Why use `thread' as an argument of Syscalls?

[MingyanGuo]

Hi all,
     I  find that  FreeBSD Syscalls  always have an `thread'
argument, for example, preadv(/sys/kern/sys_generic.c)
has a `td' argument.  But  some Syscalls may rarely  use
this argument,  and  thay ( and functions they invoke) can
get  the  `thread'  who  make  the  Syscall  _easily_  via
`curthread' macro if  needed. So the `thread' argument
seems not needed.
   Can anybody tell me why use `thread' as an argument
of Syscalls?

Thanks.

Regards,

-- 
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