Read access violation

Question

Hi , I run my cpp code and receive run-time error:
'read access violation'
I've a variable oByte with the type PUCHAR I want assign this variable to another called TEMP that its type is UCHAR .
The oByte is the pointer which its value is the address of TEMP.
I have assigned oByte to TEMP by :
TEMP=*oByte;
I don't know this way of assigment is correct or not?
Please help.
Thanks

Answer 1

I've a variable oByte with the type PUCHAR I want assign
this variable to another called TEMP that its type is UCHAR .
The oByte is the pointer which its value is the address of TEMP.
I have assigned oByte to TEMP by :
TEMP=*oByte;

Not related to any access violation, if your code does
exactly what you have described above then it serves no
useful purpose.

You say that "oByte is the pointer which its value is the
address of TEMP." You then dereference that pointer and
assign the value to TEMP. It should be obvious that what
this does is simply assign TEMP to TEMP.

Example:

UCHAR TEMP;

PUCHAR oByte = &TEMP;

TEMP = *oByte; // assigns TEMP to TEMP. Possible use
                // of uninitialized variable TEMP

• Wayne

Answer 2

The oByte pointer is initialized with 0 so it is a null pointer. At no time in your posted code is a valid address ever stored in this pointer. Therefore, dereferencing it will cause an access violation.

Answer 3

You seem to be struggling with the concept of pointers.

In your original post you said that TEMP was of type UCHAR.
But in the latest post you have TEMP as a PUCHAR which is a
pointer to UCHAR.

PUCHAR TEMP; // same as unsigned char *TEMP;

You also said that "oByte is the pointer which its value is the
address of TEMP." But in your latest post you have

PUCHAR *oByte = 0;

Which means that oByte is a pointer to a PUCHAR. So you now
have double indirection, where oByte is a pointer to a pointer
to an unsigned char.

To use code like that would require storing and dereferencing
two pointers. For example:

UCHAR iByte = 'Z';

PUCHAR p1 = &iByte;    
PUCHAR *oByteX = &p1;
unsigned char **oByteY = &p1; // equivalent

printf("%c\n", *p1);
printf("%c\n%c\n", **oByteX, **oByteY);

Output:

Z
Z
Z

You should brush up on pointers before continuing.

A TUTORIAL ON POINTERS AND ARRAYS IN C
by Ted Jensen
Version 1.2 (PDF Version)
https://pdos.csail.mit.edu/6.828/2014/readings/pointers.pdf

There are many, many other sources on the WEB. Review several
of them if you intend to continue using pointers in C (or C++
where their use is deprecated).

• Wayne