Printf malloc about raspi3-tutorial HOT 3 CLOSED

// To allocate a sufficiently large string and print into it (code correct for both glibc 2.0
// and glibc 2.1):

       #include <stdio.h>
       #include <stdlib.h>
       #include <stdarg.h>

       char *
       make_message(const char *fmt, ...)
       {
           int size = 0;
           char *p = NULL;
           va_list ap;

           /* Determine required size */

           va_start(ap, fmt);
           size = vsnprintf(p, size, fmt, ap);
           va_end(ap);

           if (size < 0)
               return NULL;

           size++;             /* For '\0' */
           p = malloc(size);
           if (p == NULL)
               return NULL;

           va_start(ap, fmt);
           size = vsnprintf(p, size, fmt, ap);
           va_end(ap);

           if (size < 0) {
               free(p);
               return NULL;
           }

           return p;
       }