Adding fpirntf() into xnu kernel tcp_xxx.c, stdio.h not found

Greeting everyone on stackoverflow!

I am trying to add fpirntf() to XNU kernel tcp_xxx.c file, so when TCP moves it can print parameters to a file, for better understanding how TCP works, but fatal error occurs like follows:

$ sudo make ARCH_CONFIGS=X86_64 KERNEL_CONFIGS=DEBUG
CC tcp_output.o
/Users/wangweikai/Desktop/xnubuilder/xnu-2422.90.20/bsd/netinet/tcp_output.c:135:10: fatal error: 'stdio.h' file not found
#include <stdio.h>
         ^
1 error generated.
make[7]: *** [tcp_output.o] Error 1
make[6]: *** [build_all] Error 2
make[5]: *** [do_all] Error 2
make[4]: *** [build_all] Error 2
make[3]: *** [build_all_recurse_into_conf] Error 2
make[2]: *** [build_all_recurse_into_bsd] Error 2
make[1]: *** [build_all_bootstrap_DEBUG^X86_64^NONE] Error 2
make: *** [all] Error 2

It looks there is no stdio.h in xnu source code, to solve this I re-installed xcode 5.0.2(only this works with xnu-2422) and outputs mach_kernel able to boot; I tried install command line tools by "xcode-select --install" in terminal, but not working neither. BTW if I add printf() without #include it gives no error, it looks print function is defined by file with name other than stdio.h.

Since my point is to log how TCP parameters(such as congestion window size and next sequence number and so on)changes in a file, so if there is any way can do this please also tell me, I am total new to xnu and C programming.

Best regards.

Solution

Standard I/O (i.e, stdio) is not available in the kernel. To output debugging information from the kernel, call printf(); its output will be visible in the kernel log. Writing data to files directly from the kernel is complicated, and is rarely advisable. (In this case, it may in fact lead to hard lockups, e.g, if the file being written to is on a network volume!)

If you are new to C programming, this is not a good first project. Kernel development is difficult, unforgiving stuff, particularly for the XNU kernel; errors will often cause your computer to crash, potentially losing data, and debugging services are often not available. If you have your heart set on examining an OS kernel, though, the Linux kernel is much easier to work with — it can be easily booted and debugged in a virtual machine, and is far better documented.