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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: New and improved version of cdecl
Date: Thu, 30 Oct 2025 13:21:39 -0700
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richard@cogsci.ed.ac.uk (Richard Tobin) writes:
> In article <10dv52b$3gq3j$1@dont-email.me>,
> Richard Heathfield  <rjh@cpax.org.uk> wrote:
>
>>$time make -j $(nproc)
>
> Eww.  How does make distinguish between j with an argument and
> j with no argument and a target?
>
> $ make -j a
> make: *** No rule to make target 'a'. Stop.
> $ make -j 3
> make: *** No targets specified and no makefile found. Stop.
> $ make 3
> cc     3.c   -o 3
>
> That's a really bad idea.

Meh.

The data structure that defines the '-j' option in the GNU make
source is:

static struct command_switch switches[] =
  {
    // ...
    { 'j', positive_int, &arg_job_slots, 1, 1, 0, 0, &inf_jobs, &default_job_slots,
      "jobs", 0 },
    //...
  };

Yes, it's odd that "-j" may or may not be followed by an argument.
The way it works is that if the following argument exists and is
(a string representing) a positive integer, it's taken as "-j N",
otherwise it's taken as just "-j".

A make argument that's not an option is called a "target"; for
example in "make -j 4 foo", "foo" is the target.  A target whose name
is a positive integer is rare enough that the potential ambiguity
is almost never an issue.  If it is, you can use the long form:
"make --jobs" or "make --jobs=N".

I think it would have been cleaner if the argument to "-j" had
been mandatory, with an argument of "0", "-1", or "max" having
some special meaning.  But changing it could break existing scripts
that invoke "make -j" (though as I've written elsethread, "make -j"
can cause problems).

It would also have been nice if the "make -j $(nproc)" functionality
had been built into make.

The existing behavior is a bit messy, but it works, and I've never
run into any actual problems with the way the options are parsed.

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */