How can I throw an exception in C?

There are no exceptions in C. In C the errors are notified by the returned value of the function, the exit value of the process, signals to the process (Program Error Signals (GNU libc)) or the CPU hardware interruption (or other notification error form the CPU if there is)(How processor handles the case of division by zero).

Exceptions are defined in C++ and other languages though. Exception handling in C++ is specified in the C++ standard "S.15 Exception handling", there is no equivalent section in the C standard.

In C you could use the combination of the setjmp() and longjmp() functions, defined in setjmp.h. Example from Wikipedia

#include <stdio.h>#include <setjmp.h>static jmp_buf buf;void second(void) {    printf("second\n");         // prints    longjmp(buf,1);             // jumps back to where setjmp                                 //   was called - making setjmp now return 1}void first(void) {    second();    printf("first\n");          // does not print}int main() {       if ( ! setjmp(buf) ) {        first();                // when executed, setjmp returns 0    } else {                    // when longjmp jumps back, setjmp returns 1        printf("main");         // prints    }    return 0;}

Note: I would actually advise you not to use them as they work awful with C++ (destructors of local objects wouldn't get called) and it is really hard to understand what is going on. Return some kind of error instead.

There's no built-in exception mechanism in C; you need to simulate exceptions and their semantics. This is usually achieved by relying on setjmp and longjmp.

There are quite a few libraries around, and I'm implementing yet another one. It's called exceptions4c; it's portable and free. You may take a look at it, and compare it against other alternatives to see which fits you most.