APUE 学习笔记(五) 进程环境

1.main函数

C程序总是从main函数开始执行，当内核执行C程序时，在调用main函数之前先调用exec函数从内核获取命令行参数和环境变量值

2.进程终止

正常终止：

(1)在main函数内执行return语句

(2)调用exit

(3)最后一个线程从其启动例程返回

(4)最后一个线程调用pthread_exit

异常中止：

(1)调用abort

(2)接收到一个信号并终止(内存越界或除0)

(3)最后一个线程对取消请求作出响应

内核使程序执行的唯一方法就是调用exec函数，进程自愿终止的唯一方法就是调用exit

3.C程序的存储空间布局

a.out中还有其他类型的段，比如符号表段、调试信息段、动态共享库段

size命令查看正文段、数据段和bss(未初始化数据段)的长度。如 size /usr/bin/cc

4. setjmp longjmp

  #include <unistd.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <setjmp.h>
  
  static jmp_buf jmpbuf;
  static int globval = 1;
  
  static void func2(void)
  {
      longjmp(jmpbuf, 1); 
  }
  
  static void func1(int i, int j, int k, int l)
  {
      fprintf(stdout, "in func1:
");
      fprintf(stdout, "globval = %d,"
              "autoval = %d,"
              "regival = %d,"
              "volaval = %d,"
              "statval = %d
", globval, i, j, k , l); 
      func2();
  }
int main(int argc, char* argv[])
  {
      int autoval = 2;
      register int regival = 3;
      volatile int volaval = 4;
      static   int statval = 5;
      if (setjmp(jmpbuf) != 0) {
          fprintf(stdout, "after longjmp:
");
          fprintf(stdout, "globval = %d,"
                  "autoval = %d,"
                  "regival = %d,"
                  "volaval = %d,"
                  "statval = %d
",
                  globval, autoval, regival, volaval, statval);
          exit(0);
      }
  
      globval = 95;
      autoval = 96;
      regival = 97;
      volaval = 98;
      statval = 99;
  
      func1(autoval, regival, volaval, statval);
      exit(0);
  }

编译时不进行优化，结果如下：

in func1:
globval = 95,autoval = 96,regival = 97,volaval = 98,statval = 99
after longjmp:
globval = 95,autoval = 96,regival = 97,volaval = 98,statval = 99

编译时进行全部优化，结果如下：

in func1:
globval = 95,autoval = 96,regival = 97,volaval = 98,statval = 99
after longjmp:
globval = 95,autoval = 2,regival = 3,volaval = 98,statval = 99

我们可以看到：全局、静态、易失变量不受优化的影响，在调用longjmp之后，他们的值是最近所呈现的值

未优化时，所有5个变量都存放在存储器中，而进行优化之后，autoval 和 regival 都存放在寄存器中，而 volatile变量仍存放在存储器中

对于 volatile 修饰符用法，这里有一篇很好的文章：http://hedengcheng.com/?p=725