| va_start | Function (Macro)

stdarg.h

void va_start (va_list &ap, &lastfix);

Initializes a pointer to a variable argument list.

Some C functions, such as sprintf have a variable number of arguments. This function, together with va_arg and va_end, provides a portable way to access these argument lists. They are used for stepping through a list of arguments when the called function does not know the number and types of the arguments being passed. Function va_start (implemented as a macro) sets ap to point to the first of the variable arguments being passed to the function. It must be used before the first call to va_arg or va_end.
va_start takes two parameters: ap and lastfix. ap is explained above, and lastfix is the name of the last fixed parameter being passed to the called function.
Note: I used notation "&ap" in the prototype description, although passing by reference does not exist in ordinary C (only in C++). However, this macro is implemented in such a way that it simulates "passing by reference".