RE: malloc and free

RE: malloc and free

[Alvarez Alberto-AALVARB1]

Hi,
	Why do you 'malloc' for token? strtok returns a pointer to an already allocated string (or a null pointer), but doesn't do a copy. So, when assigning strtok to 'token', you loose the value you had allocated.

	So, the free()'ing doesn't free your former allocation, but the piece of string returned by strtok.

Regards,

Alberto.


> -----Original Message-----
> From: Sameer Maggon [mailto:smaggon@dypatil.edu]
> Sent: lunes, 20 de mayo de 2002 13:27
> To: linux-c-programming@vger.kernel.org
> Subject: malloc and free
> 
> 
> Hi,
>   In one function i have a definiton like
> 
>   char *token;
>   token = (char *)malloc(256 * sizeof(char));
> 
>   ...
>   ...
>   token = strtok(...,"\0");
>  
>   ...
>   free(token);
> 
> 
>   Now the problem is that sometimes this runs
>   fine and sometimes it crashes on free()..
>   Why so.
>   If I remove the free(..), then the program
>   works perfectly fine
> 
>   How do i correct the problem.
>   
>   Regards
>   Sameer
> 
> 
> 
> 
> 
> --------------
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Re: malloc and free

[wwp]

Hi Sameer,


On Mon, 20 May 2002 16:56:30 +0530 (IST) Sameer Maggon <smaggon@dypatil.edu> wrote:

[snip]
>   char *token;
>   token = (char *)malloc(256 * sizeof(char));
> 
>   ...
>   ...
>   token = strtok(...,"\0");
[snip]

The problem is there, you change "token"'s value, so you
lost the value formerly returned by malloc (this one has
to be free()'ed) because strtok returns a pointer to the next
token IIRC. Then, the "token" you attempt to free is not
the good one.

For instance, if I didn't miss anything:
	char *token, *buffer;
	buffer = (char *)malloc(256 * sizeof(char));
	token = buffer;
	token = strtok(...,"\0");
	..
	free(buffer);


Regards,

-- 
wwp

malloc and free

[Sameer Maggon]

Hi,
  In one function i have a definiton like

  char *token;
  token = (char *)malloc(256 * sizeof(char));

  ...
  ...
  token = strtok(...,"\0");
 
  ...
  free(token);


  Now the problem is that sometimes this runs
  fine and sometimes it crashes on free()..
  Why so.
  If I remove the free(..), then the program
  works perfectly fine

  How do i correct the problem.
  
  Regards
  Sameer





--------------
http://www.dypatil.edu For Better tomorrow

Re: malloc and free

[Andrew Edmondson]

On Mon, 20 May 2002, Sameer Maggon wrote:
> Hi,
>   In one function i have a definiton like
>
>   char *token;
>   token = (char *)malloc(256 * sizeof(char));

No real need to cast here and in fact it could mask a missing 
#include <stdlib.h>. 

man malloc 
---< snip
RETURN VALUES
       For calloc() and malloc(), the value returned is a pointer
       to the allocated memory, which is suitably aligned for any
       kind of variable, or NULL if the request fails.
---> snip


Also, char is always size 1 so there is no need for the sizeof(). If you did 
want to use sizeof() use it with the object not the type so that if you want 
to change the type of the object you don't need to also change the malloc:

token = malloc(256 * sizeof(*token));

Also, I assume you are checking the return value of the malloc is not NULL at 
some point.

if (  ( token = malloc(256) ) == NULL  )
{
	puts("Malloc: Memory allocation failed.");
	exit(1);
}


>
>   ...
>   ...
>   token = strtok(...,"\0");

Here's the problem, you are assigning another value to token and have lost 
track of your malloc'd memory.

>
>   ...
>   free(token);

Free may now be freeing the wrong memory. 


-- 
Andrew Edmondson
Test Development Engineer
<-------------------------->
The discerning heart seeks knowledge, 
but the mouth of a fool feeds on folly.

malloc and free

[Ankit Jain]

hi

routine xyz uses malloc and free functions. it gives
accurate and correct result if called once. 

but if the function is called in a loop N number of
times then probably it gives segmentation fault.

what is the reason?  can any body guess or test code
is needed?

thanks

ankit jain

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Re: malloc and free

[Eric Bambach]

On Friday 24 December 2004 12:39 pm, you wrote:
> hi
>
> routine xyz uses malloc and free functions. it gives
> accurate and correct result if called once.
>
> but if the function is called in a loop N number of
> times then probably it gives segmentation fault.
>
> what is the reason?  can any body guess or test code
> is needed?
>
> thanks
>
> ankit jain

A common cause of failure in the malloc/free routines is memory corruption. 
The trick about memory corruption and buffer overflows is that they work 
sometimes/most of the time. Perhaps you are doing out-of-bounds reading or 
(gak!) writing with the memory. Try running it under valgrind to see if this 
is the case.

Also, although routine xyz may not corrupt the memory perhaps something else 
in your program is? Writing in strange places confuses libc and can result in 
a segfault in places that can be quite far away from the original spot of 
corruption.

----------------------------------------
--EB

> All is fine except that I can reliably "oops" it simply by trying to read
> from /proc/apm (e.g. cat /proc/apm).
> oops output and ksymoops-2.3.4 output is attached.
> Is there anything else I can contribute?

The latitude and longtitude of the bios writers current position, and
a ballistic missile.

                --Alan Cox LKML-December 08,2000 

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