Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!decwrl!labrea!csli!gandalf
From: gandalf@csli.STANFORD.EDU (Juergen Wagner)
Newsgroups: comp.lang.c
Subject: Re: modification of strings
Keywords: string, constants, pointers
Message-ID: <7429@csli.STANFORD.EDU>
Date: 5 Feb 89 00:54:07 GMT
References: <345@lakesys.UUCP>
Reply-To: gandalf@csli.stanford.edu (Juergen Wagner)
Organization: Xerox Palo Alto Research Center, Palo Alto, CA
Lines: 55

In article <345@lakesys.UUCP> chad@lakesys.UUCP (D. Chadwick Gibbons) writes:
>...
>				Insofar as I have been told, strings can not
>be modified - note, the type char *blah = "this is a string"; not the everyday
>normal strings we use.  This would appear that if you attempted to modify
>their contents, you would either get a core dump of some various flavor, or
>the program would ignore your request.

Actually, the effect you get depends on the system you're trying this on. If
your machine puts the string into text space, together with your code (HP-UX
does that), you loose when you try to change the string. Other systems usually
have a loader option to specify this behavior.

For the sake of portability, assume constant strings are read-only.

>...[TFM quote deleted]
>
>                char *blah = "meow";
>                char *tmp;
>
>                tmp = strcpy(blah, "grr, snarl, hiss");

Ok. New let's see what this piece of code does. 'blah' is a pointer to char.
It is initialized to point at the first character of the char vector
	< 'm', 'e', 'o', 'w', '\0' >
which occupies five bytes. 'tmp' is just another pointer to char but
uninitialized.

The strcpy statement copies a string of length (humph!) 16 + 1 (for the 
zero byte) into consecutive byte locations from the point 'blah' points
to on.

Hmmm.... your compiler allocated five bytes for the string but you are now
using 17 for the new string. Strcpy will just overwrite whatever follows the
string. If that happens to be another statically allocated string, it will
show changed contents. If that happens to be some data space, variables seem
to change values. If that happens to be just beyond the allocated memory page,
you get some kind of error (segmentation fault et al.). If your compiler
happily put the string in the midst of code, you will either overwrite code
or get some error like segmentation fault (text space is Read-Only). If the
string was allocated on the stack, your return address might be f***ed up.
If....

As you can see, there is a vast number of alternatives, and you tried some of
them.

As a rule of thumb, I suggest to check calls to destructive functions like
strcat, strcpy, et al. very carefully. Sometimes, they cause errors by over-
writing pieces of memory used in completely different portions of your program,
and the stuff becomes hard to debug. Allocate all the memory you need, and
don't try to overwrite static strings.

-- 
Juergen Wagner		   			gandalf@csli.stanford.edu
						 wagner@arisia.xerox.com