Re: [Haifux] Re: c with a spoon and pointer arithmetic

[Yoni]

On the same subject, can someone please explain a bit about the free() 
function ? what does it do, and how come you can only send it the memory 
address which you started allocating from ?
(I missed the last lecture because i had to study to some test :-( )
Yoni.
----- Original Message ----- 
From: "Shlomi Fish" <[EMAIL PROTECTED]>
To: <haifux@haifux.org>
Sent: Thursday, March 10, 2005 10:59 AM
Subject: [Haifux] Re: c with a spoon and pointer arithmetic


On Wednesday 09 March 2005 20:57, Diego Iastrubni wrote:
See notes inside.
Tzahi Fadida wrote:
>As we talked about in the c with a spoon lecture, I tried the
>pointer arithmetic and then free and at least for me it didn't
>work.
>gcc version egcs-2.91.66 19990314/Linux (egcs-1.1.2 release)
>
>[EMAIL PROTECTED]:~]$ gcc nis2.c
"make nis2 " quite easier no?
What difference does it make for a single-file program? What is 
disturbing,
though is that he's working as root.

>[EMAIL PROTECTED]:~]$ ./a.out
>char y[0] a
>char y[1] b
>char y[2] c
>char y[3] d
>Segmentation fault (core dumped)
of course... you are messing with wrong memory locations.
>#include <stdio.h>
>#include <stdlib.h>
>main(){
>      char *y = (char *)malloc(4*sizeof(char));
lets assume y is 100 (the allocated memory is found at "100", at least
in this run)
>      char *x = y + 2;
x = 100 + 2;
>      y[0]='a';
mem[102] = a <- ok
No. That's mem[100] = 'a'. y == 100, and is the pointer to the beginning 
of
the allocated memory.

>      y[1]='b';
mem[103] = b <- ok
mem[101] = 'b'.
>      y[2]='c';
mem[104] = c <- ok
mem[102] = 'c'.
SEGFAULT! the allocated memory is found at memory 100-103. Bad Thazi!
No, it's OK so far.
In real life Linux does 'allow' the execution of this program, as it
printed lines after this point. Can anyone explain?
>      y[3]='d';
we know whats wrong here...
This is still OK.
>      printf("char y[0] %c\n", y[0]);
>      printf("char y[1] %c\n", y[1]);
>      printf("char y[2] %c\n", y[2]);
>      printf("char y[3] %c\n", y[3]);
>
>      free(x);
free( 102 ) .... wait... 102 is not allocated! 100 was allocated!
IMHO this is the line that really segfaults.
Probably.
Regards,
Shlomi Fish
---------------------------------------------------------------------
Shlomi Fish      [EMAIL PROTECTED]
Homepage:        http://www.shlomifish.org/
Knuth is not God! It took him two days to build the Roman Empire.
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