Where is tcp.c?

Question

This question is related to my other question, which I already answered.

I want to add modify tcp.c to include my own code. The problem is I can't find tcp.c. The first thing I tried was looking in the same directory as tcp.h (C:\Users\Me\AppData\Local\Arduino15\packages\esp8266\hardware\esp82662円.5.2\tools\sdk\lwip2\include\lwip\tcp.h) but there is no tcp.c file there. So then I tried every tcp.h file on my c: drive.

c:\>dir *tcp.c /s
c:\Program Files (x86)\Arduino\hardware\arduino\avr\firmwares\wifishield\wifiHD\src
c:\Program Files (x86)\Arduino\hardware\arduino\avr\firmwares\wifishield\wifiHD\src\SOFTWARE_FRAMEWORK\SERVICES\LWIP\lwip-1.3.2\src\core
c:\Program Files (x86)\Arduino\libraries\WiFi\extras\wifiHD\src
c:\Program Files (x86)\Arduino\libraries\WiFi\extras\wifiHD\src\SOFTWARE_FRAMEWORK\SERVICES\LWIP\lwip-1.3.2\src\core
c:\Users\Me\AppData\Local\Arduino15\packages\esp8266\hardware\esp82662円.5.2\tools\sdk\lwip\src\app
c:\Users\Me\AppData\Local\Arduino15\packages\esp8266\hardware\esp82662円.5.2\tools\sdk\lwip\src\core
c:\Users\Me\AppData\Local\Arduino15\packages\esp8266\hardware\esp82662円.5.2\tools\sdk\lwip2\builder\glue-lwip

and it's none of those. I assume this because I deleted everything in those files and my program still runs and builds. Now I don't know what to do. Am I right in assuming the file would be called tcp.c? If so why doesn't it seem to be any of the tcp.c files on my computer?

• Juraj: most of the LWIP library versions in esp8266 package are provided as already compiled .a (esp8266/hardware/esp8266/2.5.2/tools/sdk/lib/liblwip2-1460.a) and without source code. source code esp8266/2.5.2/tools/sdk/lwip/src
• user875234: Thanks. How did you know that? What can I do if I want to modify and rebuild the source?
• Juraj: the lwip folders contain make files