Unresolved External Symbol with external get function

Question

I keep getting a linker error with the following setup.

I have file1.c which contains the following code

#if defined( _TEST_ENABLED )

int get_value()
{
.
.
.    
}
#endif  /*_TEST_ENABLED   */

I have file2.c which includes file2.h, which defines _TEST_ENABLED. file2.c makes a call to get_value(), however the linker isn't having any part of that.

I've exhausted a lot of different options with zero success. Now i'm asking for help :)

Answer 1

如果file1.c不包含file2.h或任何定义_TEST_ENABLED文件，则当预处理器在file1.c上运行时，将不会定义_TEST_ENABLED ， _TEST_ENABLED将不会编译int get_value() { ... } 。

Answer 2

In order to call a function in another file:

1) The files must be compiled or at least linked together. The easiest way to do this is gcc file1.c file2.c , however you can also compile both files to *.o files and then link together.

2) The calling file must have, usually through an included header, a prototype of the function. This prototype must appear before the function is used. So, if file2.h defines _TEST_ENABLED , then you must (in file2.c ) include file2.h , and then either file2.c or file2.h must include file1.h , which must contain a function prototype ( int get_value; )

For example:

file1.c

#include <file1.h>
#include <file2.h>

int main() {
  get_value();
}

file1.h

#ifndef _FILE2_H
#define _FILE2_H

#define _TEST_ENABLED

#endif

file2.c

#include <file2.h>
#include <file1.h>

#ifdef _TEST_ENABLED
int get_value() {
  return 42;
}
#endif

file2.h

#ifndef _FILE2_H
#define _FILE2_H

int get_value();

#endif

Note that for the purposes of the preprocessor, file1.c and file2.c are processed completely separately. When processing file2.c , it MUST find #define _TEST_ENABLED somewhere, which is why file2.c must include file1.h . Since this is getting a little circular, you should add " #include -guards to each header file, as shown above.

Answer 3

There are some ambiguities in your question, but given the following three files, I can compile and build in ANSI C, but I have to include the .h in both .cs:

file1.c

#include "file2.h"

int main(void)
{
    someFunc();
    get_value();
    return 0;   
}

#ifdef _TEST_ENABLED
int get_value(void)
{
    return 0;   
}
#endif

file2.c

#include "file2.h"

int someFunc(void);

int someFunc(void)
{
    get_value();
    return 0;
}

file2.h

#define _TEST_ENABLED

int get_value(void);