What does 'ap' stand for in C examples?

Question

For example, in the reference:

type va_arg (va_list ap, type)

Does ap stand for argument pointer? Something else?

Answer 1

That's actually an easy question to answer.

ap stands for argument pointer .

From The C Programming Language , Second Edition, Brian W. Kernighan and Dennis M. Ritchie:

7.3 Variable-length Argument Lists

[...] The type va_list is used to declare a variable that will refer to each argument in turn; in minprintf, this variable is called ap, for "argument pointer"

Answer 2

ap stands for Argument Pointer . In fact, it is sometimes abbreviated as arg_ptr :

type va_arg(
   va_list arg_ptr,
   type 
);

• type - Type of argument to be retrieved.
• arg_ptr - Pointer to the list of arguments.

Answer 3

That's actually a difficult question to answer.

See ouah's answer ; the phrase "argument pointer" is mentioned in K&R2.

The <stdarg.h> header was introduced by the ANSI C standard in 1989. The ANSI C Rationale doesn't show any examples, and doesn't use the name ap or arg_ptr . The standard itself (the latest draft is here uses the name ap in examples, but doesn't explain it.

I haven't been able to find an explanation of the name ap . It's plausible that it originally stood for "argument pointer", but it's also important to note that the type va_list is not (necessarily) a pointer type. In gcc, for example, it's defined as __builtin_va_list , a type that's implemented directly by the compiler. (It happens to have a size of 24 bytes on my system, but that's not particularly useful information).

Bottom line: The name ap is the conventional name for a va_list object, for no particular reason that we need to care about.