LinuxQuestions.org - [SOLVED] Value gets changed when passed to a function in C++

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Aquarius_Girl

06-10-2010 02:16 AM

Value gets changed when passed to a function in C++

Why do I see different values of character 'a', in main() and in functionA() in the following C++ code?

Code:

#include <stdio.h>

#include <string.h>

#include <iostream>



using namespace std;



void functionA (unsigned char *data)

{

    printf ("\ndata in functionA: %d\n", data[0]);

}



int main ()

{

    unsigned char *byte = new unsigned char [1];

    char          a    = 'a';

    

    memcpy (byte, &a, 1);

    printf ("\ndata in main: %d\n", byte[0]);

    

    functionA ((unsigned char*)&byte);

}

The output I am receiving is as follows:

Code:

anisha@linux-uitj:~> g++ qqqqq.cpp -Wall -Wextra

anisha@linux-uitj:~> ./a.out



data in main: 97



data in functionA: 16

Guttorm

06-10-2010 02:54 AM

Hi

You define "byte" like as a pointer to unsigned char. The paramter you send to functionA is "(unsigned char*)&byte" which means the address of the pointer typecasted to an unsigned char pointer. I guess you intend to pass the pointer to the function, not a pointer to the pointer.

rkski

06-10-2010 02:59 AM

It's because a is a char and byte is an unsigned char. Different interpretation.

Aquarius_Girl

06-10-2010 03:10 AM

Quote:

Originally Posted by Guttorm (Post 3998639)

Thanks for the answer :hattip:

I modified the code as follows and it worked.

Code:

functionA ((unsigned char*)byte);

I need to revise the pointers now :redface:

Aquarius_Girl

06-10-2010 03:12 AM

Quote:

Originally Posted by rkski (Post 3998643)

It's because a is a char and byte is an unsigned char. Different interpretation.

That may be the problem in some cases, but it was not the problem in my case.

Thanks for bothering :)

rkski

06-10-2010 03:30 AM

I was too fast to respond. that was not the problems.

You could just make the call even simply:

functionA(byte);

Aquarius_Girl

06-10-2010 03:34 AM

Quote:

Originally Posted by rkski (Post 3998675)

I was too fast to respond. that was not the problems.

Yes, I noticed you post and then edit and then delete your reply in my other thread.. That's OK :)

Quote:

Originally Posted by rkski (Post 3998675)

You could just make the call even simply:

functionA(byte);

Thanks again !

rkski

06-10-2010 03:38 AM

LOL the other thread I was responding to the wrong information, to another poster and not the values in the original question.
So i just wanted to delete to whole post but i guess i should have made an edit note.

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