Re: Into the Void

["Amit Dang" <amit_dang@intersolutions.stpn.soft.net>]

Hi George, I was just going through the code. In second code, line
((char*)memcpy((void*)*in_buf, out_buf, 4))[4] = '\0';
why does (void *)*in_buf returns NULL. I tried typecasting different type of
pointers and its always NULL.

Regards,
Amit Dang

----- Original Message ----- 
From: "Huber, George K RDECOM CERDEC STCD SRI" <George.K.Huber@us.army.mil>
To: "'Scott'" <drmemory@3rivers.net>; <linux-c-programming@vger.kernel.org>
Sent: Thursday, January 27, 2005 11:28 PM
Subject: RE: Into the Void


> Scott wrote:
>
> >Thanks for your help; I actually managed to get the program to compile
> >with no errors, but now am getting a segmentation fault which I can't
> >track down, so I'll probably be back to plague you folks some more!
>
> Even without seeing your code, I probably can guess what is happening.
> Consider the following sample program (with no error checking)
>
> ======================================================================
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> #define BUF_LEN 6
>
> int main()
> {
>     char*   in_buf = malloc(BUF_LEN*sizeof(char));
>     char*   out_buf = malloc(BUF_LEN*sizeof(char));
>
>     strcpy(out_buf, "Test");
>
>     ((char*)memcpy((void*)in_buf, out_buf, 4))[4] = '\0';
>
>     printf("%s\n", in_buf);
>
>     return 0;
> }
> ======================================================================
>
> The above program compiles cleanly and produces the expected output.
>
> However, if the above program is changed to this:
>
> ======================================================================
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
>
> #define BUF_LEN 6
>
> int main()
> {
>     char*   in_buf = malloc(BUF_LEN*sizeof(char));
>     char*   out_buf = malloc(BUF_LEN*sizeof(char));
>
>     strcpy(out_buf, "Test");
>
>     ((char*)memcpy((void*)*in_buf, out_buf, 4))[4] = '\0';
>
>     printf("%s\n", in_buf);
>
>     return 0;
> }
> ======================================================================
>
> we get a single warning (warning: cast to pointer from integer of
> different size) and the program segfaults when it is run.
>
> Make sure you are not dereferencing the the first parametere to memcpy.
>
> Also, if the above does not help, try compiling with debugging support
> turned on (i.e. gcc -g ...) and then run the program under gdb.  Using the
> gdb `where' command will show you what line you are seg-faulting on.
>
> Finally, I find it useful to alway compile using the flags
"-ansi -pedantic
> -Wall" (or "-std=c99 -pedantic -Wall) and treat any warning as errors.  I
> know it can be a pain-in-the-arse, but I find that it does minimize
run-time
> errors.
>
> George
> -
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