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From: cc100aa@prism.gatech.EDU (Ray Spalding)
Newsgroups: comp.lang.c
Subject: Mixing types in arrays
Message-ID: <22040@hydra.gatech.EDU>
Date: 14 Feb 91 19:54:59 GMT
Sender: cc100aa@prism.gatech.EDU
Organization: Georgia Institute of Technology
Lines: 70

I keep finding myself wanting to store shorts and longs (as well as
chars) in arrays of char, such as in the following simple example.

This works on all the systems I've tried, but is it "guaranteed"?  I
guess I'm worried that a pointer to char cast into a pointer to long
might somehow point to the wrong place.

I know (a) converting the value to char (e.g., ASCII) would be
guaranteed, but, I want to minimize storage space (and conversion
overhead); (b) the mixed array, if written to a file and read by
another system, would not necessarily be intelligible; and (c) the
example is not optimized (for purposes of clarity and focusing on the
issue at hand rather than other details).

But, are there other pitfalls?  Systems on which this won't work?  A
better way?

char *strcpy();

int
savevalue(key,value,bufptr)
	char *key;
	long value;
	char *bufptr;
{
	*(long *)bufptr = value;
	bufptr += sizeof value;
	(void) strcpy(bufptr,key);
	return sizeof value + strlen(key) + 1;
}

long
getvalue(key,bufptr,bufend)
	char *key;
	char *bufptr;
	char *bufend;
{
	while (bufptr < bufend) {
		if (strcmp(key,bufptr+sizeof(long)) == 0) {
			return *(long*)bufptr;
		}
		bufptr += sizeof(long);
		bufptr += strlen(bufptr) + 1;
	}
	printf("%s not found.\n",key);
	return -1;
}

main()
{
	char buffer[1000];
	char *bufptr = buffer;
	long value;

	bufptr += savevalue("First item",1234567L,bufptr);
	bufptr += savevalue("Second item",987654321L,bufptr);
	bufptr += savevalue("Third item",33333333L,bufptr);
	value = getvalue("Second item",buffer,bufptr);
	if (value == 987654321L) {
		printf("It worked.\n");
	} else {
		printf("It didn't work.\n");
	}
	return 0;
}
-- 
Ray Spalding, Technical Services, Office of Information Technology
Georgia Institute of Technology, Atlanta Georgia, 30332-0715
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