Error when compiling with GCC

Question

Every time I compile I get the following error message:

Undefined reference to ( function name )

Let's say I have three files: Main.c , printhello.h , printhello.c . Main.c calls function print_hello() , which returns "Hello World". The function is defined in printhello.c .

Now, here's the following code of printhello.h:

#ifndef PRINTHELLO_H 
#define PRINTHELLO_H

void print_hello();

#endif

I am sure this code is fine. I still don't know why is it giving me the error, though. Can you help me?

Answer 1

Undefined references are the linker errors. Are you compiling and linking all the source files? Since the main.c calls print_hello() , linker should see the definition of it.

gcc Main.c printhello.c -o a.out

Answer 2

The error is, I think, a linker error rather than a compiler error; it is trying to tell you that you've not provided all the functions that are needed to make a complete program.

You need to compile the program like this:

gcc -o printhello Main.c printhello.c

This assumes that your file Main.c is something like:

#include "printhello.h"

int main(void)
{
    print_hello();
    return 0;
}

and that your file printhello.c is something like:

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

void print_hello(void)
{
    puts("Hello World");
}

Your declaration in printhello.h should be:

void print_hello(void);

This explicitly says that the function takes no parameters. The declaration with the empty brackets means "there is a function print_hello() which returns no value and takes an indeterminate (but not variadic) list of arguments", which is quite different. In particular, you could call print_hello() with any number of arguments and the compiler could not reject the program.

Note that C++ treats the empty argument list the same as void print_hello(void); (so it would ensure that calls to print_hello() include no arguments), but C++ is not the same as C.

Answer 3

Another way to do it is to explicitly build object files for the printhello:

gcc -c printhello.c -o printhello.o
gcc -o Main main.c printhello.o

This has the added benefit of allowing other programs to use the print_hello method

Answer 4

It seems that the error is from the linker and not the compiler. You need to compile and link both the source files. I think what you are doing is simply including the header file in Main.c and you are not compiling the printhello.c

You need to:

gcc Main.c printhello.c -o myprog

or

construct the object files first

gcc -c printhello.c
gcc -c Main.c

then link them

gcc Main.o printhello.o