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Thank you _Mike. I understood well.

Thank you _Mike.

Which parameter determine dsize was 4 or 0? Does size of msg->data and buf->data is effective?

Thank you very much.

Ok, right.

I understand why using msg->dsize in the memcpy function.

By using it, can copy all buf structure to msg->data and msg->dsize simultaneously.

Thanks to all specially sganesh.

you don't ask answer of my question. I ask why using msg->dsize in first parameter of memcpy?

your written code only copy all one structure to another one.
however, i thank you for it.

Thank you.

if change size of msg->data from 100 to 2 and then try to copy buffer to msg, msg->dsize will be
invalid. I don't know what msg->dsize will has, I only know it will be invalid.

If...

memcpy function is used for copy to dst from src with size:

memcpy(dst, src, size)

If msg structure was defined as follow:

struct msg{
char data[100];
int dsize;
}