What is a void expression?

What is a void expression? -- Linux general technology-Linux programming and kernel information. The following is a detailed description. Compile a program with gcc in linux, but this error cannot be found... I don't know why

The system version is fedora 9.
What is the void expression in the last line of the message?
Hope someone else can explain it... Thank you.

The message is as follows:
In file encoded ded from udp_client_send.c: 2:
My_udp.h: In the 'link _ request' function:
My_udp.h: 51: Warning: convert incompatible pointer types when passing 'sendto''s 5th parameters
My_udp.h: 53: Warning: convert incompatible pointer types when passing the 5th 'recvfrom' parameter
My_udp.h: 53: Warning: the integer is assigned to the pointer when the first parameter of 'recvfrom' is passed, without type conversion.

My_udp.h: In the 'Section _ pack_send 'function:
My_udp.h: 161: Warning: convert incompatible pointer types when 'sendto''s 5th parameters are passed

My_udp.h: In the 'Section _ data_process 'function:
My_udp.h: 215: Warning: convert incompatible pointer types when 'sendto''s 5th parameters are passed

My_udp.h: In the 'data _ send' function:
My_udp.h: 289: Warning: convert incompatible pointer types when 'sendto''s 5th parameters are passed

My_udp.h: In the 'confirm _ pack_recv 'function:
My_udp.h: 339: Warning: converts incompatible pointer types when 'recvfrom' is passed.
My_udp.h: 339: Warning: the integer is assigned to the pointer when the 6th parameter 'recvfrom' is passed, without type conversion

Udp_client_send.c: In the 'main' function:
Udp_client_send.c: 106: error: Invalid void expression