This section contains an implementation of a minimal version of printf, to
show how to write a function that processes a variable-length argument list
in a portable way. Since we are mainly interested in the argument
processing, minprintf will process the format string and arguments but will
call the real printf to do the format conversions.
The proper declaration for printf is
int printf(char *fmt, ...)
where the declaration ... means that the number and types of these
arguments may vary. The declaration ... can only appear at the end of an
argument list. Our minprintf is declared as
void minprintf(char *fmt, ...)
since we will not return the character count that printf does.
The tricky bit is how minprintf walks along the argument list when the list
doesn't even have a name. The standard header <stdarg.h> contains a set of
macro definitions that define how to step through an argument list. The
implementation of this header will vary from machine to machine, but the
interface it presents is uniform.
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.'' The macro va_start initializes ap to point to the first unnamed
argument. It must be called once before ap is used. There must be at least
one named argument; the final named argument is used by va_start to get
started.
Each call of va_arg returns one argument and steps ap to the next; va_arg
uses a type name to determine what type to return and how big a step to
take. Finally, va_end does whatever cleanup is necessary. It must be called
before the program returns.
These properties form the basis of our simplified printf:
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