C header issue: #include and “undefined reference”

Question

Alright, I've been trying to work with this for the longest time, and I simply can't seem to get it to work right. I have three files, main.c, hello_world.c, and hello_world.h. For whatever reason they don't seem to compile nicely, and I really just can't figure out why...

Here are my source files. First hello_world.c:

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

int hello_world(void) {
  printf("Hello, Stack Overflow!\n");
  return 0;
}

Then hello_world.h, simple:

int hello_world(void);

And then finally main.c:

#include "hello_world.h"

int main() {
  hello_world();
  return 0;
}

When I put it into GCC, this is what I get:

cc     main.c   -o main
/tmp/ccSRLvFl.o: In function `main':
main.c:(.text+0x5): undefined reference to `hello_world'
collect2: ld returned 1 exit status
make: *** [main] Error 1

Anyone able to help me out? I'm really stuck on this, but I'm 99 percent sure it's a really simple fix.

Answer 1

gcc main.c hello_world.c -o main

Also, always use header guards:

#ifndef HELLO_WORLD_H
#define HELLO_WORLD_H

/* header file contents go here */

#endif /* HELLO_WORLD_H */

Answer 2

You are not including hello_world.c in compilation.

   gcc hello_world.c main.c   -o main

Answer 3

You are not linking against hello_world.c.

An easy way to do this is to run this compilation command:

cc -o main main.c hello_world.c

More complicated projects often use build scripts or make files that separate the compilation and linking commands, but the above command (combining both steps) should do fine for small projects.

Answer 4

You should link object file compled from your second .c file hello_world.c with your main.o

try this

cc -c main.c
cc -c hello_world.c
cc *.o -o hello_world

Answer 5

Ya it seems you have forgotten to link hello_world.c. I will be gcc hello_world.c main.c -o main. If the number of files are less we can use this approach but in larger projects better to use Make files or some compilation scripts.