C++ CSAPP微型shell 实验室：卡在sigprocmask

如果我点击"跳过"，它会继续运行，永远不会停止.

我复制了那个.所以让我们来看看发生了什么.

gdb -q ./tsh
(gdb) break tsh.c:191
(gdb) b tsh.c:191
Breakpoint 1 at 0x40156a: file tsh.c, line 191.

(gdb) run
Starting program: /tmp/tsh
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
tsh> aaa
[Detaching after fork from child process 182]

aaa: command not found
Breakpoint 1, eval (cmdline=0x7fffffffd930 "aaa\n") at tsh.c:191
191         sigprocmask(SIG_SETMASK, &prev, NULL);
(gdb) n

在这一点上，一切都是悬而未决的，所以我们必须被困在sigprocmask，对吗？

实际上，我们不是.

^C
Program received signal SIGINT, Interrupt.
0x00007ffff7eb5c37 in __GI___wait4 (pid=-1, stat_loc=0x7fffffffcd5c, options=3, usage=0x0)
    at ../sysdeps/unix/sysv/linux/wait4.c:30
30        return SYSCALL_CANCEL (wait4, pid, stat_loc, options, usage);
(gdb) bt
#0  0x00007ffff7eb5c37 in __GI___wait4 (pid=-1, stat_loc=0x7fffffffcd5c, options=3, usage=0x0)
    at ../sysdeps/unix/sysv/linux/wait4.c:30
#1  0x0000000000401a02 in sigchld_handler (sig=17) at tsh.c:383
#2  <signal handler called>
#3  __GI___pthread_sigmask (how=2, newmask=<optimized out>, oldmask=0x0) at pthread_sigmask.c:43
#4  0x00007ffff7e18d8d in __GI___sigprocmask (how=<optimized out>, set=<optimized out>, oset=<optimized out>)
    at ../sysdeps/unix/sysv/linux/sigprocmask.c:25
#5  0x0000000000401583 in eval (cmdline=0x7fffffffd930 "aaa\n") at tsh.c:191
#6  0x000000000040142d in main (argc=1, argv=0x7fffffffde68) at tsh.c:149

现在我们来看看actually是怎么回事.sigprocmask解锁SIGCHLD，这导致immediate传递将要返回信号just before sigprocmask.这进而调用sigchld_handler，而sigchld_handler在一个永无止境的循环中调用waitpid.

为什么循环不终止？因为代码期望waitpid在没有子级的情况下返回0，但这是不正确的:在这种情况下，waitpid返回-1.

下面的修复使tsh的工作，因为人们可能会期望:

 diff -u tsh.c.orig tsh.c
--- tsh.c.orig  2024-01-20 21:42:47.915401415 -0800
+++ tsh.c       2024-01-20 21:43:20.145996657 -0800
@@ -383,7 +383,7 @@
         pid = waitpid(-1, &status, WNOHANG | WUNTRACED);

         // 如果没有僵尸进程，则退出
-        if (pid == 0)
+        if (pid == 0 || pid == -1)
             return;

         // 如果子进程终止导致waitpid从阻塞中恢复

./tsh
tsh> aaa
aaa: command not found
tsh> bbb
bbb: command not found
tsh> 

以下是来自Linux man page的相关文本:

如果在Options中设置了WNOHANG来调用waitpid()，则它至少有一个由pid指定的子进程的状态不可用，并且状态对于任何由PID指定的进程都不可用，则返回0.否则，返回-1