stdarg(3)                                                     stdarg(3)

  stdarg()

  NAME

    stdarg - variable argument lists

  SYNOPSIS

    #include 

    void va_start (va_list ap, last)
    type va_arg (va_list ap, type)
    void va_end (va_list ap)

  DESCRIPTION

    A function may be called with a varying number of arguments of varying
    types. The include file  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(3), va_arg(3), and va_end(3).

    The va_start(3) macro initializes ap for subsequent use by va_arg(3) and
    va_end(3), and must be called first.

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

    Because the address of this parameter is used in the va_start(3) macro, it
    should not be declared as a register variable, or as a function or an
    array type.

    The va_start(3) macro returns no value.

    The va_arg(3) macro expands to an expression that has the type and value
    of the next argument in the call. The parameter ap is the va_list ap
    initialized by va_start(3). Each call to va_arg(3) modifies ap so that the
    next call returns the next argument. The parameter 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.

    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.

    The first use of the va_arg(3) macro after that of the va_start(3) macro
    returns the argument after last. Successive invocations return the values
    of the remaining arguments.

    The va_end(3) macro handles a normal return from the function whose
    variable argument list was initialized by va_start(3).

    The va_end(3) macro returns no value.

  EXAMPLES

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

    void foo(char *fmt, ...)
    {
         va_list ap;
         int d;
         char c, *p, *s;
         va_start(ap, fmt);
         while (*fmt)
              switch(*fmt++) {
              case 's':           /* string */
                   s = va_arg(ap, char *);
                   printf("string %s\n", s);
                   break;
              case 'd':           /* int */
                   d = va_arg(ap, int);
                   printf("int %d\n", d);
                   break;
              case 'c':           /* char */
                   c = va_arg(ap, char);
                   printf("char %c\n", c);
                   break;
              }
         va_end(ap);
    }

  BUGS

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

  USAGE NOTES

    The stdarg function is thread safe.

    The stdarg function is not async-signal safe.