What is responsible for creating file forward.c?

I install openwrt source code then I compiled it. A forward.c was created after compiling source code, so if can someone tell me how this file create from which file ?

Likely the same answers as you have been given in

Without knowing the directory where you found the forward.c file, it is likely from the netfilter component in the Linux kernel. Alternatively, if you made a typo (forwards.c) and that file was found in the firewall build directory, the source is from the Openwrt firewall. https://git.openwrt.org/?p=project/firewall3.git;a=summary