# va_copy

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< c‎ | variadic

Defined in header `<stdarg.h>` |  |
void va_copy( va_list dest, va_list src ); |  | (since C99)

The `va_copy` macro copies `src` to `dest`.

va_end should be called on `dest` before the function returns or any subsequent re-initialization of `dest` (via calls to va_start or va_copy).

### Parameters

dest | - | an instance of the va_list type to initialize
src | - | the source va_list that will be used to initialize `dest`

### Expanded value

(none)

### Example

Run this code

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

    double sample_stddev(int count, ...)
    {
        /* Compute the mean with args1. */
        double sum = 0;
        va_list args1;
        va_start(args1, count);
        va_list args2;
        va_copy(args2, args1);   /* copy va_list object */
        for (int i = 0; i < count; ++i) {
            double num = va_arg(args1, double);
            sum += num;
        }
        va_end(args1);
        double mean = sum / count;

        /* Compute standard deviation with args2 and mean. */
        double sum_sq_diff = 0;
        for (int i = 0; i < count; ++i) {
            double num = va_arg(args2, double);
            sum_sq_diff += (num-mean) * (num-mean);
        }
        va_end(args2);
        return sqrt(sum_sq_diff / count);
    }

    int main(void)
    {
        printf("%f\n", sample_stddev(4, 25.0, 27.3, 26.9, 25.7));
    }

Possible output:

    0.920258

### References

- C11 standard (ISO/IEC 9899:2011):

  - 7.16.1.2 The va_copy macro (p: 270)

- C99 standard (ISO/IEC 9899:1999):

  - 7.15.1.2 The va_copy macro (p: 250)

### See also

va_arg | accesses the next variadic function argument (function macro)
va_end | ends traversal of the variadic function arguments (function macro)
va_list | holds the information needed by va_start, va_arg, va_end, and va_copy (typedef)
va_start | enables access to variadic function arguments (function macro)
C++ documentation for va_copy

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