syscall to userland interface

[Alfred Perlstein]

On 5/10/13 12:31 PM, Karl Dreger wrote:
> Hello,
> I have been taking a look at a few syscalls in /usr/src/sys/kern/ and
> always find that in their actuall c definition the function names are
> preprended by a sys_. Take for example the fork system call which
> is found in /usr/src/sys/kern/kern_fork.c
>
> int
> sys_fork(struct thread *td, struct fork_args *uap)
> ...
>
> Now when I write a program from userland, that makes use of the
> fork system call, then if call it as:
>
> fork();
>
> All the syscall are part of libc, which is usually defined in
> /usr/src/lib/libc/
>
> Since the system calls are already defined in the kernel sources, they
> no longer need to be defined in /usr/src/lib/libc/. This is the reason
> why one can only find the manpages and no c files in
> /usr/src/lib/libc/sys?
> At least this is how my thinking goes.
>
> Now, when the syscalls in the kernel sources are all defined as sys_xxx
> but are invoked as xxx and the c headers also show syscall prototypes
> without any prepended sys. How does the actual user-, kernelland
> move happen? In other words, why do I invoke fork() as fork() and
> not as sys_fork()?
>
> Or is there something that I missed?
>
>
> Clarification on that point is highly welcome.

When you build the system a whole bunch of assembler files are 
automatically generated that define the functions you are looking for.

Look for .S files under the object directory.

Those assembler files have the magic to cause a system call to happen.

example: src/lib/libc/getauid.S  (note, this file is GENERATED, it's not 
part of src.)



-Alfred