Re: Oops... strange (to me) segfault problem

[Derek Martin <code pizzashack org>]

On Wed, Aug 04, 2004 at 02:23:27AM -0400, John Vetterli wrote:
> It looks like you're corrupting the free store.

A bonehead mistake... Sigh.  I was concerned about that, but I figured
that if the read returned less than BLOCK_SZ (the size of the pipe
buffer on Linux, IIRC), then the pipe must be empty, i.e. there's no
more input to read.

> I suspect the first call to read() reads less than BLOCK_SZ bytes, in
> which case offset gets incremented but the memory block doesn't get
> resized, so the next call to read() overwrites the data after the end of
> the memory block.  

You're probably right, though I'll want to do some testing to check it
out.

> Perhaps if you removed the "if ( status == BLOCK_SZ )" and resized
> the memory block unconditionally ???

I think this change would be better:

>   while ( (status = read(pd[0], (buf + offset), BLOCK_SZ)) ){
>     if ( status == -1 ){
>       fprintf(stderr, "get_keys(): read from pipe failed\n");
>       free(buf);
>       return NULL;
>     }
>
>     else if ( status == BLOCK_SZ ){
>       if ( !(buf = (char *)realloc(buf, offset + BLOCK_SZ)) ){
>         fprintf(stderr, "realloc() failed");
>         free(buf);
>         return NULL;
>       }
>     }

      else if ( !(buf = (char *)realloc(buf, offset + status)) ){
        /* handle error */
      }
        
>     offset += status;
>   }

I believe that will solve it (leaving BLOCK_SZ bytes available in the
region), and reduce memory wastage.

-- 
Derek D. Martin
http://www.pizzashack.org/
GPG Key ID: 0x81CFE75D