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:\n");
fprintf(stdout, "globval = %d,"
"autoval = %d,"
"regival = %d,"
"volaval = %d,"
"statval = %d\n", 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:\n");
fprintf(stdout, "globval = %d,"
"autoval = %d,"
"regival = %d,"
"volaval = %d,"
"statval = %d\n",
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