Why and how does GCC compile a function with a missing return statement?

Question

#include 

char toUpper(char);

int main(void)
{
    char ch, ch2;
    printf(\"lowercase input : \");
    ch = getchar();
    ch2 = toUpper(ch);
             


        

Answer 1

I have tried a small programm:

#include 
int f1() {
}
int main() {
    printf("TEST: <%d>\n",  f1());
    printf("TEST: <%d>\n",  f1());
    printf("TEST: <%d>\n",  f1());
    printf("TEST: <%d>\n",  f1());
    printf("TEST: <%d>\n",  f1());
}

Result:

TEST: <1>

TEST: <10>

TEST: <11>

TEST: <11>

TEST: <11>

I have used mingw32-gcc compiler, so there might be diferences.

You could just play around and try e.g. a char function. As long you don't use the result value it will stil work fine.

#include 
char f1() {
}
int main() {
    f1();
}

But I stil would recommend to set either void function or give some return value.

Your function seem to need a return:

char toUpper(char c)
{
    if(c>='a'&&c<='z')
        c = c - 32;
    return c;
}