Calling C code

Learn to call C functions with the import "C" block.

The preamble and C headers

The comment above import "C" (the preamble) can also contain #include lines, so you can call functions from the standard C library.

/*
#include <stdlib.h>
*/
import "C"

n := C.abs(-5)  // 5

There must be no blank line between the preamble and import "C"; with a blank line the comment isn't treated as C code.

To link an external library, #cgo lines are used in the preamble:

/*
#cgo LDFLAGS: -lm
#include <math.h>
*/
import "C"

fmt.Println(C.sqrt(16))  // 4

CFLAGS passes options to the compiler and LDFLAGS to the linker; pkg-config: can fetch library settings automatically.

Add #include <stdlib.h> in the preamble. Print the result of C.abs(-12).

Show example solution

package main

/*
#include <stdlib.h>
*/
import "C"

import "fmt"

func main() {
    fmt.Println(C.abs(-12))
}

Linker option

/*
#cgo ____: -lm
#include <math.h>
*/
import "C"

Show answer

/*
#cgo LDFLAGS: -lm
#include <math.h>
*/
import "C"

Next lesson: C data types