C: error: use of undeclared identifier 'count' (gcc: 'count' undeclared)

error: use of undeclared identifier 'count' (Clang) and error: 'count' undeclared (first use in this function) (GCC) both mean the compiler reached a name that has no declaration visible at that point. In C every variable must be declared before it is used, and a declaration only lives until the end of the block ({ ... }) that contains it. Fix the spelling, declare the variable before its first use in a scope that covers the use, or include the header that defines the constant (EXIT_SUCCESS needs <stdlib.h>).

Reproduce it

Clang 20, from the ziglang package on PyPI (Zig ships a complete Clang), run through uv:

cat > app.c <<'EOF'
#include <stdio.h>

int main(void) {
    int counter = 0;
    for (int i = 0; i < 3; i++) {
        int last = i;
        counter++;
    }
    printf("%d %d\n", count, last);
    return EXIT_SUCCESS;
}
EOF
uv run -q --no-project --python 3.13 --with ziglang==0.15.1 python -m ziglang cc -fno-color-diagnostics -c app.c -o app.o

app.c:9:23: error: use of undeclared identifier 'count'
    9 |     printf("%d %d\n", count, last);
      |                       ^
app.c:9:30: error: use of undeclared identifier 'last'
    9 |     printf("%d %d\n", count, last);
      |                              ^
app.c:10:12: error: use of undeclared identifier 'EXIT_SUCCESS'
   10 |     return EXIT_SUCCESS;
      |            ^
3 errors generated.

Three different causes in one function: count is a typo of counter, last only exists inside the for block, and EXIT_SUCCESS is a macro from <stdlib.h>, which is not included. GCC reports the same lines as 'count' undeclared (first use in this function), followed by a note that each undeclared identifier is reported only once for each function.

Fix it

cat > app.c <<'EOF'
#include <stdio.h>
#include <stdlib.h>              /* EXIT_SUCCESS, NULL */

int main(void) {
    int counter = 0;
    int last = -1;               /* declared outside the loop */
    for (int i = 0; i < 3; i++) {
        last = i;
        counter++;
    }
    printf("%d %d\n", counter, last);
    return EXIT_SUCCESS;
}
EOF
uv run -q --no-project --python 3.13 --with ziglang==0.15.1 python -m ziglang cc -fno-color-diagnostics app.c -o app && ./app

3 2

Common identifiers and their headers

Globals from another file

A global defined in config.c is not visible in main.c until it is declared there. Put an extern declaration in a header and include it in both files:

extern int verbose;     /* declaration: no storage */

#include "config.h"
int verbose = 0;        /* definition: exactly one .c file */