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B.7 Variable Argument Lists: <stdarg.h>

The header <stdarg.h> provides facilities for stepping through a list of function arguments of unknown number and type.

Suppose lastarg is the last named parameter of a function f with a variable number of arguments. Then declare within f a variable of type va_list that will point to each argument in turn:

• va_list ap;

ap must be initialized once with the macro va_start before any unnamed argument is accessed:

• va_start(va_list ap, lastarg);

Thereafter, each execution of the macro va_arg will produce a value that has the type and value of the next unnamed argument, and will also modify ap so the next use of va_arg returns the next argument:

• type va_arg(va_list ap, type);

The macro

• void va_end(va_list ap);

must be called once after the arguments have been processed but before f is exited.