my pointers are not getting changed when passing into a function...

[C++ Newbie]

I have the following function that takes in 2 pointers, does copies and then returns. The problem I have is that when the function returns I want the pointers to have the modified values but they retain the original values. Can someone tell me why & how to fix it?



unsigned char *p_source_buf;// {I set this to a memory buffer which read in a file};
unsigned char *pInBuf = new char[200];
unsigned int num_bytes_to_insert;

void main(void){
byte_copy(m_pInBuf, p_source_buf, num_bytes_to_insert);
}


const unsigned int BITMASK_FOR_LEAST_SIGNIFICANT_TWO_BITS = 0x3;
inline void byte_copy(unsigned char *p_in_buf, unsigned char *p_source_buf, unsigned int num_bytes_to_insert){
//Transfer remaining bytes individually
unsigned int remaining_bytes = static_cast<unsigned int>(num_bytes_to_insert & BITMASK_FOR_SIGNIFICANT_TWO_BITS);
for(unsigned int i=0;i<remaining_bytes;i++)
*p_in_buf++ = *p_source_buf++;
}

[Rigel]

What values are you passing into the function as parameters?

[golanshahar]

the code you posted looks fine, meaning the pointers in the function should changed, unless the remaining_bytes is qual to 0 and the loop is never performed ;)


if i helped dont forget to rate :-)
Cheers

[C++ Newbie]

I showed how i called the function.

[golanshahar]

you showed the function ineed but you didnt show the paramters you passing to it if num_bytes_to_insert == 0 then the loop never get perform
casue the next line

unsigned int remaining_bytes = static_cast<unsigned int>(num_bytes_to_insert & BITMASK_FOR_SIGNIFICANT_TWO_BITS);

will be zero!


unsigned int num_bytes_to_insert;


this paramter is never initialzied and its zero!
so like i said the loop never performed hence pointers not moving.

Cheers

[C++ Newbie]

Oh I see what you are saying. That is not the problem. I had to cut and paste this code from a larger framework so what I showed you is essentially what is going on. I stepped thru the debugger and it is passing between 2 & 300 bytes at a time. The pointers look like they increment properly within the routine but when the routine returns, the data in the buffers has changed but not the pointers themselves.

[D_Drmmr]

I think this is because the pointers are passed to the function by value. That is, when p_in_buf is incremented, m_pInBuf is not, because they are not the same pointers.

It's the same as this:

void myfunc(int a, int b)
{
a = b;
b++;
}

void main() {
int x = 0, y = 1;
myfunc(x, y);
cout << x << ", " << y << endl;
}


The output will be: "0, 1"
This example can be rewritten as:

typedef int something;

void myfunc(something a, something b)
{
a = b;
b++;
}

void main() {
something x = 0, y = 1;
myfunc(x, y);
cout << x << ", " << y << endl;
}


Now consider what happens when you replace the first line with

typedef unsigned char* something;


Do you understand?

[qynum123]

what you really want to do? I debug the code it seems no problem.Do you want to change the value of the buffer ? or you want do other ?
here is my code:

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

unsigned char p_source_buf[] = "This is a test";// {I set this to a memory buffer which read in a file};
unsigned char *pInBuf = new unsigned char[200];
unsigned int num_bytes_to_insert = 15;

inline void byte_copy(unsigned char *p_in_buf, unsigned char *p_source_buf, unsigned int num_bytes_to_insert){
//Transfer remaining bytes individually
//unsigned int remaining_bytes = static_cast<unsigned int>(num_bytes_to_insert & BITMASK_FOR_SIGNIFICANT_TWO_BITS);
for(unsigned int i=0;i<num_bytes_to_insert;i++)
*p_in_buf++ = *p_source_buf++;
}
void main(void){
byte_copy(pInBuf, p_source_buf, num_bytes_to_insert);
printf("%s\n",pInBuf);
}

Do we misunderstand you ?

[NMTop40]

Firstly use code tags.

Secondly, main returns int not void.

Thirdly, you are only copying up to 3 bytes. I think though you wanted to copy a 4-byte word at a time until you reached the last part of the buffer at which point you are copying the remaining bytes. However I don't see the part where you are transferring 4 bytes at a time.

Fourthly, you are not modifying soruce_buf so that one should be const unsigned char *. But const unsigned void* would be even better.

If you are going to do it that way for performance reasons then you'd want to put the boundary at 4 bytes, not necessarily the start of the buffer. And then which 4-byte boundary, the source or the target?

Are you sure this is going to be faster than memcpy? (Probably not, and probably therefore not worth the effort).

[HighCommander4]

?? I never new there was such thing as "unsigned void*" ...