Weird ioctl_readwrite! behaviour for values above 10

[mekanix]

I'm trying to implement crate to make OSS (audio on FreeBSD) handling easier, and this is the part of code that is behaving strangely

const SNDCTL_DSP_MAGIC: u8 = b'P';
const SNDCTL_DSP_COOKEDMODE: u8 = 30;
nix::ioctl_readwrite!(oss_set_cooked, SNDCTL_DSP_MAGIC, SNDCTL_DSP_COOKEDMODE, i32);

Later I call it like this

let mut cooked: i32 = 0;
oss_set_cooked(fd, &mut cooked).expect("Failed to disable cooked mode");

When I use truss to look at the syscalls, I get this

# truss target/debug/deps/oss-10d0d16316d09bcd --ignored 2>&1 | grep -A1 dsp
openat(AT_FDCWD,"/dev/dsp4",O_WRONLY|O_CLOEXEC,00) = 4 (0x4)
ioctl(4,0xc004501e { IORW 0x50('P'), 30, 4 },0x2fa2c9fc85ec) ERR#22 'Invalid argument'

But if I change SNDCTL_DSP_COOKEDMODE to 9 for example, this is what I get

# truss target/debug/deps/oss-10d0d16316d09bcd --ignored 2>&1 | grep -A1 dsp
openat(AT_FDCWD,"/dev/dsp4",O_WRONLY|O_CLOEXEC,00) = 4 (0x4)
ioctl(4,SNDCTL_DSP_SUBDIVIDE,0x157e572ad5ec)	 ERR#22 'Invalid argument'

Can you help me debug this, please? I'm new to Rust, but not new to FreeBSD. Thank you!

[asomers]

Hi Goran! I think you're close, but you should use ioctl_write! instead of ioctl_readwrite!. That affects the calculation of the second argument of ioctl.

[mekanix]

If I use nix::ioctl_write_int!(oss_set_cooked, SNDCTL_DSP_MAGIC, SNDCTL_DSP_COOKEDMODE); I get the same result.