Go-C conversion

Learn conversion between Go string and C.CString.

String conversion

Go strings carry their length, while C strings end with \0. C.CString(s) copies a Go string into C memory; C.GoString(p) turns a *C.char into a Go string.

cs := C.CString("merhaba")
defer C.free(unsafe.Pointer(cs))
fmt.Println(C.strlen(cs))  // 7

Byte slices

For byte data, C.CBytes(b) copies a []byte into C memory and C.GoBytes(p, n) copies n bytes from C memory into a Go slice. All these conversions copy; with large data their cost matters.

Exercise String length

Add #include <stdlib.h> and #include <string.h> in the preamble. Create a C.CString for "gopher", free it with defer C.free(unsafe.Pointer(cs)) and print C.strlen(cs).

Show example solution

package main

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

import (
    "fmt"
    "unsafe"
)

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

Test yourself

Who frees the memory allocated by <code>C.CString</code>?
- You, with C.free
- The Go garbage collector
- It is freed automatically on exit only
- The C.GoString call
Show answer

You, with C.free — C.CString allocates C memory with malloc; the GC can't see it, so it leaks unless freed with C.free.

Convert a C string to Go

p := C.CString("go")
s := C.____(p)

Show answer

p := C.CString("go")
s := C.GoString(p)

Conversion functions

C.CString

Go → C string
C.GoString

C → Go string
C.CBytes

Go → C bytes
C.GoBytes

C → Go bytes

Next lesson: cgo and memory

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