Brainlag
  1. Library
  2. Debugging
  3. C

undeclaredWhat does "undeclared" mean?

The C compiler stops with "undeclared" when a line uses a name it has not seen yet: a typo, a missing #include, or a function written further down.

C5 min

What the compiler is telling you

Output
main.c(6): `bonsu' undeclared
    int total = score + bonsu;
                        ^~~~~

The compiler reads your file from the top, once. Every name must be declared before the line that uses it. This message means: "on line 6 you used bonsu, and I have never seen that name".

The usual reasons:

  • A typo. bonsu instead of bonus. Capitals count too: Score is not score.
  • Declared too late. A function written below main, with no prototype above it.
  • Out of scope. The variable was declared inside another { } block, or another function, so it no longer exists here.
  • A missing #include. Library names like strlen only exist after #include <string.h>.

How to find it

The message gives the line in brackets, and the ^~~~ marks the exact name. Search the file for where that name is declared. If you find it spelled differently, fix the spelling. If it is below this line, or inside a different block, move it up or add a prototype. If it is a library function, check its #include.

Run it: one letter swapped

Press Run and read the error from the bottom up.

main.c to run
#include <stdio.h>

int main(void) {
    int score = 120;
    int bonus = 30;
    int total = score + bonsu;
    printf("total %d\n", total);
    return 0;
}

The basket total nobody can see

This adds up a shopping basket in pence. It does not compile. Read the message, then fix it so it prints basket 1819.

exercise_1.c to run
#include <stdio.h>

int main(void) {
    int prices[4] = {250, 450, 120, 999};
    for (int i = 0; i < 4; i++) {
        int total = 0;
        total += prices[i];
    }
    printf("basket %d\n", total);
    return 0;
}