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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.lang.c
Subject: Re: Whaddaya think?
Date: Mon, 17 Jun 2024 07:48:39 +0200
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On 17.06.2024 02:06, Keith Thompson wrote:
> Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
> [...]
>> #include <stdlib.h>
>> #include <stdio.h>
>>
>> void main (int argc, char * argv[])
>> {
>>     int chunk = 10;
>>     int bufsize = chunk+1;
>>     char * buf = malloc(bufsize);
>>     char * anchor = buf;
>>     while (fgets(buf, chunk+1, stdin) != NULL)
>>         if (realloc(anchor, bufsize += chunk) != NULL)
>>             buf += chunk;
>>     puts(anchor);
>> }
> 
> realloc() can return the pointer you pass to it if there's enough room
> in the existing location.  (Or it can relocate the buffer even if there
> is enough room.)
> 
> But if realloc() moves the buffer (copying the existing data to it), it
> returns a pointer to the new location and invalidates the old one.  You
> discard the new pointer, only comparing it to NULL.
> 
> Perhaps you assumed that realloc() always expands the buffer in place.
> It doesn't.

No, I didn't assume that. I just missed that 'anchor' will get lost.
Thanks!

> 
> If the above program worked for you, I suspect that either realloc()
> never relocated the buffer, or you continued using the original buffer
> (and beyond) after realloc() invalidated it. [...]

Yes, that was certainly the case. (I did no thorough test with large
data sets, just a simple ad hoc test.)


Elsethread I suggested to merge the malloc() with the realloc() call.
The resulting code would be simpler (and might address that problem).

    int chunk = 10;
    int bufsize = 1;
    char *  anchor = NULL;
    while ((anchor = realloc (anchor, bufsize += chunk)) != NULL  &&
            fgets (anchor+bufsize-chunk-1, chunk+1, stdin) != NULL)
        ;
    puts (anchor);


Do you see the exposed problem (or any other issues) here, too?

Janis