NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/2734

Text File | 1996-08-05 | 2.4 KB | 69 lines

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From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: [Q] Porting to ANSI C
Date: Tue, 23 Jan 96 13:22:31 GMT
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In article <[e-mail]> john "John Muller" writes:

>
>Hi,
>
>We are attempting to port existing code to ANSI C.  However, we have
>many instances of something like this:
>
>typedef struct {
>        :
>        int     len;
>        char    data[0];
>}

This looks like an example of what is known as the 'struct hack'. This
idea is to allocate (using malloc etc.) a variable sized structure depending
on the size of the data array. Strictly you can't do this in ANSI C because
every object type must have a length that is known at compile time (there
has been much debate on this in the past on comp.std.c). However even though
it is not strictly ANSI C you can still do something like this very
portably (section 2.6 of the FAQ covers this question). Possibly the best
approach is something like the following:

#include <stddef.h>

#define HACK_MAX_DATA_SIZE   1000     /* Or whatever value is appropriate */
#define HACK_ALLOC_SIZE(datasize)   (offsetof(struct hack, data) + (datasize))

struct hack {
    ...
    int     len;
    char    data[HACK_MAX_DATA_SIZE];
};


Then in the program you can use something like:

   struct hack *hptr = malloc(HACK_ALLOC_SIZE(numchars));

The reason for making the declared size of .data HACK_MAX_DATA_SIZE instead
of, say, 1 is to tell the compiler what range of offsets it has to support
for pointer arithmetic. It would be possible (and legal) for a smart
compiler to notice that an object is 'small' and use some sort of efficient
'small' pointer arithmetic calculation.

Again, this has been deemed not strictly conforming by the language committee
but even so you should have no trouble using it in practice, so long
as you don't ever do anything that relies on sizeof(struct hack) such as
structure assignment or making it part of another struct/union/array.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
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