On Tue, May 11, 2021 at 11:50:20AM -0500, Peng Yu via Libc-help wrote:
> https://man7.org/linux/man-pages/man3/printf.3.html
> 
> It is not clear what "ap" stands for in printf(3). Could anybody
> explain what it stands for? Thanks.

This is for the v*printf group. The ap is a va_list in the sense of
stdarg. Let me quote the man page:

  "The functions vprintf(), vfprintf(), vdprintf(), vsprintf(),
   vsnprintf() are equivalent to the functions printf(),
   fprintf(), dprintf(), sprintf(), snprintf(), respectively,
   except that  they  are called  with a va_list instead of a
   variable number of arguments.  These functions do not call
   the va_end macro.  Because they invoke the va_arg macro,
   the value of ap is undefined after the call.  See stdarg(3)."

They are typically used if you want to somehow leverage or extend
printf: you unwrap the parameters "the stdarg way" and pass on
whatever is left to v*printf.

Cheers
 - t