how to access unaligned address of memory?

nesta · 05-11-2007, 07:23 AM

hi all,
i have a kit with arm processor and i have simulated my actual problem with the following code:

Code:

#include <stdio.h>
#include<stdlib.h>

int main ()
{
	char* temp = malloc(100);
	*temp = 'a';
	temp++;
	*(int*)temp = 13;
	temp +=4;
	return 0;
}

as u can see from the previous code:i want to store an int variable into a memmory address that is starting with odd number i.e. here is the layout of the data stored in my memory:
suppose that temp start address is 0x08 for instance,
then i saved the ascii value of a in this address (0x08) afterwards i increased the temp to be 0x09 and now wanna write an integer value in this odd address (0x09)but this does not happen as i found the processor start saving this value from the even address (0x08)i.e. it overrides the ascii value of a.

i want to force the processor to store the int value from address (0x09)?
how can i do that, note that i m using arm toolchain not gcc
and the previous code is just a simulation to my problem.
thanks in advance.

jim mcnamara · 05-11-2007, 09:28 AM

Do you have the equivalent of memcpy()?

Code:

  int i=13;
.........
  memcpy(temp,&i, sizeof(int));

nesta · 05-12-2007, 11:22 AM

i have manged from accessing the unaligned memory address by using __packed before the pointer as follows:

Code:

	
        int main ()
     {
        char* temp;
	int x = 13;
	temp = malloc(100);
	*temp = 'a';
	temp++;
	*(__packed int*)temp = x;
	temp +=4;
	*temp = 'c';
	return 0;}

so i want to ask 2 questions:
1. does __attribute__ ((packed)) have the same effect of __packed ? i.e.

Code:

packed int* x;

is equivalent to

Code:

int* x __attribute__ ((packed));

or not?

2. when i tried to compile the following simple code :

Code:

int main ()
{
	__packed char* temp;

	return 0;
}

when i tried to compile it using arm-elf-gcc i got the following error:

Code:

main.c: In function `main':
main.c:28: `__packed' undeclared (first use in this function)
main.c:28: (Each undeclared identifier is reported only once
main.c:28: for each function it appears in.)
main.c:28: parse error before "char"
make: *** [main.o] Error 1

while when i tried to compile it using armcc ,
it was compiled suucessfully?

so should i search for the equivalence of __packed in arm-elf-gcc which is the toolchain i m wroking on, or there is something that should i include before using __packed.

thanks in advance