Re: [9fans] the declaration of main()

["Douglas A. Gwyn" <gwyn@arl.army.mil>]

Boyd Roberts wrote:
> yep, IIRC, sounds like the 7th ed /lib/crt0.s which eventually
> called _exit() [the syscall].  exit() was a library function
> which did the above and then called _exit().

I think there was a linker trick (similar to the floating-point
trick) to get the appropriate runtime startup behavior depending
on whether stdio functions had been invoked or not; at least,
when I tried the experiment on 7th Ed. UNIX at the time I
posted (to make sure I remembered correctly), the flushing
version of exit() got invoked from the startup function.

Note that in PDP-11 UNIX C, C externs have an underscore
prefixed to them at the linker level; so _exit in assembler
corresponds to exit() in C.  There is also _exit() in C, i.e.
__exit in assembler, which is the plain system call you
described, but from assembler one would not call that extern
function but would rather use a "sys" (TRAP) instruction
with the proper subcode to select the exit syscall.  There
were (optional) assembler macros for the syscall subcodes.