code:trap1 - ikarishinjieva/unixV6-code-analyze-chs GitHub Wiki

2826

2827 /*

2828  * Call the system-entry routine f (out of the

2829  * sysent table). This is a subroutine for trap, and

2830  * not in-line, because if a signal occurs

2831  * during processing, an (abnormal) return is simulated from

2832  * the last caller to savu(qsav); if this took place

2833  * inside of trap, it wouldn’t have a chance to clean up.

2834  *

2835  * If this occurs, the return takes place without

2836  * clearing u_intflg; if it’s still set, trap

2837  * marks an error which means that a system

2838  * call (like read on a typewrite) got interrupted

2839  * by a signal.

2840  */

2841 trap1(f)

2842 int (*f)();

2843 {

2844

2845     u.u_intflg = 1;

• 置 u.u_intflg（系统调用 出错标志）
• 参看 后面的注释

2846     savu(u.u_qsav);

• 保存R5 R6到 u.u_qsav

2847     (*f)();

2848     u.u_intflg = 0;

• 清 u.u_intflg（系统调用 出错标志）

关于此处的说明

• 如果 系统调用处理函数 正常，则u.u_intflg=0
• 若 系统调用处理函数 运行中 收到信号
  • 检测信号 时 直接从u.u_qsav恢复R5,R6(sleep 2106)
  • 函数返回到trap（SP已经恢复）
  • 此时，u.u_intflg=1
  • 在 trap 2773 抛出错误

2849 }

2850 /* ------------------------- */

• 中断

• code:trap