How can I write functions that take a variable number of arguments? in C programming

How can I write functions that take a variable number of arguments?

Use <stdarg.h>. This defines some macros that let your program deal with variable numbers of arguments.

NOTE
The “variable arguments” functions used to be in a header file known as <varargs.h> or some such. Your compiler might or might not still have a file with that name; even if it does have the file now, it might not have it in the next release. Use <stdarg.h>.

There’s no portable way for a C function, with no constraints on what it might be passed, to know how many
arguments it might have gotten or what their types are. If a C function doesn’t take a fixed number of arguments (of fixed types), it needs some convention for what the arguments are. For example, the first argument to printf is a string, which indicates what the remaining arguments are:

printf(“Hello, world!\n”); /* no more arguments */
printf(“%s\n”, “Hello, world!”); /* one more string argument */
printf(“%s, %s\n”, “Hello”, “world!”); /* two more string arguments */
printf(“%s, %d\n”, “Hello”, 42); /* one string, one int */

Listing XII.3 shows a simple printf-like function. The first argument is the format; from the format string, the number and types of the remaining arguments can be determined. As with the real printf, if the format doesn’t match the rest of the arguments, the result is undefined. There’s no telling what your program will do then (but probably something bad).

A simple printf-like function.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdarg.h>
static char *
int2str(int n)
{
int minus = (n < 0);

static char buf[32];
char *p = &buf[31];
if (minus)
n = -n;
*p = ‘\0’;
do {
*--p = ‘0’ + n % 10;
n /= 10;
} while (n > 0);
if (minus)
*--p = ‘-’;
return p;
}
/*
* This is a simple printf-like function that handles only
* the format specifiers %%, %s, and %d.
*/
void
simplePrintf(const char *format, ...)
{
va_list ap; /* ap is our argument pointer. */
int i;
char *s;
/*
* Initialize ap to start with the argument
* after “format”
*/
va_start(ap, format);
for ( ; *format; format++) {
if (*format != ‘%’) {
putchar(*format);
continue;
}
switch (*++format) {
case ‘s’:
/* Get next argument (a char*) */
s = va_arg(ap, char *);
fputs(s, stdout);
break;
case ‘d’:
/* Get next argument (an int) */
i = va_arg(ap, int);
s = int2str(i);
fputs(s, stdout);
break;
case ‘\0’:
format--;
break;
default:
putchar(*format);
break;
}

}
/* Clean up varying arguments before returning */
va_end(ap);
}
void
main()
{
simplePrintf(“The %s tax rate is %d%%.\n”,
“sales”, 6);
}

Cross Reference:

XII.2: What header files do I need in order to define the standard library functions I use?