mmap examples

README.md

identical mmap programs in C and go

reader.c

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/mman.h>

int main () {
  const char* filename = "main.c";
  int offset = 10;

  int fd = open(filename, O_RDONLY);
  if (fd == -1) {
    printf("cannot open %s\n", filename);
    return -1;
  }

  struct stat sbuf;
  if (stat(filename, &sbuf) == -1) {
    printf("cannot stat %s\n", filename);
    return -1;
  }
  int size = sbuf.st_size;

  char* data = mmap((caddr_t)0, size, PROT_READ, MAP_SHARED, fd, 0);
  if (data == -1) {
    printf("cannot mmap %s\n", filename);
    return -1;
  }

  printf("byte at offset %d is %c\n", offset, data[offset - 1]);

  int err = munmap(data, size);
  if (err == -1) {
    printf("cannot munmap %s\n", filename);
    return -1;
  }

  return 0;
}

reader.go

package main

import (
	"fmt"
	"os"
	"syscall"
)

func main() {
	filename, offset := "main.c", 10

	f, err := os.Open(filename)
	if err != nil {
		panic(err)
	}
	fd := int(f.Fd())

	stat, err := f.Stat()
	if err != nil {
		panic(err)
	}
	size := int(stat.Size())

	b, err := syscall.Mmap(fd, 0, size, syscall.PROT_READ, syscall.MAP_SHARED)
	if err != nil {
		panic(err)
	}

	fmt.Printf("%c\n", b[offset-1])

	err = syscall.Munmap(b)
	if err != nil {
		panic(err)
	}
}

writer.go

package main

import (
	"os"
	"syscall"
)

func main() {
	filename, offset, length := "testgo.out", 10, 20

	f, err := os.Create(filename)
	if err != nil {
		panic(err)
	}
	f.Write(make([]byte, length))
	fd := int(f.Fd())

	b, err := syscall.Mmap(fd, 0, length, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
	if err != nil {
		panic(err)
	}

	b[offset-1] = 'x'

	err = syscall.Munmap(b)
	if err != nil {
		panic(err)
	}
}