Re: string initilization WHY?

From: Keith Thompson (kst-u_at_mib.org)
Date: 01/30/05 

Date: Sun, 30 Jan 2005 08:33:58 GMT

"puzzlecracker" <ironsel2000@gmail.com> writes:
> Why would this work? - and it does! any thoughts?
>
>
>
> #include<stdio.h>
>
> void init(char **str){ *str="Awesome"; }
>
> main(){
>
>
> char * str;
>
> printf("address: %d\n", str);
> init(&str);
> printf("address: %d\n", str);
> printf("string:\'%s\' has %d characters \n", str,strlen(str));
> }

It "works" because behaving as you expect is one of the infinitely
many possible consequences of undefined behavior.

Here's a corrected version of your program that (almost) avoids the
undefined behavior:

#include <stdio.h>
#include <string.h>

void init(char **str){ *str="Awesome"; }

int main(void)
{
    char *str;
    printf("address: %p\n", (void*)str);
    init(&str);
    printf("address: %p\n", (void*)str);
    printf("string:\'%s\' has %d characters \n", str, (int)strlen(str));
    return 0;
}

I say "almost" because it still uses the value of an unitialized (and
therefore indeterminate) variable in the first printf() call.

You could probably get away without the void* casts, since void* and
char* are effectively compatible as parameters, but IMHO it's good
style to use it anyway. (I've argued before that all casts should be
considered suspicious. Arguments to printf-like functions are a rare
case where casts are often required, because there is no implicit
conversion to the proper type.)

But as you said elsewhere in this thread, none of the problems I
corrected are directly relevant to what you're asking about.

The variable str is a pointer to char; effectively it points to a
string. The init() call changes the value of str, so it points to a
string whose value is "Awesome" (with a trailing '\0', of course).
(The string itself isn't copied.) This string is statically
allocated, so there's no problem referring to it after leaving the
init() function. 

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center             <*>  <http://users.sdsc.edu/~kst>
We must do something.  This is something.  Therefore, we must do this.

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