It come across to me that function like printf() have not limited the number of parameters.
But when debugging program on Solaris, I noticed it will push at most 5 parameters into stack, common register will be used if there are more than 5 parameters.
So what will happen if even common register is not enough in function like printf ? Did compiler do something for me ?
The behaviour is controlled by the ABI for the platform. If there are more parameters than fit in the registers, then they will be handled in a different way. There isn't a simple upper limit on the number of arguments that can be passed, so the compiler and the ABI define a mechanism that works on the hardware in question. What works on SPARC does not necessarily work on, for example, Intel IA32.
Normally platforms where the ABI uses registers for argument passing switch to a different calling convention for variadic functions, whereby everything is passed on the stack. This is why the C standard assigned undefined behavior to calling a variadic function without a prototype; without a prototype, on such platforms the compiler will generate an incorrect call.
It should be noted that some platforms use more complicated (uselessly complicated, I would say) methods of passing arguments to variadic functions, such as constructing a sort of linked list and passing a hidden pointer to that list, which the implementation of va_start is then somehow able to obtain. As a programmer, you should just treat the whole stdarg.h stuff as a black box that does what's expected, and pray that you never have to see the gorey details of some of the uglier implementations...