Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!elf115!rec
From: rec@elf115.uu.net (Roger Critchlow)
Newsgroups: comp.lang.c
Subject: Re: Another silly question
Summary: jmp_buf is passed by reference
Message-ID: <96@elf115.uu.net>
Date: 25 May 89 22:26:40 GMT
References: <17812@cup.portal.com> <607@kl-cs.UUCP> <749@mccc.UUCP> <13269@haddock.ima.isc.com>
Organization: ELF, Sea Cliff, NY
Lines: 46



In <105998@sun.Eng.Sun.COM>
jamesa@arabian.Sun.COM (James D. Allen) had written:
>                                         But a logical array can be
>promoted to a first-class citizen by just putting it in a structure:
>
>		typedef struct {
>			jmp_buf j;
>		} first_class_jmpbuf;
>
>Any idea why this wasn't done for jmp_buf's?

In <13269@haddock.ima.isc.com>
karl@haddock.ima.isc.com (Karl Heuer) responded:
>In article <105998@sun.Eng.Sun.COM> jamesa@arabian.Sun.COM (James D. Allen) writes:
>>Any idea why this wasn't done for jmp_buf's?
>Originally?  Probably shortsightedness.  In ANSI C?  Backward compatibility.

I believe that struct's were still second class objects when
jmp_buf was first declared.

The declaration of a jmp_buf as an array means that the jmp_buf
is passed by reference instead of by value.  This is essential
to many implementations of setjmp().  It's also a useful trick
to remember if you want user declared data objects to be passed
to your library routines by reference.

In <105998@sun.Eng.Sun.COM>
jamesa@arabian.Sun.COM (James D. Allen) continued:
>I think the "second-classedness" of arrays helps give C its elegant
>syntax.  Any other examples of the "problems" it causes?

I think of C arrays as syntactic sugar for initialized pointers.
Thus

	char foo[] = "I am an anonymous char *";

is an abbreviation for

	register char *const foo = "I am an anonymous char *";

I reason 'const' because the value of the pointer cannot be changed,
and 'register' because the address of the pointer cannot be taken.

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