Basic and Advance C Question:

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How can I write a function that takes a format string and a variable number of arguments, like printf, and passes them to printf to do most of the work?

Use vprintf, vfprintf, or vsprintf. These routines are like their counterparts printf, fprintf, and sprintf, except that instead of a variable-length argument list, they accept a single va_list pointer.
As an example, here is an error function which prints an error message, preceded by the string ``error: '' and terminated with a newline:
#include <stdio.h>
#include <stdarg.h>

void error(const char *fmt, ...)
{
va_list argp;
fprintf(stderr, "error: ");
va_start(argp, fmt);
vfprintf(stderr, fmt, argp);
va_end(argp);
fprintf(stderr, "n"); }