Error handling with return codes

C has no try/except

• Some languages throw exceptions when something goes wrong. C does not have exceptions.
• Instead, a C function reports trouble through its return value — a status code.
• The caller checks that code after every call and decides what to do.

Return a status code

• A common rule: return 0 for success and a negative number like -1 for failure.
• So int do_thing(...) returns 0 when it worked, -1 when it could not.
• The real "answer" is then handed back a different way (see the next step).

Reporting the answer through an out-pointer

• If the return value is a status code, where does the real result go? Through an out-pointer.
• The caller passes the address of a variable; the function writes the answer with *.
• On failure, the function returns -1 and simply leaves that variable alone.

Sentinels for "no value"

• A sentinel is a special value that means "nothing here": -1 for an index, NULL for a pointer.
• Functions use sentinels so the caller can tell success from failure with a simple check.
• Always check for the sentinel before you trust the result.

#include <stdio.h>

int safe_div(int a, int b, int *result) {
    if (b == 0) return -1;          // failure: cannot divide by zero
    *result = a / b;                // success: hand back the answer
    return 0;
}

int main(void) {
    int r;
    if (safe_div(10, 2, &r) == 0) {
        printf("%d\n", r);          // 5
    }
    return 0;
}

Common mistakes

• Return a non-zero code from main to signal an error.
• Check the return value of any function that can fail.

Now you try

• Return 0 on success and -1 on failure, and write the real answer through the out-pointer.
• On failure, do not touch the out-pointer. Do not write a main — the checker provides one.