What is wrong with this mmap system call?

This is an attempt to mmap a file and write a single byte:

package main
import (
  "fmt"
	"os"
	"syscall"
)
func main() {
	file, _ := os.Open("/tmp/data")
	mmap, _ := syscall.Mmap(int(file.Fd()), 0, 100, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
	fmt.Printf("cap is %d", cap(mmap))
	mmap[0] = 0
	syscall.Munmap(mmap)
}

Despite length is set to 100, mmap capacity is always 0. What went wrong in the system call?

Always check for error!

os.Open opens a file for reading only, however mmap call asks to map the file read/write, thus giving a permission denied error, and as a result mapped region size is 0.

Is the file /tmp/data empty? If so:

I think you cannot pass arbitrary length parameter (like 100 in your case) to Mmap. I think this parameter must be <= file.Size(), ie the the size of the file referred to by fd. If that's the case then try to make your data file non-empty and try again.