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using memset to erase buffer within a structure

 
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Liamthesnake



Joined: 10 Sep 2010
Posts: 1

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using memset to erase buffer within a structure
PostPosted: Fri Sep 10, 2010 10:10 am     Reply with quote
Hi all

I'm erasing a buffer using memset. If this was a normal buffer it would be

Code:
memset(buffer,0,sizeof(buffer));


and to erase a structure it would be

Code:
memset(&structure_test,0,sizeof(structure_test));


but what happens if it's a buffer within a structure? do you use the address symbol or not?

Code:
memset(&structure_test.buffer,0,sizeof(structure_test.buffer));


Thanks in advance,
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Fri Sep 10, 2010 12:55 pm     Reply with quote
According to the .LST file, the compiler generates the same address for
either method (with vs. 4.112).
Code:

.... addr = structure_test.buffer;
0022:  MOVLW  02   // Load addr with 0x0283
0024:  MOVWF  06
0026:  MOVLW  83
0028:  MOVWF  05
.... 
.... addr = &structure_test.buffer;
002A:  MOVLW  02  // Load addr with 0x0283
002C:  MOVWF  06
002E:  MOVLW  83
0030:  MOVWF  05

// Symbol table
000     @SCRATCH
001     @SCRATCH
001     _RETURN_
002     @SCRATCH
003     @SCRATCH
004     rs232_errors
005-006 main.addr         // 'addr' is at 0x0005
280-28C structure_test    // Structure is at 0x280
F83     PSP_DATA
FBB     CCP_2_LOW

Test program:
Code:

#include <18F452.h>
#fuses XT,NOWDT,PUT,BROWNOUT,NOLVP
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

struct
{
int8 a;
int16 b;
int8 buffer[10];
}structure_test;

#locate structure_test = 0x280

//====================================
void main()
{
int16 addr;

addr = structure_test.buffer;

addr = &structure_test.buffer;

while(1);
}


The same results occur for a C program compiled in MSVC++ 6.0:
Quote:

structure_test.buffer = 004237D4

&structure_test.buffer = 004237D4


Test program for MSVC++ 6.0 (C program, not C++):
Code:

#include <stdio.h>

struct
{
unsigned char a;
unsigned short b;
unsigned char buffer[10];
}structure_test;

//============================
void main(void)
{
printf("structure_test.buffer = %p \n", structure_test.buffer);

printf("&structure_test.buffer = %p \n", &structure_test.buffer);
}
Wayne_



Joined: 10 Oct 2007
Posts: 681

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PostPosted: Mon Sep 13, 2010 1:47 am     Reply with quote
It also depends on the buffer, if it is a defined array then the compiler knows the size of the overall structure and should clear all memory, if it is a dynamic buffer (pointer) then you would need to free this as clearing the structure will nullify the pointer, you would then re-allocate a new buffer to your pointer.
Ttelmah



Joined: 11 Mar 2010
Posts: 19552

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PostPosted: Mon Sep 13, 2010 3:32 am     Reply with quote
But of course, far more efficient, to use pointers, and reset these, leaving the contents uncleared. Saved a huge amount of processing to do it this way.....

Best Wishes
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