Realloc() problem with Efence

Realloc() problem with Efence

[Alain Birtz]

What is wrong with this code ?
Code is executed correctly without Electric Fence
But linked with Efence library, dbg complaint:

   Electric Fence 2.0.5 Copyright (C) 1987-1998 Bruce Perens.
   ...
   ...
   Program received signal SIGSEGV, Segmentation fault.

when trying to write the first B, just after the first memory allocation
buffer
(at the instruction (*c_ptr++ = 'B'))


void tst ()
{
 int i;
 char *c_buf, *c_ptr, *temp_c_buf;

 c_buf = (char *)malloc(5000);
 if (c_buf != NULL)
  {
  c_ptr = c_buf;
  for (i = 0; i < 5000; i++)
   *c_ptr++ = 'A';
  temp_c_buf = c_buf;
  realloc(temp_c_buf, 5000 + 5000);
  if (temp_c_buf != NULL)
   {
   for (i = 0; i < 5000; i++)
    *c_ptr++ = 'B';
   }
  }
}




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Re: Realloc() problem with Efence

[Nicholas T Ingolia]

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Hello...

realloc() may move the memory being realloated.  To quote from the man page,

       realloc() returns a pointer to the newly allocated memory,
       which is suitably aligned for any kind of variable and may
       be  different from ptr...

It doesn't (and couldn't) mutate the pointer temp_c_buf directly.

Thus, you probably want to re-write your code to do

temp_c_buf = realloc(temp_c_buf, 5000 + 5000);

- --Nicholas Ingolia
ingolia@mit.edu
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Realloc() problem with Efence

[Jerry Quinn]

Alain Birtz <abz@videotron.ca> writes:
Alain> What is wrong with this code ?
Alain> Code is executed correctly without Electric Fence
Alain> But linked with Efence library, dbg complaint:
Alain> 
Alain> 	  Electric Fence 2.0.5 Copyright (C) 1987-1998 Bruce Perens.
Alain> 	  ...
Alain> 	  ...
Alain> 	  Program received signal SIGSEGV, Segmentation fault.
Alain> 
Alain> when trying to write the first B, just after the first memory allocation
Alain> buffer
Alain> (at the instruction (*c_ptr++ = 'B'))
Alain> 
Alain> 
Alain> void tst ()
Alain> {
Alain> 	int i;
Alain> 	char *c_buf, *c_ptr, *temp_c_buf;
Alain> 
Alain> 	c_buf = (char *)malloc(5000);
Alain> 	if (c_buf != NULL)
Alain> 	 {
Alain> 	 c_ptr = c_buf;
Alain> 	 for (i = 0; i < 5000; i++)
Alain> 	  *c_ptr++ = 'A';
Alain> 	 temp_c_buf = c_buf;
Alain> 	 realloc(temp_c_buf, 5000 + 5000);
Alain> 	 if (temp_c_buf != NULL)
Alain> 	  {
Alain> 	  for (i = 0; i < 5000; i++)
Alain> 	   *c_ptr++ = 'B';
Alain> 	  }
Alain> 	 }
Alain> }
Alain> 

realloc can (and did) move the block.  Therefore, when you start writing 'B's, 
they are going into invalid memory.  The fact that it works without Electric
Fence is luck.

To do this, you need to set the pointer to the whole block to the output of
realloc and then you can start writing B's at the new location + 5000.

-- 
Jerry Quinn                             Tel: (514) 761-8737
jquinn@nortelnetworks.com               Fax: (514) 761-8505
Speech Recognition Research


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