"Undefined symbols for architecture x86_64: "_add_number", referenced from: (C)

Question

In macOS With Visual Studio Code, I tried to compile C source code. (There are 3 parts of source code bellow)

head.h(edited)

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

int add_number(int a, int b);

body.c

#include "head.h"
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

int add_number(int a, int b)
{
    return a + b;
}

main.c

#include "head.h"
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int result = add_number (1, 2);
    printf("%d", result);
    return 0;
}

AND

gcc main.c

However, when I pushed F5 to compile this c code, following error occured:

Undefined symbols for architecture x86_64:
  "_add_number", referenced from:
      _main in main-49215f.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)

Is it link fail messages? Is anyone help me to fix this errors? for example, vsc setting or ...

P.S. my vsc's tasks.json is as bellow:

{
    "version": "2.0.0",
    "tasks": [
        {
            "type": "shell",
            "label": "gcc build",
            "command": "/usr/bin/gcc",
            "args": [
                "-g",
                "-o",
                "${fileDirname}/${fileBasenameNoExtension}",
                "${file}"
            ],
            "options": {
                "cwd": "/usr/bin"
            },
            "problemMatcher": 
            { 
                "fileLocation": [ 
                "relative", "${workspaceRoot}" 
                ], 
                "pattern": 
                { 
                    // The regular expression. 
                    //Example to match: helloWorld.c:5:3: warning: implicit declaration of function 'prinft' 
                    "regexp": "^(.*):(\\d+):(\\d+):\\s+(warning error):\\s+(.*)$", 
                    "file": 1, 
                    "line": 2, 
                    "column": 3, 
                    "severity": 4, 
                    "message": 5 
                }
            },
        "group": "build"
        },
    {
        "label": "execute", 
        "command": "cd ${fileDirname} && ./${fileBasenameNoExtension}", 
        "group": "test"
    }
]
}

Answer 1

Yes, it's an error from the linker. The reason is that you built your application by means of

gcc main.c

which has a declaration of add_number (from head.h), but no definition because the module / compilation unit which implements that function is missing. A quick fix is to

gcc body.c main.c

Answer 2

When you

inline int add_number(int a, int b); // head.h

int add_number(int a, int b) { ... } // head.c

extern int add_number(int a, int b); // main.c

Then the problem is still the same: The code works because there is an implementation of add_number provided you compile like in the other answer, i.e. with

gcc body.c main.c

If you return to your previous compile that did only

gcc main.c

then the implementation of add_number is still missing. If you want to have the function inline, then remove the implementation from head.c and do it in head.h like so:

static inline int add_number (int a, int b)
{
    return a + b;
}

and remove the prototype from main.c (in general it is a bad idea to have external declarations in C-modules, they should go to the headers so that all modules including the header agree on the prototype).

In your special case, after moving add_number from head.c to an inline function in head.h, the C file will be empty so that

gcc main.c

Works as expected.