系统级程序设计笔记(4)

Memory Layout and Allocation

前言

本篇博客整理了系统级程序设计课程中的lecture5.

待更新

static
库函数的详细定义
Memory Bugs

Several Uses of Memory

Static Allocation

Static Allocation
- happen at compile time and link time
- exist at a fixed address throughout the execution of the program

Static Allocation vs. C++ static Declarations

Static Allocation

//a statically allocated variable int my_var[128];
int my_var[128];
static bool my_var_initialized = false;

int my_fn(int x){
  if (my_var_initialized) return; 
  my_var_initialized = true; 
      
  for(int i=0;i<128;i++)
    my_var[i] = 0;
}

C++ static Declarations

//a statically allocated variable int my_var[128];
int my_var[128];

int my_fn(int x){
  static bool my_var_initialized = false;
      
  if (my_var_initialized) return; 
  my_var_initialized = true; 
      
  for(int i=0;i<128;i++)
    my_var[i] = 0;
}

static 变量，内存？？？
[简答题] 为什么要使用动态内存分配？
- Precautions For Static Allocation
- Good reasons to use D.A. of variables

Dynamic Allocation

A Function Call Using the Stack
- Activation Record or Stack Frame
- TOS = Top of Stack
- Frame = Activation Record Base
- PC = Program Counter

函数调用的过程，详见[深入理解C语言的函数调用过程] .

Eg:

   
int find(char *str, char *pat) {
  int i, j, str_max, pat_max;
  pat_max = strlen(pat);
  str_max = strlen(str) - pat_max; 
    
  for (i = 0; i < str_max; i++){
    for (j = 0; j < pat_max; j++){
      if (str[i + j] != pat[j]) break;
    }
    // Did loop complete? If so, we found a match.
    if (j == pat_max) return i; 
  }
    
  return -1; 
}

void main() {
  printf("find(\"this is a test\", \"is\") -> %d\n", find("this is a test", "is"));
  printf("find(\"this is a test\", \"IS\" -> %d\n", find("this is a test", "IS"));
  getchar();
}

P31 ？？？

Heap Allocation

Common Memory Bugs
- Forget to free memory
- Memory leak
- Dangling pointer problem
- C++ never tries to debug the above
Explicit vs. Implicit

Program Memory Layout

Heap memory management

库函数

<stdlib.h>
- malloc()
- calloc()
- realloc()
- free()
<unistd.h>
- sbrk()

Allocator

[简答题]

分配器的要求和目标

教材P590
碎片

教材P591
策略选择

教材P592
实例应用：隐式空闲链表
- 放置已分配的块
- 分割空闲块
- 获取额外的堆内存
- 合并空闲块
- 带边界标记的合并
实例应用：一个简单的分配器

Memory Bugs

Code Examples

size_t

无符号数
P94-96？？？

Off-by-One Bug

#define array_size 100
int *a = (int *) malloc(sizeof(int *) * array_size); 
for (int i = 0; i <= array_size; i++)
  a[i] = NULL;

Allocate too less memory

#define array_size 100
int *a = (int *) malloc(array_size);
a[99] = 0; // this overwrites memory beyond the block

Data will be overwritten due to Too Less Memory allocated

char *heapify_string(char *s) {
  int len = strlen(s);
  char *new_s = (char *) malloc(len); strcpy(new_s, s);
  return new_s;
}

Referencing a Pointer instead of the Object it points to

void dec_positive(int *a) {
  *a--; // decrement the integer
  if (*a < 0) *a = 0; // make sure a is positive 
}

Never return a Local Pointer

int *ptr_to_zero() {
  int i = 0;
  return &i; 
}

char q[] = "do not overflow"; char r[] = " memory";
char s[16];
strcpy(s, q);
strcat(s, r);

Eg
```
strcat("don't overflow", "memory");
```

Twice Free

Memory Leaks

Eg1

 void my_function(char *msg) {
   // allocate space for a string
   char *full_msg = (char *) malloc(strlen(msg) + 100); 
   strcpy(full_msg, "The following error was encountered: "); 
   strcat(full_msg, msg);
   if (!display(full_msg)) return;
   free(full_msg); 
}

Eg2

 typedef struct {
   char *name;
   int age;
   char *address; 
   int phone;
} Person;

void my_function() {
  Person *p = (Person *) malloc(sizeof(Person)); 
  p->name = (char *) malloc(128);
  p->address = (char *) malloc(128);
  free(p);
}

Eg3

指导书分章Unit3 练习2

实例应用

为C语言的动态内存分配添加出错预警.