stdarg -- variable argument lists
#include <stdarg.h>
void
va_start(va_list ap, last);
type
va_arg(va_list ap, type);
void
va_copy(va_list dest, va_list src);
void
va_end(va_list ap);
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_copy(), and va_end().
The 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.
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.
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 copies a variable argument list, previously initialized
by va_start(), from src to dest. The state is preserved such that
it is equivalent to calling va_start() with the same second argument used
with src, and calling va_arg() the same number of times as called with
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().
The va_end() macro returns no value.
The function foo takes a string of format characters and prints out the
argument associated with each format character based on the type.
void foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *s;
va_start(ap, fmt);
while (*fmt)
switch(*fmt++) {
case 's': /* string */
s = va_arg(ap, char *);
printf("string %s\n", s);
break;
case 'd': /* int */
d = va_arg(ap, int);
printf("int %d\n", d);
break;
case 'c': /* char */
/* Note: char is promoted to int. */
c = va_arg(ap, int);
printf("char %c\n", c);
break;
}
va_end(ap);
}
The va_start(), va_arg(), va_copy(), and va_end() macros conform to
ISO/IEC 9899:1999 (``ISO C99'').
These macros are not compatible with the historic macros they replace. A
backward compatible version can be found in the include file <varargs.h>.
Unlike 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).
FreeBSD 5.2.1 October 25, 2002 FreeBSD 5.2.1 [ Back ] |
