cgo

file.go

// A simple cgo example for writing to a file.
package main

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

import (
	"errors"
	"fmt"
	"unsafe"
)

// FileReadWrite means we can read and/or write to the file.
const FileReadWrite = "w+"

// ErrWrite indicates a write did not complete successfully.
var ErrWrite = errors.New("failed to write")

// File is any file in the file system.
type File struct {
	fp   *C.FILE
	name string
}

// Open returns a new File for writing.
func Open(filename string) (*File, error) {
	cFilename := C.CString(filename)
	cMode := C.CString(FileReadWrite)
	defer C.free(unsafe.Pointer(cFilename))
	defer C.free(unsafe.Pointer(cMode))

	fp, err := C.fopen(cFilename, cMode)
	if err != nil {
		return nil, err
	}
	return &File{
		fp:   fp,
		name: filename,
	}, nil
}

// Close frees up the File's memory and closes the file descriptor.
func (f *File) Close() {
	C.fclose(f.fp)
}

// Write prints a string into the file.
func (f *File) Write(content string) error {
	size := C.size_t(len(content))
	cContent := unsafe.Pointer(C.CString(content))
	defer C.free(cContent)

	n := C.fwrite(cContent, size, 1, f.fp)
	if n == 0 {
		return ErrWrite
	}
	return nil
}

func main() {
	f, err := Open("tmp.txt")
	if err != nil {
		fmt.Printf("failed to open file: %s", err)
	}
	defer f.Close()
	f.Write("hello\n")
}