sizeof operator

[John Z]

So wondering -

if
LVCOLUMNW TableCol;       // Make Column struct

then is it better to use
memset(&TableCol, 0, sizeof(TableCol));   // Reset Column

or
memset(&TableCol, 0, sizeof(LVCOLUMNW));   // Reset Column

or
Both are OK to use w/o distinction

?

John Z

[MrBcx]

ChatGPT says this and I agree.

Both options are technically correct and will have the same effect since sizeof(TableCol) and sizeof(LVCOLUMNW) are equivalent.

However, using sizeof(TableCol) is generally preferred because it directly ties the memset operation to the actual variable you're working with. This approach improves code maintainability. If the type of TableCol ever changes, the code will still work correctly without needing to update the memset call.

So, while both are okay, using sizeof(TableCol) is often considered better practice.

[John Z]

Great - Thanks MrBcx!

I was using TableCol but thought I'd ask to be sure.

Appreciate it!

John Z

[WiiLF23]

I always use this when initializing LVITEMW struct

Code Select Expand


LVITEMW lvItemFileName;
memset(&lvItemFileName, 0, sizeof(LVITEMW));
...


You can use memset() or LVITEMW lvItemFileName = { 0 }. I prefer memset().

[MrBcx]

I always use this when initializing LVITEMW struct

Code Select Expand


LVITEMW lvItemFileName;
memset(&lvItemFileName, 0, sizeof(LVITEMW));
...


You can use memset() or LVITEMW lvItemFileName = { 0 }. I prefer memset().

For simple datatypes (ints, floats, etc), ={0}; works great.

For user-defined types, some compilers may issue warnings, in which case we can explicitly
zero each member using, for example, this approach:     struct MyStruct mstr = { 0, 0, 0.0, 0.0 };

[TimoVJL]

Code Select Expand

={0};optimizer can generate smaller code, so useful in small code 

[WiiLF23]

This is more than sufficient

Code Select Expand


LVITEMW lvi = { 0 };


Unless you are working with a RECT, you dont need to input extra parameters. memset is more than enough.

[HellOfMice]

I have read that using memset is not always generated by the compiler prefer to use RtlZeroMemory


From Pelle's

Code Select Expand

Fills an object with a value. 
Syntax: void __stosb(unsigned char *dst, unsigned char val, size_t num);
void __stosw(unsigned short *dst, unsigned short val, size_t num);
void __stosd(unsigned long *dst, unsigned long val, size_t num);
void __stosq(unsigned long long *dst, unsigned long long val, size_t num);  (X64 only)

Declared in: <intrin.h>

The __stosb, __stosw, __stosd and __stosq function fills the object at dst with num byte, word, double-word, or quad-word values from val using the X86/X64 instruction REP STOSB, REP STOSW, REP STOSD, or REP STOSQ, respectively.

The __stosb, __stosw, __stosd and __stosq functions are only available as intrinsics.

Returns: Nothing
#code
https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtlzeromemory
https://learn.microsoft.com/en-us/windows-hardware/drivers/ddi/wdm/nf-wdm-rtlfillmemory



[Vortex]

Reading \PellesC\Include\win\winnt.h :

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#define RtlZeroMemory(Destination,Length) (void)memset((Destination),0,(Length))

[TimoVJL]

optimizer can also replace memset to rep stosb

Code Select Expand

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

int __cdecl main(void)
{
//char b[80] = {0};
char b[80];
memset(b, 63, 80);
b[5] = 0;
printf("size: %d %s\n", sizeof(b), b);
return 0;
}


[HellOfMice]

Vortex, I know that but what I read was that in the form the macro RtlZeroMemory is created the compiler cannot remove or replace it as it can do with memset. If I find the link I post it. memset with its stosb is not very efficient and could be easily replaced with __stosXX macro family


memset(b,63,80) can be replace __stosq((unsigned long long *) b, (unsigned long long) 0x3f3f3f3f, sizeof(b) / sizeof(__int64)) ; just 10 loops rather than 80the compiler sets rcx to 80 / 8, sets 0x3f3f3f3f to rax and creates rep stosq instruction


[0000000140001CDB] 31C0                         xor               eax,eax
[0000000140001CDD] B907000000              mov               ecx,7
[0000000140001CE2] F348AB                      rep stosq         


__stosq((unsigned long long *) &_Lvc,0,sizeof(LVCOLUMN) / 8

[Vortex]

Hi HellOfMice,

It works as you described, here is a quick example :

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#include <stdio.h>
#include <intrin.h>

int main(void)
{
char p[]={0,1,2,3,4,5,6,7,0,8,9};
__stosq((unsigned long long *)p, 0x4141414141414141,1);
printf("%s\n",p);
return 0;

}

Examining the assembly code :

Code Select Expand


    lea     rdi,[rsp+25H]
    mov     rax,4141414141414141H
    mov     ecx,1               
    rep     stosq


[HellOfMice]

Vortex you are an expert, not me


What we said can be applied to memset, memcpy...