Using xcode to create static library with C, Error: "Undefined symbols for architecture i386"!

Question

I am using xcode to create a static library with C, I seem to get an error Undefined symbols for architecture i386.

The static library project, include three files: fun.c, testFun.cpp, testFun.h

Here is testFun.cpp

#include "testFun.h"
extern void test_c_fun();
void TestFun::test()
{
    printf("# TestFun c++ # ");
    test_c_fun();
}

and here is the fun.c

#include <stdio.h>
void test_c_fun() 
{ 
    printf("# test_c_fun #"); 
}

When I build with "IOS Device" and "iPhone Retina(4-inch)", I get two x.a file.

Using lipo tool with -create param to output the new x.a, supporting arm and i386.

add the x.a to my project, and include the testFun head file now code:

TestFun tf;
tf.test();

Then building it, I get these errors

Undefined symbols for architecture i386: "test_c_fun()", referenced from: TestFun::test() in libstatistic.a(testFun.o) ld: symbol(s) not found for architecture i386

When I hide the c-fun call (test_c_fun), then the build is successful!

look like:

#include "testFun.h"
extern void test_c_fun();
void TestFun::test()
{
    printf("# TestFun c++ # ");
    //test_c_fun();
}

Why can it not work with C file?

Answer 1

In testFun.cpp, declare C functions with extern C

as

extern "C" void test_c_fun();

C style function have different name mangling rules. When you declare a C function in .cpp file, it will treat that function as a C++ function.

When you will add extern "C" before declaration, it will see that function as a C function.