[C] print Fatal Error message

#include <stdio.h>      /* printf */
#include <stdarg.h>     /* va_list, va_start, va_arg, va_end */
#include <string.h>     /* strdup - duplicate a string */
#include <stdlib.h>     /* exit(0) */
#include <errno.h>
#include <unistd.h>     /* STDOUT_FILENO */


#define MAXLINE 4096

/* Print a message and return to caller.
 * Caller specifies "errnoflag". */

static void err_doit(int errnoflag, const char *fmt, va_list ap) {

        int             errno_save;
        char    buf[MAXLINE];

        errno_save = errno;             /* value caller might want printed */
        vsprintf(buf, fmt, ap);
        if (errnoflag)
                sprintf(buf+strlen(buf), ": %s", strerror(errno_save));
        strcat(buf, "\n");
        fflush(stdout);         /* in case stdout and stderr are the same */
        fputs(buf, stderr);
        fflush(NULL);           /* flushes all stdio output streams */
        return;

}


/* Fatal error related to a system call.
 * Print a message and terminate. */

void err_sys(const char *fmt, ...) {

        va_list         ap;

        va_start(ap, fmt);
        err_doit(0, fmt, ap);
        va_end(ap);
        exit(1);
}

int main ()
{
  err_sys("===============================================================================\n");
  return 0;
}

Result

err_doit(1, fmt, ap);
===============================================================================
: Success


err_doit(0, fmt, ap);
===============================================================================

Reference:
1. [OpenSource] ipband-0.8.1