hi i use wattcp to program network Dos programs but when i use the
received function in pcpkt.c library it always give me null
in spite of that there are many packets are received and i do not know why that function do not gve me any packet

Lemme guess, you're real OS is actually Win2K, but you're clinging to some crappy old 16 bit compiler and pretending that you're using DOS.

I guess the DOS emulation on your machine isn't up to supporting that particular network hackery code (yes, networking in 16-bit real mode was always a hack).

UNLESS you're copying the DOS executable to a real physical machine running real 6.x DOS, then expect problems if you push hard on the emulation layer in any win32 operating system.

Are you still going to be using DOS when grandchild of Vista comes out in say 2020 ?

Bite the bullet and get yourself upgraded to something more modern, and leave 16 bit programming to the archaeologists.