Why doesn't gcc emit a format warning?

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#include <stdio.h>

void a(signed char a) {
    printf("%u\n", a);
}

void b(short b) {
    printf("%u\n", b);
}

void c(int c) {
    printf("%u\n", c);
}

void d(long d) {
    printf("%u\n", d);
}

void e(long long e) {
    printf("%u\n", e);
}

int main() {
    a(-1); //no warning
    b(-1); //no warning
    c(-1); //no warning
    d(-1); //warning
    e(-1); //warning
    return 0;
}

Compiled and tested with gcc 11.2.0 using gcc -std=c17 -pedantic -Wall -Wextra test.c and g++ -std=c++17 -pedantic -Wall -Wextra test.cpp. Both don't give any warning to a(), b(), and c(). Is this intended, or a bug?

gcc c c++
1 Answers

Short answer: C warnings are a mystery. Use -Wformat-signedness if you want warnings here.

Note that -Wformat-signedness requires -Wformat, which is already enabled by -Wall.

Apparently, the compiler only checks for sign mismatches when -Wformat-signedness is used. -Wall and -Wextra don't include -Wformat-signedness. I don't know why this is. Someone said it's because it would result in too many warnings, but that tells me it really needs to be used if it's such a common error!

So, all that's being checked are size mismatches. The two that warn (d and e) warn because a value of a type that's potentially larger than int is being passed. As for the other two, signed char and short int values are promoted to int values when passed to variadric (...) functions like printf, so a and b are equivalent to c.

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