CSAPP Chapter3 3.7 Procedures

概述

过程其实是软件中一种很重要的抽象。就像我们在写代码时，一个函数，传进去入参和一个可选的返回值定义了某个功能。不同的语言过程的形式多样，比如函数、方法、子例程、等等。

机器对过程的实现提供了下面几个支持，为了方便大家理解，例如：过程P调用过程Q，Q执行后返回到P，这些动作包含如下一个或多个机制：

传递控制。在进入Q时，PC要设置为Q代码的起始地址，然后在返回时，要设置为P调用Q后那条指令的地址；
传递数据。P必须向Q提供一个或多个参数，Q必须向P提供一个返回值；
分配和释放内存。Q在执行时需要为局部变量分配空间，当返回时，销毁这些空间；

演示代码

main.c

#include <stdio.h>

void multstore(long, long, long *);

int main() {
	long d;
	multstore(2, 3, &d);
	printf("2 * 3 --> $ld\n", d);
	return 0;
} 

long mult2(long a, long b) {
	long s = a * b;
	return s;
}

mstore.c

long mult2(long, long);

void multstore(long x, long y, long *dest)
{
    long t = mult2(x, y);
    *dest = t;
}

执行gcc命令, 生成可执行文件prog(windows平台该文件为prog.exe)

gcc -Og -o prog main.c mstore.c

文件变成了131k，因为它不仅包含了两个过程的代码，还包含了用来启动和终止程序的代码，以及用来和操作系统交互的代码

现在我们反汇编prog(windows平台该文件为prog.exe)文件:
linux平台

objdump -d prog

windows平台

objdump -d prog.exe

NOTE:
3.10 Options That Control Optimization
-Og
Optimize debugging experience. -Og enables optimizations that do not interfere with debugging. It should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.

3.2 Options Controlling the Kind of Output
-o file
Place output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.