What version of Go are you using (go version)?

$ go version
go version go1.20.3 linux/amd64
$ go list -m golang.org/x/sys
golang.org/x/sys v0.8.1-0.20230523194307-b5c7a0975ddc

What operating system and processor architecture are you using (go env)?

GOARCH=s390x
GOOS=linux

(using QEMU on an amd64 host)

What did you do?

package main

import (
	"fmt"
	"os"

	"golang.org/x/sys/unix"
)

func main() {
	tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0)
	if err != nil {
		panic(err)
	}
	pgid, err := unix.IoctlGetInt(int(tty.Fd()), unix.TIOCGPGRP)
	fmt.Println(pgid, unix.Getpgrp())
}

What did you expect to see?

Same number twice, as on amd64:

$ go run pgrp.go
101981 101981

What did you see instead?

$ GOARCH=s390x go run pgrp.go
438060894388224 101994

The first number is 101994<<32. I believe this happens because IoctlGetInt passes a pointer to a Go int to the system call:

func IoctlGetInt(fd int, req uint) (int, error) {
        var value int
        err := ioctlPtr(fd, req, unsafe.Pointer(&value))
        return value, err
}

The ioctl system call expects a pointer to a C int. Passing a Go int means it stores the pgid in the upper 32 bits.

Note that IoctlSetPointerInt passes a pointer to an int32:

func IoctlSetPointerInt(fd int, req uint, value int) error {
        v := int32(value)
        return ioctlPtr(fd, req, unsafe.Pointer(&v))
}