程序空间(Program memory)

The computer program memory is organized into the following:

Data Segment (Data + BSS + Heap)
Stack
Code segment

Data

The data area contains global and static variables used by the program that are explicitly initialized with a non-zero (or non-NULL) value. This segment can be further classified into a read-only area and read-write area. For instance, the string defined by char s[] = "hello world" in C and a C statement like int debug=1 outside the "main" would be stored in initialized read-write area. And a C statement like const char* string = "hello world" makes the string literal "hello world" to be stored in initialized read-only area and the character pointer variable string in initialized read-write area. Ex: both static int i = 10 and global int i = 10 will be stored in the data segment.

data段包含程序使用的全局变量和静态变量(变量明确的使用非0或非NULL初始化).data段可以进一步的分类到read-only区和read-write区.

例如在C语言的"main"函数之外通过char s[] = "hello world"定义的字符串和通过int debug=1声明的变量debug将会存储在初始化的read-write区。

类似const char* string = "hello world"这样的声明,字符串"hello world"存储在初始化的read-only区, 指针变量string存储在read-write区.

BSS

The BSS segment, also known as uninitialized data, is usually adjacent to the data segment and contains all global variables and static variables that are initialized to zero or do not have explicit initialization in source code. For instance a variable declared static int i; would be contained in the BSS segment.

BSS段也被称为未初始化数据，它通常邻进data段，包含所有的全局变量和静态变量(变量被初始化为0 或者没有在代码中进行显示初始化).

例如使用static int i;语句声明的变量i会被包含在BSS段.

Heap

The heap area commonly begins at the end of the .bss and .data segments and grows to larger addresses from there. The heap area is managed by malloc, realloc, and free, which may use the brk and sbrk system calls to adjust its size (note that the use of brk/sbrk and a single "heap area" is not required to fulfill the contract of malloc/realloc/free; they may also be implemented using mmap to reserve potentially non-contiguous regions of virtual memory into the process' virtual address space). The heap area is shared by all threads, shared libraries, and dynamically loaded modules in a process.

Heap(堆)段通常在.bss段和.data段的结尾处开始,并且开始向打的地址增长.

Heap段是使用函数malloc\realloc\free等函数进行管理,这些函数可能会使用brk\sbrk系统调用调整Heap段的大小.

(需要注意的是一个heap区不是只能使用brk/sbrk来调整大小, 也可以使用mmap保留潜在的不连续区域的虚拟内存到进行的虚拟地址空间)

Heap段是一个进程中所有线程、共享库、动态加载的模块所共享的。

Stack

The stack area contains the program stack, a LIFO structure, typically located in the higher parts of memory. A "stack pointer" register tracks the top of the stack; it is adjusted each time a value is "pushed" onto the stack. The set of values pushed for one function call is termed a "stack frame". A stack frame consists at minimum of a return address. Automatic variables are also allocated on the stack.

The stack area traditionally adjoined the heap area and they grew towards each other; when the stack pointer met the heap pointer, free memory was exhausted. With large address spaces and virtual memory techniques they tend to be placed more freely, but they still typically grow in opposite directions. On the standard PC x86 architecture the stack grows toward address zero, meaning that more recent items, deeper in the call chain, are at numerically lower addresses and closer to the heap. On some other architectures it grows the opposite direction.

栈段包含了程序的栈,他是一个LIFO(后进先出)结构,通常位于内存较高的部分.一个栈指针寄存器保存着栈顶位置。

每次有值压栈，栈指针寄存器就会进行调整。为了进行函数调用而压栈一组数值这种方式叫做stack frame（栈帧：栈帧也叫过程活动记录，是编译器用来实现函数调用的一种数据结构，从逻辑上讲，栈帧就是一个函数执行的环境：函数参数、函数的局部变量、函数执行完后返回到哪里等等。）

一个栈帧最少也会包含一个返回地址。自动变量也在堆上进行分配。

栈段习惯上邻接heap 段，他们向着对方的方向增长。当栈指针和堆指针相遇就意味着free memory耗尽了。

随着地址空间的增大和虚拟内存技术的出现，趋向于更自由的安排堆和栈的存放放置，但是他们还是向相反的方向增长。

在标准x86结构的PC上，栈向0增长，意味着：越是新的项，在调用链中越深，地址越低，更靠近heap段。

在其他结构中栈朝着相反的方向增长。

Code

In computing, a code segment, also known as a text segment or simply as text, is one of the sections of a program in an object file or in memory, which contains executableinstructions.

It has a fixed size and is usually read-only. If the text section is not read-only, then the particular architecture allows self-modifying code. Fixed-position or position independent code may be shared in memory by several processes in segmented or paged memory systems.

As a memory region, a code segment may be placed below the heap or stack in order to prevent heap and stack overflows from overwriting it.

在电脑方面,code段也被称为text段,或者只是简单的称为text.

code段是目标文件或内存中程序的其中一个section(节),其中包含了可执行的命令.

code段有固定的大小,并且通常是read-only的.

如果code段不是read-only的,那么这个特殊的体系结构中允许self-modifying(自修改)代码.

固定位置的代码或者与位置无关的代码可以在段存储内存系统和页存储内存系统中被几个进程共享.

//example.cpp

#include <string.h> //for strcpy

#include <stdlib.h> //for malloc

int a = ; //BSS

char *p1;  //BSS

int b = ; //Data

int main(void)

{

    int c;            //Stack

    char s[] = "abc"; //Stack

    char *p2;         //Stack

    char *p3 = "";  //123456'\0' Data段，p3在Stack上。

    static int d = ;     //BSS

    p1 = (char *)malloc(); //Heap

    strcpy(p1, "");    //123456'\0'Data段，编译器可能会将它与p3所指向的"123456"优化成一个地方。

    free(p1);

    return ;

}

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参考资料：

http://bbs.csdn.net/topics/390737887

http://en.wikipedia.org/wiki/Data_segment

http://en.wikipedia.org/wiki/Code_segment