In APUE, the following example shows the meaning of nonblocking IO.

#include "apue.h"
#include <errno.h>
#include <fcntl.h>

char buf[500000];

int main(void) {
   int ntowrite, nwrite;
   char *ptr;
   ntowrite = read(STDIN_FILENO, buf, sizeof(buf));
   fprintf(stderr, "read %d bytesn", ntowrite);
   set_fl(STDOUT_FILENO, O_NONBLOCK); /* set nonblocking */

   ptr = buf;
   while (ntowrite > 0) {
      errno = 0;
      nwrite = write(STDOUT_FILENO, ptr, ntowrite);
      fprintf(stderr, "nwrite = %d, errno = %dn", nwrite, errno);
      
      if (nwrite > 0) {
         ptr += nwrite;
         ntowrite -= nwrite;
      }
   }
   clr_fl(STDOUT_FILENO, O_NONBLOCK); /* clear nonblocking */
   exit(0);
}

My question is, is it neccssary, to define ptr variable and update it or is it handled automatically by write system call?