CrossWorks for ARM User Guide

- Variable argumentsThe header file defines a number of macros to access variable parameterlists.Functionsva_endva_argva_endva_copyStart access to variable argumentsGet variable argument valueFinish access to variable argumentsCopy va_arg structureva_argSynopsisDescription#include type va_arg(va_list ap, type);va_arg expands to an expression that has the specified type and the value of the typeargument. The ap parameter must have been initialized by va_start or va_copy,without an intervening invocation of va_end. You can create a pointer to a va_list andpass that pointer to another function, in which case the original function may makefurther use of the original list after the other function returns.Each invocation of the va_arg macro modifies ap so that the values of successivearguments are returned in turn. The parameter type must be a type name such that thetype of a pointer to an object that has the specified type can be obtained simply bypostfixing a ‘*’ to type.If there is no actual next argument, or if type is not compatible with the type of theactual next argument (as promoted according to the default argument promotions), thebehavior of va_arg is undefined, except for the following cases:•one type is a signed integer type, the other type is the corresponding unsignedinteger type, and the value is representable in both types;•one type is pointer to void and the other is a pointer to a character type.The first invocation of the va_arg macro after that of the va_start macro returns thevalue of the argument after that specified by parmN. Successive invocations return thevalues of the remaining arguments in succession.ExamplesWhen calling va_arg, you must ensure that type is the promoted type of the argument,not the argument type. The following will not work as you expect:char x = va_arg(ap, char);Because characters are promoted to integers, the above must be written:278 Chapter 28 Standard C Library Reference