Reading error messages

Error messages look frightening, but they all have the same shape:

marks.c:6:5: error: expected ';' before 'return'

File marks.c, line 6, column 5, the kind (error or warning), then the message. Most editors let you click it to jump to the line.

Three rules

1. Fix the first error first, then compile again. One missing ; can confuse the compiler into reporting ten more errors that are not real.
2. Look at the line before. "Expected ; before return" on line 6 means the ; is missing at the end of line 5.
3. Read the whole message, including the lines with ^ under your code: they point at the exact spot.

Common messages

When the program compiles but is wrong

#include <stdio.h>

int main(void) {
    int marks[3] = {70, 85, 90};
    int total = 0;
    for (int i = 0; i <= 3; i++) {
        total += marks[i];
    }
    printf("total %d\n", total);
    return 0;
}

This program compiles without an error, and with -Wall -Wextra gcc does not even warn (only with -O2 does it sometimes notice). But the loop runs while i <= 3, so it reads marks[3], one past the end. Compiled with gcc, it prints a wrong total (whatever was in memory next to the array) or crashes with Segmentation fault, and it can behave differently on another computer. Press Visualize: StepVioz stops at the exact line and shows you the array. On your own computer, gcc -fsanitize=address -g (Linux and macOS) gives you similar checks.

A compiler checks that your code is legal C. Only running it, testing it and watching it (the next lessons) can show that it is right.

Mark lesson complete