Grazie Oregon l'ho riscritto seguendo i tuoi consigli.
Ora sembra funzionare, ma non capisco perchè quando inserisco il valore 343 mi restituisce il valore (Total) -18...... , perchè mi compare il segno "-".
Codice PHP:
#include <stdio.h>
#include <stdlib.h>
#include <cctype>
#includ#ifndef SIZE_MAX
#define SIZE_MAX ((size_t)-1)
#endif
#define BUFSIZE 100

int main()e <errno.h>

{     size_t q; /*quantiy*/
      char buff[BUFSIZE];
      int n = 0;
      char* end_ptr;
      bool flag;
      for(;;){
         size_t p = 10000000 ;/*price*/
         size_t t = 0;     /*total*/
         printf("%s","Suppose the article costs 10.000.000 E \n");
         printf("%s","Enter quantity: ");
         int char_count = 0;
         char ch;
         ch = getchar();
         if(ch == EOF )
               perror("NO INPUT");
         char_count = 0;
            while((ch != '\n')  &&  (char_count <BUFSIZE)) {
                flag = true;
                buff[char_count++] = ch;
                if(isdigit(ch)==0){
                   flag = false;
                }
                ch = getchar();
                if(ch == EOF )
                      perror("NO INPUT");
            }
            buff[char_count] = 0x00;      /* null terminate buffer */
            if(flag == true){
                    errno = 0;
                    q = (int) strtoul(buff,NULL,0);
                    if (ERANGE == errno) {
                               if (puts("number out of range\n") == EOF) {
                                }
                    }
                    else if (end_ptr == buff) {
                         if (puts("not valid numeric input\n") == EOF) {
                         }
                    }
                    else {
                         if(q==0){
                           return 0; /*with 0 we terminate*/
                         }
                         if(q>0 && SIZE_MAX/p>=q){
                           t = q*p;
                           printf("%s %d","Total:",t);
                           printf("%s %s","\n","\n");
                         }
                         else{
                           printf("%s", "TOO BIG \n");
                         }
                    }
           }
      }
      system("PAUSE");
      return 0;
}