[TUHS] C entry keyword

[TUHS] C entry keyword

[Steve Simon]

“entry” was a reserved word in K&R Ed.1,
my personal favourite C trivia. I have never seen it used outside fortran on mainframes though.

-Steve

Re: [TUHS] C entry keyword

[Lars Brinkhoff]

Steve Simon wrote:
> “entry” was a reserved word in K&R Ed.1,
> my personal favourite C trivia. I have never seen it used outside
> fortran on mainframes though.

It's there in Snyder's C compiler, but not used.  From the grammar:

    statement:
    ...
        | ENTRY identifier ':' statement       {val=aentry(#2,#4);}

But the supporting code in the compiler:

    aentry (idn, stmt)
        {return (stmt);}    /* not implemented */

Re: [TUHS] C entry keyword

[Angelo Papenhoff]

On 30/10/18, Steve Simon wrote:
> 
> “entry” was a reserved word in K&R Ed.1,
> my personal favourite C trivia. I have never seen it used outside fortran on mainframes though.

I think PL/1 on Multics uses it, which is probably how it "got into" C.

aap

Re: [TUHS] C entry keyword

[Paul Winalski]

On 10/30/18, Angelo Papenhoff <aap@papnet.eu> wrote:
> On 30/10/18, Steve Simon wrote:
>>
>> “entry” was a reserved word in K&R Ed.1,
>> my personal favourite C trivia. I have never seen it used outside fortran
>> on mainframes though.
>
> I think PL/1 on Multics uses it, which is probably how it "got into" C.
>
PL/1 inherited the concept of routines with multiple entry points from Fortran.

Did the early K&R C compilers actually implement multiple entry point
routines, or was the keyword simply reserved for future use?

-Paul W.

Re: [TUHS] C entry keyword

[Warner Losh]

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I'd imagine you'd use it like so (in more modern C):

void *memcpy(void *src, void *dst, size_t len)
{
char *rv = dst;
top:
while (len--) *dst++ =*src; /* not always a trivial body */
return(rv);
entry bcopy:
rv = dst; /* Swap args and jump to memcpy */
dst = src;
src = rv;
goto top:
}

Variations on this theme are often done in assembler for these routines,
though the example is an attempt to cope with the diversity of interfaces
that grew up after v6 unix was released in the world, so it's not likely
this particular example inspired it...

Warner

On Tue, Oct 30, 2018 at 4:00 AM Angelo Papenhoff <aap@papnet.eu> wrote:

> On 30/10/18, Steve Simon wrote:
> >
> > “entry” was a reserved word in K&R Ed.1,
> > my personal favourite C trivia. I have never seen it used outside
> fortran on mainframes though.
>
> I think PL/1 on Multics uses it, which is probably how it "got into" C.
>
> aap
>

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Re: [TUHS] C entry keyword

[Paul Winalski]

In Fortran secondary entry points served two main purposes.  It
allowed for sharing of code for similar routines (as in Angelo
Papenhoff's C example), which was important when your program had to
fit in only a few K of memory.  Secondly, it provided partial relief
for Fortran's very restrictive variable-scoping rules.

It's a pain in the butt for compiler optimizers, although certain
modern interprocedural optimizations emit code that is the moral
equivalent of a routine with multiple entry points.

-Paul W.