Why must I use gnu99 instead of c99 to compile a kernel module?

Question

I am used to using -std=c99 to enable c99 features when compiling application code.

Recently I have been following some basic kernel module examples, and added ccflags-y := -std=c99 to the makefile. However this resulted in 17K lines of errors when I tried to make. gnu99 works perfectly.

What is the difference between gnu99 and c99 that kernel code relies on?

Answer 1

The linux kernel uses a lot of GCC extensions i.e., the C language used in the kernel is not standards compliant, it is a superset which includes GCC extension. Hence GNU99 is the compiler option which needs to be passed.