-> Click here to learn how to get live help <-
NAMEstdarg - variable argument lists
DESCRIPTIONA 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.
va_startThe 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.
va_argThe 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.
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.
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 ap is passed to a function that uses va_arg(ap,type) then the value of ap is undefined after the return of that function.
va_endEach invocation of va_start must be matched by a corresponding invocation of va_end in the same function. After the call va_end(ap) the variable ap is undefined. Multiple transversals of the list, each bracketed by va_start and va_end are possible. va_end may be a macro or a function.
va_copyAn obvious implementation would have a va_list a pointer to the stack frame of the variadic function. In such a setup (by far the most common) there seems nothing against an assignment
EXAMPLESThe function foo takes a string of format characters and prints out the argument associated with each format character based on the type.
CONFORMING TOThe va_start, va_arg, and va_end macros conform to ANSI X3.159-1989 (``C89''). C99 defines the va_copy macro.
COMPATIBILITYThese macros are not compatible with the historic macros they replace. A backward compatible version can be found in the include file varargs.h.
COMPARISONThe historic setup is:
BUGSUnlike the varargs macros, the stdarg macros do not permit programmers to code a function with no fixed arguments. This problem generates work mainly when converting varargs code to stdarg code, but it also creates difficulties for variadic functions that wish to pass all of their arguments on to a function that takes a va_list argument, such as vfprintf(3).