cgo: function scope variable and package scope variable

code1 output.txt

Driver Capability:
    Driver: "uvcvideo"
    Card: "UCAM-DLN130T series"
    Bus: "usb-0000:00:14.0-2"
    Capabilities: 84000001

code1.go

package main

/*
#include <linux/videodev2.h>
*/
import "C"
import (
	"fmt"
	"os"
	"syscall"
	"unsafe"
)

func main() {
	vd, err := os.OpenFile("/dev/video0", os.O_RDWR, 0660)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer vd.Close()

	if err = printInfo(vd); err != nil {
		fmt.Println(err)
		return
	}
}

func ioctl(fd *os.File, op uintptr, arg uintptr) error {
	_, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd.Fd(), op, arg)
	if err != 0 {
		return err
	}
	return nil
}

func printInfo(vd *os.File) error {
	// get capability info
	var caps C.struct_v4l2_capability

	err := ioctl(vd, C.VIDIOC_QUERYCAP, uintptr(unsafe.Pointer(&caps)))
	if err != nil {
		return err
	}

	fmt.Printf("Driver Capability:\n"+
		"    Driver: \"%s\"\n"+
		"    Card: \"%s\"\n"+
		"    Bus: \"%s\"\n"+
		"    Capabilities: %08x\n",
		caps.driver,
		caps.card,
		caps.bus_info,
		caps.capabilities)
	return nil
}

code2 output.txt

bad file descriptor

code2.go

package main

/*
#include <linux/videodev2.h>
*/
import "C"
import (
	"fmt"
	"os"
	"syscall"
	"unsafe"
)

var vd *os.File

func main() {
	vd, err := os.OpenFile("/dev/video0", os.O_RDWR, 0660)
	if err != nil {
		fmt.Println(err)
		return
	}
	defer vd.Close()

	if err = printInfo(); err != nil {
		fmt.Println(err)
		return
	}
}

func ioctl(fd *os.File, op uintptr, arg uintptr) error {
	_, _, err := syscall.Syscall(syscall.SYS_IOCTL, fd.Fd(), op, arg)
	if err != 0 {
		return err
	}
	return nil
}

func printInfo() error {
	// get capability info
	var caps C.struct_v4l2_capability

	err := ioctl(vd, C.VIDIOC_QUERYCAP, uintptr(unsafe.Pointer(&caps)))
	if err != nil {
		return err
	}

	fmt.Printf("Driver Capability:\n"+
		"    Driver: \"%s\"\n"+
		"    Card: \"%s\"\n"+
		"    Bus: \"%s\"\n"+
		"    Capabilities: %08x\n",
		caps.driver,
		caps.card,
		caps.bus_info,
		caps.capabilities)
	return nil
}