These historic interfaces are provided to support compilation of existing programs only. New code should use the
stdarg(3) interfaces.
A function may be called with a varying number of arguments of varying types. The include file
<varargs.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 name an argument
va_alist, which marks the start of the variable argument list, and which is naturally the last argument named. It is declared by
va_dcl, which should not be followed by a semicolon. The called function also 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.
It is possible for
va_alist to be the only parameter to a function, resulting in it being possible for a function to have no fixed arguments preceding the variable argument list.
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
va_end() macro returns no value.