When including a header file in C, does it automatically include the .c file of the same name?

Question

I've looked around online as well as in my textbook and this is confusing me.

Say you have some functions for stacks in stack.c, and you put their prototypes in stack.h. Your main program, say, test.c has #include "stack.h" at the top. This is how all the examples show.

So it includes the prototypes, but how does it get their implementations? The header files don't seem to require that you #include stack.c with them. Does it just search all the .c files in the same folder and try to find them?

Answer 1

No; it includes just the header.

You compile the source separately, and link that with your code that uses it.

For example (toy code):

stack.h

extern int pop(void);
extern void push(int value);

stack.c

#include "stack.h"
#include <stdio.h>
#include <stdlib.h>

enum { MAX_STACK = 20 };
static int stack[MAX_STACK];
static int stkptr = 0;

static void err_exit(const char *str)
{
     fprintf(stderr, "%s\n", str);
     exit(1);
}

int pop(void)
{
    if (stkptr > 0)
        return stack[--stkptr];
    else
        err_exit("Pop on empty stack");
}

int push(int value)
{
    if (stkptr < MAX_STACK)
        stack[stkptr++] = value;
    else
        err_exit("Stack overflow");
}

test.c

#include <stdio.h>
#include "stack.h"

int main(void)
{
    for (int i = 0; i < 10; i++)
        push(i * 10);

    for (int i = 0; i < 10; i++)
        printf("Popped %d\n", pop());
    return(0);
}

Compilation

c99 -c stack.c
c99 -c test.c
c99 -o test_stack test.o stack.o

Or:

c99 -o test_stack test.c stack.c

So, you compile the source files (optionally producing object files) and link them. Often, the stack.o file would be placed into a library (other than the standard C library) and you'd link with that library. This is what happens with the standard C library functions as well, of course. The C compiler automatically adds the C library (usually -lc) to the linking command.