C/C++: multiple definition of `x'; first defined here — ld returned 1 exit status

Meaning

Each .cpp is compiled separately; if a header contains int counter = 0; or a non-inline function body, every file that includes it gets its own copy and the linker refuses to merge them. Declare in the header, define once in a .cpp, or mark it inline / static / constexpr.

Common causes

• Variable or function defined (not declared) in a header included by several .cpp files
• Two .cpp files define the same function
• Including a .cpp file in another one that is also compiled
• Same library linked twice

Detailed fix by platform

Code examples

// config.h
#pragma once
extern int g_retries;                 // declaration only
inline int clamp0(int v) { return v < 0 ? 0 : v; }   // inline: allowed in headers

// config.cpp
#include "config.h"
int g_retries = 3;                    // the single definition

How to diagnose

1. Symbol — Which name does the linker print?
2. Files — Which two object files define it?
3. Header — Is it defined in a header?

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