I want to know the process of function call and download source codes of glibc. I find the function __printf in /stdio-common/printf.c. It calls vfprintf. And in vprintf I cannot continue this process. It seems to call Xprintf. But the definition of Xprintf is like this define Xprintf(n) __printf_##n. I cannot understand its meaning. It calls printf again?

I also check out putchar. It calls _IO_putc_unlocked and then __putc_unlocked_body,by *(_fp)->_IO_write_ptr++ = (_ch) to stdout. I cannot find the the way in printf.

Also I want to know atfer writing the parameter to stdout, what’s the next process?