such statements.

So here is the code:

unsigned strlen(char *string)
{
  printf("ignored by gcc");
  unsigned count = 0;
  for(; *string++; count++);
  return count;
}

int main()
{
  char string[] = "something";
  strlen(string);

  return 0;
}

Thanks in advance.

Answer 1

It is perfectly legal to ignore the return value of a function. You probably do it more than 99% of the times you invoke printf() (which returns an int).

However, as several persons have said in the comments, you called your function strlen() after a standard library function. This is illegal according to C99’s 7.1.3:2:

If the program declares or defines an identifier in a context in which
it is reserved (other than as allowed by 7.1.4), or defines a reserved
identifier as a macro name, the behavior is undefined.

Here the compiler warns unexpectedly, and either calls the standard function instead of yours, or calls no function at all (since it knows its strlen() is supposed to be without side-effects). This is one of the things undefined behavior can do.