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I am trying to properly understand how to call C functions from another source file in C. I have a simple example that I am trying to get working. I have two functions, function1 and function2, which are contained in file.c. I am attempting to call function 2 from main.c bur receive the error "undefined reference to function 2".

main.c

#include <stdio.h>
#include <stdlib.h>
#include "file.h"
int main()
{
    int result = function2(3);
    printf("Result = %d  ", result);
    return 0;
}

file.c

int function1(int a)    /* Function 1  definition */
{
    int val = a*a;

    return val;
}

int function2(int val)    /* Function 2 definition */
{
    return val +5;
}

file.h

#ifndef FILE_H_INCLUDED
#define FILE_H_INCLUDED
extern int function1(int a) ;
extern int function2(int val);


#endif // FILE_H_INCLUDED

(I know that function1 and function2 can be combined into one function but I want to understand why this example isn't working! - thanks)

Jabberwocky
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Sjoseph
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    how are you compiling? – Sourav Ghosh Nov 28 '17 at 15:46
  • I am using Codeblocks and compiling and running main.c with all the source files contained within the same project. – Sjoseph Nov 28 '17 at 15:47
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    You check the options, you need to get those object files linked together. – Sourav Ghosh Nov 28 '17 at 15:48
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    Not related to your problem, but you should include `file.h` at the beginning of `file.c` as well. This will allow you to spot errors if the declaration in `file.h` are not the same as in `file.c`. – Jabberwocky Nov 28 '17 at 15:51
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    Just having the source files in the same directory won't cause Codeblocks to automatically compile and link them with `main.c`. You need, as @SouravGhosh says, to check your compile/link options. – lurker Nov 28 '17 at 15:51
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    1) In your case you don't need the `extern` keyword in the declarations; 2) You should include the corresponding header (for example, `a.c` includes `a.h` before anything else) - it's not always necessary, but it's good practice; 3) I guess you need to add `file.c` to your Codeblocks project - not just leave it in the same directory; – jweyrich Nov 28 '17 at 15:52
  • Great - thanks for the advice! I will look into that now. – Sjoseph Nov 28 '17 at 15:52

2 Answers2

2
  • In Codeblocks, File -> New -> Project.
  • Pick console application. Next.
  • Pick language C. Next.
  • Pick a project title path etc. Next.
  • Compiler should be GCC, don't touch any settings. Finished.
  • In the project window, add the relevant files to the project. You already get a main.c, so create a file.c and add it. It is convenient to add the h file too, although not necessary.
  • Build & run.
  • Output: Result = 8

Works fine with copy/paste of your code into main.c file.c file.h

Misc good advice not related to the problem:

  • Settings -> Compiler -> Compiler flags tab. Check the options corresponding to -Wall -Wextra and -pedantic-errors. If there is no C11 option, add one. See this.

  • As someone pointed out, you should always #include "file.h" from file.c

Lundin
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1

Your source code is correct. What you are probably not doing is linking the two object files together to create an executable.

From the command line, compile both files into object files

cc -Wall -std=c11 -c main.c

Creates main.o

cc -Wall -std=c11 -c file.c

Creates file.o

Then link them

cc -o my-executable main.o file.o

Then you will have a runnable binary called my-executable. If you have an IDE, there will be an equivalent way to do the above, but it will be IDE dependent.

JeremyP
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