test2.c

#include <stdio.h>

int main(int argc, char *argv[]) {
    int c;
    int curr_line_length;
    while ((c = getc(stdin)) != EOF) {
        if (c == '\n') {
            printf("%d\n", curr_line_length);
            curr_line_length = 0;
        } else {
            curr_line_length++;
        }
    }
    if (curr_line_length > 0) {
            printf("%d\n", curr_line_length);
    }

    return 0;
}

clang -o t -Werror -Wconditional-uninitialized test2.c
test2.c:8:28: error: variable 'curr_line_length' may be uninitialized when used here [-Werror,-Wconditional-uninitialized]
            printf("%d\n", curr_line_length);
                           ^~~~~~~~~~~~~~~~
test2.c:5:25: note: initialize the variable 'curr_line_length' to silence this warning
    int curr_line_length;
                        ^
                         = 0
1 error generated.