Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!mimsy!oddjob!gargoyle!ihnp4!chinet!john
From: john@chinet.UUCP (John Mundt)
Newsgroups: comp.lang.c
Subject: Re: Allocation for pointers
Message-ID: <1486@chinet.UUCP>
Date: Sat, 5-Sep-87 17:50:58 EDT
Article-I.D.: chinet.1486
Posted: Sat Sep  5 17:50:58 1987
Date-Received: Sun, 6-Sep-87 09:08:04 EDT
References: <1357@faline.bellcore.com> <27282@sun.uucp>
Reply-To: john@chinet.UUCP (John Mundt)
Organization: Chinet - Public Access Unix
Lines: 56

In article <27282@sun.uucp> guy%gorodish@Sun.COM (Guy Harris) writes:
>> main()
>> {	char *a;
>> 	a = "text";
>> 	recurs(a);
>> }
>> 
>> void recurs(a);
>> char *a;
>> {	strcat(a,"more text");
>> 	if (some conditional test) recurs(a);
>> }
>> 
>> As I understand it, I'm passing the address of the variable a into
>> the routine recurs.

When the program was compiled, "text" was placed into the static area
of the object code along with other static variables.  a was then 
assigned to point to the beginning of the string.

When you use strcat to append more text, you will merrily rub out
whatever static data was kept beyond the 5 bytes assigned to store
"text\0".  Strange and wonderful things will happen when the static
area is overrun and you get out into other sections of the program.

The only safe way to do what you want is to either pre-assign a with
enough memory to hold whatever you'll put into it, as in 

char a[BUFSIZ];

or to get the size of the present a, the size of the piece you want
to append + 1 byte for the \0, and call malloc() to hold the new
string.  You should probably free the old string.

in recurs(a) 
char *a;
{
...
int len;
char *ptr, malloc();
	len = strlen(a)	
	len += strlen(string_you_want_to_add);
	ptr = malloc(len + 1);
	strcpy(ptr, a);
	strcat(ptr, string_you_want_to_add);
	free(a)
	return(ptr);
...
}

Up in main, you have to make a = recurs(a) each time you call it.
You should add error checking to see that malloc returns a valid
pointer too.

John Mundt                    ...ihnp4!chinet!john
Teachers' Aide, Inc.	      ...ihnp4!chinet!teachad!fred