Mac OS X Manual Page For stdarg(3) (original) (raw)

STDARG(3) BSD Library Functions Manual STDARG(3)

NAME stdarg -- variable argument lists

SYNOPSIS #include <stdarg.h>

 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 <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 parameter last is the name of the last parameter before the variable
 argument list, i.e. the last parameter of which the calling function
 knows the type.

 Because the address of this parameter is 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_start**() macro returns no value.

 The **va_arg**() 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**().  Each call to **va_arg**() 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 speci-fied specified
 fied 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 argu-ment argument
 ment promotions), random errors will occur.

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

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

 The **va_end**() 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);
       }

STANDARDS The va_start(), va_arg(), and va_end() macros conform to ANSI X3.159-1989 (``ANSI C89'').

COMPATIBILITY These macros are not compatible with the historic macros they replace. A backward compatible version can be found in the include file <varargs.h>.

BUGS Unlike the 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 argu-ments arguments ments on to a function that takes a va**_**list argument, such as vfprintf(3).

BSD June 5, 1993 BSD