Passing variable number of arguments around?

Question

Say I have a C function which takes a variable number of arguments: How can I call another function which expects a variable number of arguments from inside of it, passing all the arguments that got into the first function?Example: void format_string(char *fmt, ...); void debug_print(int dbg_lvl, char *fmt, ...) { format_string(fmt, /* how do I pass all the arguments from '...'? */); fprintf(stdout, fmt); }

Answer 1

The caller initializes the va_list and passes it.

Specifically, the calling function does va_start( valist, x);

and then passes it to the helper function.

The helper function must CLOSE it off by doing va_end(valist);

or else you have a memory leak;

Also the helper function(s) must be declared AHEAD of the caller, or else

the compiler complains, unless you do a forward reference (or put them in a header)

Here is an example:

#include <stdio.h>

#include <stdarg.h>

double average(int num,va_list valist)

{

double sum = 0.0;

int i;

/* accesses all the arguments assigned to valist */

for (i = 0; i < num; i++)

{

sum += va_arg(valist, int); //updates running total

}

/* cleans memory reserved for valist */

va_end(valist);

return sum/num; //returns the average

}

double max(int num, va_list valist)

{

double max=0.0;

int i;

double x;

max = -32767;

for (i=0; i<num; i++)

{

x = va_arg(valist,int);

if (x>max) { max = x; }

}

va_end(valist);

return max;

}

double min(int num, va_list valist)

{

double min=0.0;

int i;

double x;

min = 32768;

for (i=0; i<num; i++)

{

x = va_arg(valist,int);

if (x<min) { min = x; }

}

va_end(valist);

return min;

}

double driver ( int opFlag, int num,...)

{

va_list valist;

double dReturn = 0;

//initializes the variable argument list

va_start(valist, num);

switch (opFlag)

{

case 1: //average

{

dReturn = average(num,valist);

break;

}

case 2: //max

{

dReturn = max(num,valist);

break;

}

case 3: //min

{

dReturn = min(num,valist);

break;

}

default:

{

printf(" say what???? \n" );

break;

}

}//switch

return(dReturn);

}

int main()

{

printf("Average of 2, 3, 4, 5 = %f\n", driver(1,4, 2,3,4,5));

printf("Average of 5, 10, 15 = %f\n", driver(1,3, 5,10,15));

printf(" max of 4,8, 6, 12, 5 = %f\n", driver(2,5,4,8,6,12,5));

printf(" min of 4,8, 6, 12, 5 = %f\n", driver(3,5,4,8,6,12,5));

printf(" max of 2 , 3, 1 = %f\n", driver(2,3,2,3,1));

printf(" min of 2, 3, 1 = %f\n", driver(3,3,2,3,1));

}