How to Avoid Invalid Argument Errors When Using IOCTL in Go?

How to IOCTL Properly in Go

When porting C code involving ioctl() to Go, developers frequently encounter challenges, such as invalid argument errors. This guide addresses the issues and provides solutions for proper IOCTL handling in Go.

Consider the following C code snippet:

#define MAJOR_NUM 100
#define IOCTL_MBOX_PROPERTY _IOWR(MAJOR_NUM, 0, char *)
static int mbox_property(int file_desc, void *buf){

   int ret_val = ioctl(file_desc, IOCTL_MBOX_PROPERTY, buf);
   return ret_val;
}

The corresponding Go equivalent would be:

func mBoxProperty(f *os.File, buf [256]int64) {
        err := Ioctl(f.Fd(), IOWR(100, 0, 8), uintptr(unsafe.Pointer(&buf[0])))

        if err != nil {
                log.Fatalln("mBoxProperty() : ", err)
        }

}

func Ioctl(fd, op, arg uintptr) error {
        _, _, ep := syscall.Syscall(syscall.SYS_IOCTL, fd, op, arg)
        if ep != 0 {
                return syscall.Errno(ep)
        }
        return nil
}

func IOWR(t, nr, size uintptr) uintptr {
        return IOC(IocRead|IocWrite, t, nr, size)
}
func IOC(dir, t, nr, size uintptr) uintptr {
        return (dir << IocDirshift) | (t << IocTypeshift) | (nr << IocNrshift) | (size << IocSizeshift)
}

However, this code can result in an invalid argument error. To resolve this, consider the following recommendations:

1. Use Go's ioctl Wrappers:

The "golang.org/x/sys/unix" package provides ioctl wrappers such as unix.IoctlSetInt, which may be suitable for your needs.

2. Manage Kernel Memory Carefully:

When handing memory buffers to the kernel, such as in the example provided, ensure that the buffer remains pinned in memory. Garbage collection can move or deallocate the buffer, leading to errors. Consider using cgo with malloc() to create an appropriate buffer.

3. Use CGO Extension:

Consider writing a small extension using cgo that can allocate a suitable buffer and manage its memory. This approach ensures the buffer is not subject to Go's garbage collection and eliminates the risk of memory errors.

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