1.1.3.1 malloc

• That is a long story. In C language, if you forget a header file, what will happen? (I have add answer to my evernote). In one word, It will(c99) or will not produce(c89) warning, and compiler will assume a "implicit int" rule. That is to say, It will think that the function without prototype (declaring in the header file) just return int type.
• continue: if you have code below: you comment stdlib.h. then compiler will assume that malloc return a int. This code will run on 32 bit computer, because int and int* have same length. but on some 64 bit computer, int* is 64 bit and int is 32 bit. so half data will be lost, and you code will crash. And if you add (int*) cast, it will suppress the warning message.(You kill the only clue, it’s bad).
   // include "stdlib.h" 
  // with cast, it will NOT produce a (missing protype)warning. 
  int *sieve = (int *)malloc(sizeof(int)*length); 
   
  // without cast, will produce a warning. 
  int *sieve = malloc(sizeof(int)*length);
• continue: So don’t recommend to use (int*) cast at all. Next question is that malloc actually return void*. Can void* automatically implicit be cast to int* or other pointer type in assignment? the answer is YES.
• continue: In C language:
  #define NULL ( (void*) 0) 
  int *p = NULL;  //legal 
  FILE * f = NULL;  //legal
• continue: In C++, int *p = (void *) 0 is not legal any more.(C++ is type safe language). So in C++ NULL is literal 0. But It produce another ambiguity problem. So In C++11, we define nullptr to resolve this problem.
  #define NULL 0 
  int *p = NULL //legal 
  -------------------------------------- 
  f(int i) 
  f(int * p); 
   
  f(NULL);  //which one will be called. Answer is f(int i). 
  f(nullptr) //will call f(int *p)