A function may be called with a varying number of arguments of varying types. The include file
.In 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, va_copy, 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 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 macro after that of the va_start macro returns the argument after last. Successive invocations return the values of the remaining arguments.
The va_copy macro copies a variable argument list, previously initialized by va_start, from src to dest. The state is preserved such that it is equivalent to calling va_start with the same second argument used with src, and calling va_arg the same number of times as called with src.
The va_copy macro returns no value.
The va_end macro handles a normal return from the function whose variable argument list was initialized by va_start.
The va_end macro returns no value.
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, ...)
char c, *s;
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 */
/* Note: char is promoted to int. */
c = va_arg(ap, int);
printf("char %c\n", c);
These macros are not compatible with the historic macros they replace. A backward compatible version can be found in the include file
.In varargs.h .
The va_start, va_arg, va_copy, and va_end macros conform to -isoC-99.
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 arguments on to a function that takes a va_list argument, such as vfprintf(3).