Linker error: Multiple definition of a function

Question

I'm trying to write a program that includes .cpp and .h files.

Here is my code:

main.cpp:

#include "beep.h"
#include "movecursor.h"

beep.h:

#include <Windows.h>
#include <mmsystem.h>

DWORD WINAPI BeepSec(LPVOID parameters);

beep.cpp:

#include "beep.h"
#include "random.h"

DWORD WINAPI BeepSec(LPVOID parameters)
{

}

movecursor.h:

#include <Windows.h>
#include "beep.h"

DWORD WINAPI MoveCursor(LPVOID parameters);

movecursor.cpp:

#include "movecursor.h"
#include "random.h"

DWORD WINAPI MoveCursor(LPVOID parameters)
{

}

random.h:

#include <Windows.h>

int random() {
    HCRYPTPROV prov;
    if (prov == NULL)
        if (!CryptAcquireContext(&prov, NULL, NULL, PROV_RSA_FULL, CRYPT_SILENT | CRYPT_VERIFYCONTEXT))
    ExitProcess(1);

    int out;
    CryptGenRandom(prov, sizeof(out), (BYTE *)(&out));
    return out & 0x7fffffff;
}

But I got stuck with this linker error:

movecursor.cpp:(.text+0x0): multiple definition of 'random()' beep.cpp:(.text+0x0): first defined here

`int random()` - you implement it in `random.h` and then include it to `movecursor.cpp` and `beep.cpp` - so in both this files the same `int random()` exist. you need create `random.cpp` for example and implement `int random()` here. — RbMm, Nov 03 '19 at 11:38
Does this answer your question? [multiple definition: error at link time](https://stackoverflow.com/questions/32504570/multiple-definition-error-at-link-time). Also same problem: [multiple definition linker error after adding a function to a previously linking file](https://stackoverflow.com/questions/3136616/multiple-definition-linker-error-after-adding-a-function-to-a-previously-linking) — BDL, May 12 '20 at 11:55
You have multiple definition of your `random` function, since multiple files include that header, which causes the linker to fail. Either put your random function in a separate cpp file, or mark it inline. Both links provided by BDL explain this. — ChrisMM, May 12 '20 at 14:52

score 2 · Accepted Answer · edited Apr 04 '20 at 04:54

2

Put the definition of random() in a .cpp source file.

Use include guards in your .h header files:

#ifndef RANDOM_H
#define RANDOM_H
extern int random();
...
...
#endif

edited Apr 04 '20 at 04:54

Peter Mortensen

30,738
21
105
131

answered Nov 03 '19 at 11:41

suspectus

16,548
8
49
57

Or mark the definition [inline](https://en.cppreference.com/w/cpp/language/inline). – IInspectable Nov 03 '19 at 16:18

Linker error: Multiple definition of a function

1 Answers1