Trying to generate a bus error

This is a discussion on Trying to generate a bus error within the C Programming forums, part of the General Programming Boards category; Hi. I am trying to generate a bus error using unaligned memory but it's not working. I tried it myself ...

  1. #1
    Registered User Tommo's Avatar
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    Trying to generate a bus error

    Hi. I am trying to generate a bus error using unaligned memory but it's not working. I tried it myself first but couldn't get it to work, so I used the code from the wikipedia entry:

    Code:
    #include <stdlib.h>
    int main (void) {
      int x, *iptr;
    
      char *cptr;
    
      cptr = (char*)malloc(33);
      if (!cptr) return 1;
    
      cptr++;
    
      x = *cptr;
    
      iptr = (int *) cptr;
      x = *iptr;
    
      return 0;
    }
    It seems to run fine. The version of gcc is 4.1.2. It is supposed to generate a bus error so why isn't it?

  2. #2
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    The following links indicate that on Intel boxes, unaligned access normally causes performance degradation but not a bus error.

    http://www.thescripts.com/forum/thread161809.html
    http://www.thescripts.com/forum/thread213140.html

    Edit: Here's a good one:

    http://msdn.microsoft.com/library/de...fx86x86-64.asp
    Last edited by robatino; 08-21-2007 at 05:51 PM.

  3. #3
    Chinese pâté foxman's Avatar
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    This will not always lead to error. Some processor tolerates unaligned acces to memory. And in some case, the compiler will add extra instruction for the memory to be accesed in a proper way (but i don't think it's the case in your example). I did my own code to test on my own computer and there's no errror while executing. I thought the one you showed wasn't really good (not even freeing memory)

    Code:
    #include <stdlib.h>
    #include <stdio.h>
    
    int main()
    {
    	void *ptr;
    	int i;
    
    	ptr = malloc(2 * sizeof(int) - 1);
    	if (malloc != NULL)
    	{
    		for (i = 0; i < sizeof(int); i++)
    		{
    			* (int *) ((int) ptr + i);
    		}
    
    		free(ptr);
    
    		printf("Looks like no memory alignement error happened\n");
    	}
    
    	else
    	{
    		printf("Error while allocating memory\n");
    	}
    
    	getchar();
    
    	return 0;
    }
    Last edited by foxman; 08-22-2007 at 09:40 AM.

  4. #4
    Captain Crash brewbuck's Avatar
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    An x86 can handle unaligned access. It's just slow.

    Try this on a SparcStation instead.

  5. #5
    Registered User Tommo's Avatar
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    OK, thanks for the help guys. FWIW, it wasn't my code. I got it from the wikipedia entry. You should maybe change it if you don't think it's a suitable example to show everyone.

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