memcpy with struct and pointer

[neeraj_rn]

Hi everyone.

I am trying the following.

//*********************
typedef struct{
 unsigned char Dlength;
 unsigned char *Value;
 } AttributeA;

 //In the main
 unsigned char userData[] = {0x01,0x02,0x03,0x04,0x5};
 
 AttributeA buffSrc;
 AttributeA buffDst;
 buffSrc.Dlength = 5;
 buffDst.Dlength = 5;
 buffSrc.Value = userData;

 memcpy(buffDst.Value, buffSrc.Value, buffDst.Dlength);
 
//**********************

printf of buffSrc.Value gives me the output = 0x01,0x02,0x03,0x04,0x5;
But printf of buffDst.Value does not give the expected output (as buffSrc.Value).

I wonder what am I doing wrong?
I have even tried
memcpy(&buffDst.Value[0], &buffSrc.Value[0], buffDst.Dlength);

I shall appreciate.

[Bayint Naung]

You have a pointer inside struct.
You need to allocate enough memory before copying anything to it.

[BillyTKid]

try at the end:

buffDst = buffSrc;

and it works.

[phantomotap]

O_o

Define "works"?

Soma

[maxorator]

buffDst.Value is an uninitialized pointer, so I'm wondering why it doesn't crash for you. It has to point to some allocated memory of size at least buffDst.Dlength.

[neeraj_rn]

Bayint & maxorator.
I got it.
Thank you.

[BillyTKid]

O_o

Define "works"?

Soma

 unsigned char userData[] = {0x01,0x02,0x03,0x04,0x5};

 AttributeA buffSrc;
 AttributeA buffDst;

 buffSrc.Dlength = 5;
 buffSrc.Value = userData;
 fwrite(buffSrc.Value,buffSrc.Dlength,1,stdout),fflush(stdout);

 buffDst=buffSrc;
 fwrite(buffDst.Value,buffDst.Dlength,1,stdout),fflush(stdout);

 assert(5==buffSrc.Dlength && 5==buffDst.Dlength && !memcmp(buffSrc.Value,buffDst.Value,5));

You see, no new memory needed.

[maxorator]

OP wanted to make a copy of the data. Your code does NOT make a copy of the data - it uses the same data for both structures, not 2 separate copies of it.

[BillyTKid]

printf of buffSrc.Value gives me the output = 0x01,0x02,0x03,0x04,0x5;
But printf of buffDst.Value does not give the expected output (as buffSrc.Value).

The question was only an expected output for buffDst like buffSrc, therefore to make an extra copy is overdesigned.

[phantomotap]

You see, no new memory needed.

O_o

Your code is... broken? That doesn't seem to be the right word. Stupid? Crazy? Ignorant? Foolish? Dangerous?

I'll leave that to the other philosophers, but safe to say, your "it works" code certainly does not do anything useful.

Oh, and the source and destination "point" to the same location so the `memcmp' is pointless (probably a "NOP").

The question was only an expected output for buffDst like buffSrc, therefore to make an extra copy is overdesigned.

o_O

I think I'll go with "foolish" on this one.

Soma

[Salem]

> The question was only an expected output for buffDst like buffSrc, therefore to make an extra copy is overdesigned.
Try applying that logic to your file system, when you try to make a backup copy of a file.

"It's only a copy, I'll just point at it, no need to waste disk space".

"WTF - my backup's been trashed!"

Do you now understand why shallow copies are a PITA?