NetNews Usenet Archive 1992 #20/NN_1992_20.iso/spool/comp/std/c/2599

Internet Message Format | 1992-09-10 | 1.9 KB

Path: sparky!uunet!spool.mu.edu!yale.edu!think.com!barmar
From: [e-mail] (Barry Margolin)
Newsgroups: comp.std.c
Subject: Re: calloc()
Date: 10 Sep 1992 16:41:03 GMT
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 43
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: telecaster.think.com

In article <[e-mail]> [e-mail] (Richard A. O'Keefe) writes:
>Now I wonder:  if I want to allocate an array with elements of known size
>but with number of elements not known until run-time, and if I don't really
>care what the initial contents are, _is_ there something in the standard to
>make
>    p = calloc(NumberOfElements, sizeof *p)
>better than
>    p = malloc(NumberOfElements * sizeof *p) ?

calloc() could compatibly be defined as:

void *calloc (size_t nelem, size_t size) {
    void *result;
    result = malloc(nelem * size);
    if (result)
        memset(result, 0, nelem * size);
    return result;
}

>I thought I knew how C works, but if one is using a C that checks
>subscripts at run-time, what _might_ it do with a dynamic array allocated
>using malloc()?  I'm not asking so much about actual systems as about what
>the standard requires/permits.

An array allocated with malloc() is conceptually an array of N 1-byte
elements.  If calloc() were the primitive, malloc() could be defined as:

void *malloc (size_t size) {
    return calloc(size, 1);
}

(this zeroes the returned memory unnecessarily, but there's nothing that
says that malloc() can't return zeroed memory).

C's rules for automatic conversion between an array and a pointer to its
first element, and its rules for equivalence of array[subscript] and
array+subscript, guarantee that there's no real need to distinguish whether
a block of memory was allocated as an array or not.
--
Barry Margolin
System Manager, Thinking Machines Corp.

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