Question

I have the following files:

f.c:

#include <stdlib.h>

double f(){
     return 12345;
}

main.c:

#include <stdlib.h>
#include <stdio.h>
int main(){
    double ret = f();
    printf("%f\n", ret);
}

I first compile the f.c into an object file using

gcc -c f.c -o f.o

then combine them using

gcc f.o main.c -o a.out

I’m expecting the output to be 12345.000000, but instead I got 0.000000.

Which step went wrong?

Answer 1

You didn’t compile with enough warnings.

Because you didn’t say:

extern double f(void);

in the main program, the C compiler was forced to assume the function would return an integer. This mangles the result as the compiler treats 4 bytes of the double as if it were an int, and then converts that into a double again.

Your compiler should have warned you about no prototype in scope, unless you’re using a C89 compiler rather than a C99 or C2011 compiler. You should turn on more compiler warnings, so that you get told about such mistakes and don’t run into such problems.

You could provide yourself with a header that declares the function f and include that header in both code modules. The header acts as a go between, ensuring that the definition is consistent with the usage.