C/C++: Functions with variable numbers of arguments

A function usually takes an immutable number of arguments whose types are fixed when its code is compiled. But sometimes it is desirable to implement a function where the number of arguments is not constant or not known beforehand, when the function is written.

For example, the printf function is a special type of routine that takes a variable number of arguments. The declaration requires a special syntax to indicate the fact that beyond a certain argument, the number and type of the parameters cannot be checked at compile time. Instead, the number and type of the parameters has to be computed at run-time. Using ellipsis in the signature denotes a variable argument list.

eg., void printf(const char* format, ... /* variable args */);

Three special macros va_start, va_arg & va_end are defined in sys/stdarg.h, to process the variable arguments.

void va_start(va_list pvar, void name);

va_start() is called to initialize pvar to the beginning of the variable argument list. va_start() must be invoked before any access to the unnamed arguments. The parameter name is the identifier of the rightmost parameter in the variable parameter list in the function definition (the one just before the ellipses ie., ", ...").

(type *) va_arg(va_list pvar, type);

pvar is a pointer to variable argument list; and type is the type name of the next argument to be returned.

va_arg() expands to an expression that has the type and value of the next argument in the call. The parameter pvar must be initialized by va_start(). Each invocation of va_arg() modifies pvar so that the values of successive arguments are returned in turn. The parameter type is the type name of the next argument to be returned.

void va_end(va_list pvar);

pvar is a pointer to variable argument list

The va_end() macro is used to clean up. It invalidates pvar
for use (unless va_start() is invoked again).

Example:
The following example creates a variable length argument routine to return the maximum of the data being passed to the routine, maximum.

% more maximum.c 
#include <stdio.h>
#include <stdarg.h>

float maximum (const char *format, ...) {
        int maxint = 0, temp_maxint = 0;
        float maxfloat = 0.0, temp_maxfloat = 0.0;

        va_list ap; /* declare an argument pointer to a variable arg list */
        va_start(ap, format); /* initialize arg pointer using last known arg */

        const char *p = format;

        while (*p) {
            switch(*p) {
                case 'i':
                        temp_maxint = va_arg(ap, int);
                        if (temp_maxint > maxint) {
                                maxint = temp_maxint;
                        }
                        ++p;
                        continue;

                case 'f':
                        temp_maxfloat = va_arg(ap, float);
                        if (temp_maxfloat > maxfloat) {
                                maxfloat = temp_maxfloat;
                        }
                        ++p;
                        continue;
                        
                default:
                        ++p;
                        continue;
           } // switch
        } // while
        
        va_end(ap); /* restore any special stack manipulations */

        if (*format == 'i') {
                return (maxint);
        } else if (*format == 'f') {
                return (maxfloat);
        }

        return (0);
} // maximum


int main() {
        float max;

        max = maximum("iiiii", 431, 276, 3982, 5, 54);
        printf("\nMaximum of 431, 276, 3982, 5, 54 = %d", (int)max);

        max = maximum("fff", 12.2, 1.3, 54.43);
        printf("\nMaximum of 12.2, 1.3, 54.43 = %f", max);
        
        max = maximum("iiiiiiiiii", 43, 45, 65, 12, 32, 78, 9, 28, 14, 44);
        printf("\nMaximum of 43, 45, 65, 12, 32, 78, 9, 28, 14, 44 = %d", (int)max);

        return (0);
} // main

% CC -o maximum maximum.c

% ./maximum
Maximum of 431, 276, 3982, 5, 54 = 3982
Maximum of 12.2, 1.3, 54.43 = 54.430000
Maximum of 43, 45, 65, 12, 32, 78, 9, 28, 14, 44 = 78

The caller knows the number of arguments pushed onto the stack, but the called routine maximum does not, so it has to compute them at run-time. The va_start macro computes the (saved frame pointer + offset) value following the argument past the last known argument (ie., const char* format)). The rest of the arguments are then computed by calling va_arg, where the argument to va_arg is some (saved frame pointer + offset) value.

Reference:
Man page of va_start, va_arg, va_end