*nux编程的108种奇淫巧计-5(rbp寄存器)

昨天在家浏览了纽约大学的一个课件，找到一个例子，很有趣，我稍微修改了一下，支持32位和64位环境：如下。

以后该系列采用这样的模式，首篇只贴代码，续篇进行解释和分析，期间希望读者朋友能自己做实验。

理解该代码就算对rbp，rsp寄存器以及一些调用有了直接的接触。

里面有两个奇怪的数字17和23，这是为什么呢？续篇给予解答。

编译方法：

64位环境：

g++ -g test.cpp -o test_64 //64位代码

g++ -g -m32 test.cpp -o test_32 //32位代码

32位环境：

g++ -g test.cpp -o test_32

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/time.h>

int done = 0;

int thread1( void )
{
printf( "Hello from thread1 \n" );
while ( !done );
printf( "\nThread1 is quitting\n" );
exit(0);
}

int thread2( void )
{
printf( "Hello from thread2 \n" );
while ( !done );
printf( "\nThread2 is quitting\n" );
exit(0);
}

int thread3( void )
{
printf( "Hello from thread3 \n" );
while ( !done );
printf( "\nThread3 is quitting\n" );
exit(0);
}

int thread4( void )
{
printf( "Hello from thread4 \n" );
while ( !done );
printf( "\nThread4 is quitting\n" );
exit(0);
}

typedef int (*THREAD)(void);

THREAD ready[] = { thread1, thread2, thread3, thread4 };
const int thread_count = sizeof( ready )/sizeof( THREAD );

int next_thread = 0; // array-index for 'ready' list

void upon_signal( int signum )
{
unsigned long *tos; // this is used for addressing the stack-frame
#if defined(__i386__)
asm(" movl %%ebp, %0 " : "=m" (tos) ); // initializes frame-pointer
tos[17] = (unsigned long)ready[ next_thread ];
#elif defined(__x86_64__)

asm(" mov %%rbp, %0 " : "=m" (tos) ); // initializes frame-pointer
tos[23] = (unsigned long)ready[ next_thread ];
#endif
// This was useful during development -- to display the stack elements
//for (int i = 0; i < 30; i++) printf( "before:tos[%d]=%16X \n", i, tos[i] );

next_thread = ( 1 + next_thread ) % thread_count; // "circular" array

if ( signum == SIGINT ) done = 1; // to terminate on <CONTROL>-C
}

int main( int argc, char **argv )
{
signal( SIGINT, upon_signal );

struct itimerval it;
it.it_value.tv_sec = 1;
it.it_value.tv_usec = 0;
it.it_interval.tv_sec = 0;
it.it_interval.tv_usec = 200000;
setitimer( ITIMER_REAL, &it, NULL );
signal( SIGALRM, upon_signal );

while ( !done );

printf( "\nQuitting\n" );
}