Summary - cgo

Combine cgo to call a C math function.

cgo summary

In this group you called C code with import "C" and the preamble, linked libraries with #cgo, converted C.int/C.double, moved text with C.CString/C.GoString and released memory with C.free.

Remember: cgo calls are expensive and complicate cross-compiling; prefer a pure-Go alternative when one exists.

A complete example

Processing a Go string in a C function and getting the result back:

/*
#include <ctype.h>
void buyut(char* s) { for (; *s; s++) *s = toupper(*s); }
*/
import "C"

cs := C.CString("go")
defer C.free(unsafe.Pointer(cs))
C.buyut(cs)
fmt.Println(C.GoString(cs))  // GO

Exercise Convert to upper case

Add stdlib.h and ctype.h in the preamble and write void buyut(char* s) using toupper. Create a C.CString for "gopher", set up freeing, call C.buyut and print with C.GoString.

Show example solution

package main

/*
#include <stdlib.h>
#include <ctype.h>
void buyut(char* s) { for (; *s; s++) *s = toupper(*s); }
*/
import "C"

import (
    "fmt"
    "unsafe"
)

func main() {
    cs := C.CString("gopher")
    defer C.free(unsafe.Pointer(cs))
    C.buyut(cs)
    fmt.Println(C.GoString(cs))
}

Test yourself

Which allocates C memory and requires <code>C.free</code>?
- C.int(5)
- float64(C.f())
- C.CString("x")
- C.GoString(p)
Show answer

C.CString("x") — C.CString makes a copy with malloc; the others either produce Go memory or are just type conversions.

Disable cgo

GOOS=linux ____=0 go build -o app .

Show answer

GOOS=linux CGO_ENABLED=0 go build -o app .

Group summary

import "C"

Gateway to C
#cgo LDFLAGS

Linking a library
C.CString / C.GoString

Moving text
C.free

Releasing memory

Next lesson: The plugin package

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