---
name: "stdarg"
section: "3"
platform: "macos"
package: "Libc"
version: "1752.120.2"
release: "macOS 26.5"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/stdarg"
---

# stdarg(3)

variable argument lists

## 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 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 must be called first, and it initializes *ap*, which can be passed to **va\_arg**() for each argument to be processed. Calling **va\_end**() signals that there are no further arguments, and causes *ap* to be invalidated. Note that each call to **va\_start**() must be matched by a call to **va\_end**(), from within the same function.

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**(). 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 the state of the variable argument list,  *src*, previously initialized by **va\_start**(), to the variable argument list, *dest*, which must not have been previously initialized by **va\_start**(), without an intervening call to **va\_end**(). The state preserved in *dest* is equivalent to calling **va\_start**() and **va\_arg**() on *dest* in the same way as was used on *src*. The copied variable argument list can subsequently be passed to **va\_arg**(), and must finally be passed to **va\_end**() when through with it.

After a variable argument list is invalidated by **va\_end**(), it can be reinitialized with **va\_start**() or made a copy of another variable argument list with **va\_copy**().

## 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, ap2;
	int d;
	char c, *s;
	va_start(ap, fmt);
	va_copy(ap2, ap);
	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);
	...
	/* use ap2 to iterate over the arguments again */
	...
	va_end(ap2);
}
```

## 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”).

## 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)](/vfprintf/3.md).