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From: turk@apple.UUCP (Ken "Turk" Turkowski)
Newsgroups: comp.lang.c
Subject: Re: help needed about dynamic space allocation
Message-ID: <332@apple.UUCP>
Date: Wed, 26-Nov-86 16:33:00 EST
Article-I.D.: apple.332
Posted: Wed Nov 26 16:33:00 1986
Date-Received: Wed, 26-Nov-86 22:07:00 EST
References: <13802@amdcad.UUCP> <359@prairie.UUCP> <1046@lifia.UUCP> <715@dg_rtp.UUCP>
Reply-To: turk@apple.UUCP (Ken "Turk" Turkowski)
Organization: Apple Computer Inc., Cupertino, USA
Lines: 38

>> fano@lifia.UUCP (Rampa*)
>> I need advice on dynamic space allocation for multidimensionnal arrays:
>>
>>   float **sig;                          /* matrix declaration     */
>>   sig = (int *)malloc(n * sizeof(int)); /* allocates space for sig */

There have been numerous responses to get the declarations consistent, so I
won't belabor them here.

However, I would like to point out the use of a function that can speed up the
program considerably when you are only allocating temporary variables.  The
function is alloca(), and allocates memory from the stack, rather than from
the heap.  The result is that allocation is simpler, and vanishes automatically
when you exit the routine.

The purpose of alloca() is to compensate for the deficiency in C that does not
allow you to do something like this:

	func(a, n)
	int *a, n;
	{
		int b[n+1];	/* <- C doesn't allow this */
		. . .
	}

by doing this:

	func(a, n)
	int *a, n;
	{
		int *b;
		b = (int *)(alloca((n+1) * sizeof(int)));
		. . .
-- 
Ken Turkowski @ Apple Computer, Inc., Cupertino, CA
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