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 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_end macro handles a normal return from the function whose variable argument list was initialized by va_start.
The function foo takes a string of format characters and prints out the argument associated with each format character based on the type.