Re: strange behavious

[Mohammed Khalid Ansari <khalid@ncst.ernet.in>]

On Tue, 24 Dec 2002, Stephen Satchell wrote:

> At 10:50 AM 12/24/02 +0530, Mohammed Khalid Ansari wrote:
> > > > #include <stdio.h>
> > > >
> > > > void parse (char **);
> > > > char        *words[] = {"mohammed", "khalid", "ansari"};
> > >
> > > While the array of pointers will be mutable, the strings themselves
> > > aren't; if you attempt to modify them, you will get a segmentation
> > > fault.
> > >
> >
> >Is this in ANSI C?  Will you please elaborate why is it so with this array
> >only and how it is different from the example you have given below because
> >ultimately both examples are same ie array of pointers.
> >
> >As far as I know if there is an string in a code with sufficient memory,
> >it can be overwritten unless it is static.
> 
> Look at the type for string constants.  It is "const array of char".  

I haven't defined them to be const, please see above.

And, 
> based on the number of compilers that assign this type to a string 
> constant, it is ANSI C.  (It was true for the original ANSI specification, 
> I don't know about the latest ones.)
> 
> This is at variance with K&R C, and has been documented.
> 
> Because the string constant has the "const" property, the compiler is free 
> to include it in a read-only space.  In modern computers with worthwhile 
> operating systems, the hardware will then enforce the "const"-ness of the 
> string.
> 
> This is a good thing, because it avoids some of the more astonishing 
> results that happens when you change a string constant.  Older Unix 
> compilers would store separate instances of each constant string; modern 
> compilers will re-use the constant for multiple instances.
> 
> The GCC compiler has switches to modify the behavior such that the old 
> Unix-style mechanisms can be used.  RTFM.
> 
> Or learn how to deal with the new rules -- which is what I had to do, and 
> have found my programming life made a little easier by them.  They make sense.
> 
> Satch
>