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(),
va_end(), and, optionally,
va_copy().
The
va_start() macro initializes
ap for subsequent use by
va_arg(),
va_copy() 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.
If the type in question is one that gets promoted, the promoted type should be used as the argument to
va_arg(). The following describes which types are promoted (and to what):
–
short is promoted to int
–
float is promoted to double
–
char is promoted to int
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 makes
dest a copy of
src as if the
va_start() macro had been applied to it followed by the same sequence of uses of the
va_arg() macro as had previously been used to reach the present state of
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() or
va_copy().
The
va_end() macro returns no value.