Question

// File foo1.c :
#include <stdio.h> // once
void foo1(void);
void foo1(void){
    puts("foo1");
}

// File foo2.c :
#include <stdio.h> // again
void foo2(void);
void foo2(void){
    puts("foo2");
}

// File foomain.c :
#include <stdio.h> // yet again
void foo1(void); // again
void foo2(void); // again
int main(void){
    foo1();
    foo2();
    puts("foomain");
    return 0;
}

// create object files
gcc -fPIC foo1.c -o foo1.o // 1 stdio.h
gcc -fPIC foo2.c -o foo2.o // 1 stdio.h

// create shared library
gcc -fPIC -shared foo1.o foo2.o -o foo.so // foo.so contains stdio.h 2 times ?

// build entire program
gcc foo.so foomain.c -o foomain // foomain contains 1 stdio.h plus the 2 from foo.so ?

1. Why does the entire program contain 3 stdio.h ? Seems redundant, why not just 1 ? Shouldn’t the compiler need only 1 ?

2. It makes sense for the object files to contain a prototype but why do they have to be specified again in foomain.c ? Shouldn’t the compiler know they are already specified in foo.so ?

Answer 1

That’s because each file is compiled separately, so each time the compiler should know the signatures of all functions used to perform compile-time checks. So, each file has to contain all declarations used, which are included by the preprocessor before the file is compiled.