What is cgo?

Learn the ability to call C code with cgo.

What is cgo?

cgo is the tool that lets a Go program call C functions. The comment block right above the import "C" line is compiled as C code, and you reach its functions as C.name.

/*
int topla(int a, int b) { return a + b; }
*/
import "C"

fmt.Println(C.topla(2, 3))  // 5

This is how mature C libraries such as SQLite and OpenSSL are used from Go.

The cost of cgo

cgo calls are far more expensive than normal Go calls (tens of nanoseconds of overhead). Cross-compiling also gets harder and producing a single static binary becomes difficult. Hence the saying: "cgo is not Go". If a pure-Go alternative exists (e.g. modernc.org/sqlite), it is considered first.

Exercise Call a C function

In the preamble comment define int kare(int x) { return x * x; }. In main, print the result of C.kare(7) with fmt.Println.

Show example solution

package main

/*
int kare(int x) { return x * x; }
*/
import "C"

import "fmt"

func main() {
    fmt.Println(C.kare(7))
}

Test yourself

What is needed to build a package that uses cgo?
- Only the Go compiler
- A Java virtual machine
- Docker
- A C compiler on the system (gcc/clang)
Show answer

A C compiler on the system (gcc/clang) — cgo calls gcc or clang to compile the C code; with CGO_ENABLED=0 files that use cgo are not built.

Import the C package

/*
int iki() { return 2; }
*/
import "____"

Show answer

/*
int iki() { return 2; }
*/
import "C"

cgo parts

import "C"

Enables cgo
The preamble comment

C source code
C.topla

A C function call
CGO_ENABLED=0

Disables cgo

Next lesson: Calling C code

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