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       stdarg, va_start, va_arg, va_end, va_copy - variable argument lists


       #include <stdarg.h>

       void va_start(va_list ap, last);
       type va_arg(va_list ap, type);
       void va_end(va_list ap);
       void va_copy(va_list dest, va_list src);


       A function may be called with a varying number of arguments of varying types.  The include
       file <stdarg.h> declares a type va_list and defines three macros for  stepping  through  a
       list of arguments whose number and types are not known to the called function.

       The  called  function  must  declare an object of type va_list which is used by the macros
       va_start(), va_arg(), and va_end().

       The va_start() macro initializes ap for subsequent use by va_arg() and va_end(), and  must
       be called first.

       The argument last is the name of the last argument before the variable argument list, that
       is, the last argument of which the calling function knows the type.

       Because the address of this argument may be used in the va_start() macro, it should not be
       declared as a register variable, or as a function or an array type.

       The  va_arg()  macro  expands  to  an  expression  that has the type and value of the next
       argument in the call.  The argument ap is the va_list ap initialized by va_start().   Each
       call  to  va_arg()  modifies  ap  so  that  the  next call returns the next argument.  The
       argument type is a type name specified so that the type of a pointer to an object that has
       the specified type can be obtained simply by adding a * to type.

       The  first  use  of  the  va_arg()  macro  after  that of the va_start() macro returns the
       argument after last.  Successive invocations return the values of the remaining arguments.

       If there is no next argument, or if type is not compatible with the  type  of  the  actual
       next  argument  (as  promoted according to the default argument promotions), random errors
       will occur.

       If ap is passed to a function that uses va_arg(ap,type), then the value of ap is undefined
       after the return of that function.

       Each invocation of va_start() must be matched by a corresponding invocation of va_end() in
       the same function.  After the call va_end(ap) the  variable  ap  is  undefined.   Multiple
       traversals  of the list, each bracketed by va_start() and va_end() are possible.  va_end()
       may be a macro or a function.

       The va_copy() macro copies the (previously initialized)  variable  argument  list  src  to
       dest.   The behavior is as if va_start() were applied to dest with the same last argument,
       followed by the same number of va_arg() invocations that was used  to  reach  the  current
       state of src.

       An  obvious  implementation  would  have  a va_list be a pointer to the stack frame of the
       variadic function.  In such a setup (by far the most common) there seems  nothing  against
       an assignment

           va_list aq = ap;

       Unfortunately, there are also systems that make it an array of pointers (of length 1), and
       there one needs

           va_list aq;
           *aq = *ap;

       Finally, on systems where arguments are passed in  registers,  it  may  be  necessary  for
       va_start()  to allocate memory, store the arguments there, and also an indication of which
       argument is next, so that va_arg() can step through the list.  Now va_end() can  free  the
       allocated  memory  again.   To  accommodate this situation, C99 adds a macro va_copy(), so
       that the above assignment can be replaced by

           va_list aq;
           va_copy(aq, ap);

       Each invocation of va_copy() must be matched by a corresponding invocation of va_end()  in
       the  same  function.   Some  systems  that do not supply va_copy() have __va_copy instead,
       since that was the name used in the draft proposal.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue           │
       │va_start(), va_end(), │ Thread safety │ MT-Safe         │
       │va_copy()             │               │                 │
       │va_arg()              │ Thread safety │ MT-Safe race:ap │


       The  va_start(),  va_arg(), and va_end() macros conform to C89.  C99 defines the va_copy()


       Unlike the historical varargs macros, the stdarg macros do not permit programmers to  code
       a  function  with  no fixed arguments.  This problem generates work mainly when converting
       varargs code to stdarg code, but it also creates difficulties for variadic functions  that
       wish  to  pass all of their arguments on to a function that takes a va_list argument, such
       as vfprintf(3).


       The function foo takes  a  string  of  format  characters  and  prints  out  the  argument
       associated with each format character based on the type.

       #include <stdio.h>
       #include <stdarg.h>

       foo(char *fmt, ...)   /* '...' is C syntax for a variadic function */

           va_list ap;
           int d;
           char c, *s;

           va_start(ap, fmt);
           while (*fmt)
               switch (*fmt++) {
               case 's':              /* string */
                   s = va_arg(ap, char *);
                   printf("string %s\n", s);
               case 'd':              /* int */
                   d = va_arg(ap, int);
                   printf("int %d\n", d);
               case 'c':              /* char */
                   /* need a cast here since va_arg only
                      takes fully promoted types */
                   c = (char) va_arg(ap, int);
                   printf("char %c\n", c);


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                                            2019-05-09                                  STDARG(3)