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Liamthesnake
Joined: 10 Sep 2010 Posts: 1
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using memset to erase buffer within a structure |
Posted: Fri Sep 10, 2010 10:10 am |
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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, |
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Fri Sep 10, 2010 12:55 pm |
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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
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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);
}
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The same results occur for a C program compiled in MSVC++ 6.0:
Quote: |
structure_test.buffer = 004237D4
&structure_test.buffer = 004237D4
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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);
} |
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Wayne_
Joined: 10 Oct 2007 Posts: 681
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Posted: Mon Sep 13, 2010 1:47 am |
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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. |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19369
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Posted: Mon Sep 13, 2010 3:32 am |
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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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