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STDARG(3) FreeBSD Library Functions Manual STDARG(3)
NAME
stdarg - variable argument lists
SYNOPSIS
#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);
DESCRIPTION
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
four 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(),
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_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() or va_copy(). 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
RETURN VALUES
The va_arg() macro returns the value of the next argument.
The va_start(), va_copy(), and va_end() macros return no value.
EXAMPLES
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);
}
COMPATIBILITY
These macros are not compatible with the historic macros they replace. A
backward compatible version can be found in the include file <varargs.h>.
STANDARDS
The va_start(), va_arg(), va_copy(), and va_end() macros conform to
ISO/IEC 9899:1999 ("ISO C99").
HISTORY
The va_start(), va_arg() and va_end() macros were introduced in ANSI
X3.159-1989 ("ANSI C89"). The va_copy() macro was introduced in ISO/IEC
9899:1999 ("ISO C99").
BUGS
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 14.0-RELEASE-p11 February 25, 2020 FreeBSD 14.0-RELEASE-p11