va_arg, va_copy, va_end, va_start

Article
04/28/2015

Accesses variable-argument lists.

type va_arg(
   va_list arg_ptr,
   type 
);
void va_copy(
   va_list dest,
   va_list src
); // (ISO C99 and later)
void va_end(
   va_list arg_ptr 
);
void va_start(
   va_list arg_ptr,
   prev_param 
); // (ANSI C89 and later)
void va_start(
   arg_ptr 
);  // (Pre-ANSI C89 standardization version)

Parameters

type
Type of argument to be retrieved.
arg_ptr
Pointer to the list of arguments.
dest
Pointer to the list of arguments to be initialized from src
src
Pointer to the initialized list of arguments to copy to dest.
prev_param
Parameter that precedes the first optional argument.

Return Value

va_arg returns the current argument. va_copy, va_start and va_end do not return values.

Remarks

The va_arg, va_copy, va_end, and va_start macros provide a portable way to access the arguments to a function when the function takes a variable number of arguments. There are two versions of the macros: The macros defined in STDARG.H conform to the ISO C99 standard; the macros defined in VARARGS.H are deprecated but are retained for backward compatibility with code that was written before the ANSI C89 standard.

These macros assume that the function takes a fixed number of required arguments, followed by a variable number of optional arguments. The required arguments are declared as ordinary parameters to the function and can be accessed through the parameter names. The optional arguments are accessed through the macros in STDARG.H (or VARARGS.H for code that was written before the ANSI C89 standard), which sets a pointer to the first optional argument in the argument list, retrieves arguments from the list, and resets the pointer when argument processing is completed.

The C standard macros, defined in STDARG.H, are used as follows:

va_start sets arg_ptr to the first optional argument in the list of arguments that's passed to the function. The argument arg_ptr must have the va_list type. The argument prev_param is the name of the required parameter that immediately precedes the first optional argument in the argument list. If prev_param is declared with the register storage class, the macro's behavior is undefined. va_start must be used before va_arg is used for the first time.
va_arg retrieves a value of type from the location that's given by arg_ptr, and increments arg_ptr to point to the next argument in the list by using the size of type to determine where the next argument starts. va_arg can be used any number of times in the function to retrieve arguments from the list.
va_copy makes a copy of a list of arguments in its current state. The src parameter must already be initialized with va_start; it may have been updated with va_arg calls, but must not have been reset with va_end. The next argument that's retrieved by va_arg from dest is the same as the next argument that's retrieved from src.
After all arguments have been retrieved, va_end resets the pointer to NULL. va_end must be called on each argument list that's initialized with va_start or va_copy before the function returns.

C++ Note
The macros in VARARGS.H are deprecated and are retained only for backwards compatibility with code that was written before the ANSI C89 standard. In all other cases, use the macros in STDARGS.H.

When they are compiled by using /clr (Common Language Runtime Compilation), programs that use these macros may generate unexpected results because of differences between native and common language runtime (CLR) type systems. Consider this program:

#include <stdio.h>
#include <stdarg.h>

void testit (int i, ...)
{
    va_list argptr;
    va_start(argptr, i);

    if (i == 0)
    {
        int n = va_arg(argptr, int);
        printf("%d\n", n);
    }
    else
    {
        char *s = va_arg(argptr, char*);
        printf("%s\n", s);
    }
}

int main()
{
    testit(0, 0xFFFFFFFF); // 1st problem: 0xffffffff is not an int
    testit(1, NULL);       // 2nd problem: NULL is not a char*
}

Notice that testit expects its second parameter to be either an int or a char*. The arguments being passed are 0xffffffff (an unsigned int, not an int) and NULL (actually an int, not a char*). When the program is compiled for native code, it produces this output:

-1
(null)

However, when the program is compiled by using /clr:pure, the type mismatches cause it to generate an exception. The solution is to use explicit casts:

int main()
{
   testit( 0, (int)0xFFFFFFFF ); // cast unsigned to int
   testit( 1, (char*)NULL );     // cast int to char*
}

Requirements

Header: <stdio.h> and <stdarg.h>

Deprecated Header: <varargs.h>

Libraries

All versions of the C run-time libraries.

Example

// crt_va.c
/* Compile with: cl /W3 /Tc crt_va.c
 * The program below illustrates passing a variable
 * number of arguments using the following macros:
 *      va_start            va_arg              va_copy
 *      va_end              va_list
 */

#include <stdio.h>
#include <stdarg.h>
#include <math.h>

double deviation(int first, ...);

int main( void )
{
    /* Call with 3 integers (-1 is used as terminator). */
    printf("Deviation is: %f\n", deviation(2, 3, 4, -1 ));

    /* Call with 4 integers. */
    printf("Deviation is: %f\n", deviation(5, 7, 9, 11, -1));

    /* Call with just -1 terminator. */
    printf("Deviation is: %f\n", deviation(-1));
}

/* Returns the standard deviation of a variable list of integers. */
double deviation(int first, ...)
{
    int count = 0, i = first;
    double mean = 0.0, sum = 0.0;
    va_list marker;
    va_list copy;

    va_start(marker, first);     /* Initialize variable arguments. */
    va_copy(copy, marker);       /* Copy list for the second pass */
    while (i != -1)
    {
        sum += i;
        count++;
        i = va_arg(marker, int);
    }
    va_end(marker);              /* Reset variable argument list. */
    mean = sum ? (sum / count) : 0.0;

    i = first;                  /* reset to calculate deviation */
    sum = 0.0;
    while (i != -1)
    {
        sum += (i - mean)*(i - mean);
        i = va_arg(copy, int);
    }
    va_end(copy);               /* Reset copy of argument list. */
    return count ? sqrt(sum / count) : 0.0;
}

Output

Deviation is: 0.816497
Deviation is: 2.236068
Deviation is: 0.000000

.NET Framework Equivalent

System::ParamArrayAttribute Class

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