Summary - cgo
- Go course
- Advanced
- Lesson 18/68
- 4 min
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)) // GOPractice 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.
Starter code
package main
/*
// C kodu buraya
*/
import "C"
import (
"fmt"
"unsafe"
)
func main() {
// buraya yaz
}Sample 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))
}Expected output
GOPHER
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Which allocates C memory and requires <code>C.free</code>?
- C.int(5)
- float64(C.f())
- C.CString("x") Correct answer
- C.GoString(p)
Explanation: C.CString makes a copy with malloc; the others either produce Go memory or are just type conversions.
2 Disable cgo
GOOS=linux ____=0 go build -o app .GOOS=linux CGO_ENABLED=0 go build -o app .Explanation: CGO_ENABLED=0 builds pure Go; it is used for cross-compiling and static binaries.
3 Group summary
| Term | Meaning |
|---|---|
import "C" |
Gateway to C |
#cgo LDFLAGS |
Linking a library |
C.CString / C.GoString |
Moving text |
C.free |
Releasing memory |