char buffer[1024];

  ....(тут recv  через сокет), в буфере при отладке "С:/6.pro"
  FILE *in = fopen(buffer, "r");
  if(in == NULL) printf("Error opening file");
  else printf("File was successfully opened");

fgets(buff, BUF_SIZE, stdin);
 n = strlen(buff);
 if (buff[n-1] == 'n') --n;
 send(my_socket, buff, n, 0);

char *str;
...

if(in == NULL) {
 str = strerrno(errno);
 printf("Error opening file - %s",str);
}