Struct Waitmsg 

#[repr(C)]
pub struct Waitmsg {
    pub pid: u64,
    pub exit_code: i32,
    pub _pad: i32,
    pub msg: [u8; 64],
}

Plan 9-inspired exit message written to userspace by SYS_PROC_WAIT.

Layout (C-compatible, 80 bytes total):

pid       u64   — task ID of the exited child
exit_code i32   — numeric exit code (0 = success)
_pad      i32   — padding for alignment
msg       [u8; 64] — null-terminated exit description

The msg field follows Plan 9 convention:

• "" (empty, or first byte = 0) → process exited normally (code 0)
• "exit <N>" → process exited with code N ≠ 0
• "killed" → process was killed by signal

Fields

pid: u64

exit_code: i32

_pad: i32

msg: [u8; 64]