调试意外终止的守护进程

Question

我正在 linux 上用 c 语言编写一个守护进程。它捕获信号 SIGHUP、SIGTERM、SIGINT 和 SIGQUIT，使用 syslog 记录它们并退出。如果它收到 SIGSEGV，它会转储核心。当这些发生时，一切都会按预期发生，但偶尔会退出......不会干净地退出，不会记录信号，也不会留下核心转储。我很难过，不知道如何调试问题。除了这些信号，它还能通过哪些方式退出？有没有明显的答案，我错过了什么？对于在守护进程中调试这种看似零星的问题，您还推荐了哪些其他调试实践？

Answer 1

如果您的守护程序正在使用网络套接字，则很可能是SIGPIPE - 如果您尝试写入已被另一端关闭的套接字（或管道），您会得到这个。请注意，即使您在写入之前检查套接字是否可写（例如使用select()），它也始终可以在检查和写入之间关闭。

Answer 2

您可以让守护进程的父进程留下并等待它，然后让父进程记录守护进程退出的原因（即，它是收到信号还是退出）。

Answer 3

将gdb附加到它

gdb -p <pid>

确保使用 -g 标志进行编译，并在退出后立即进行回溯。 祝你好运！

Answer 4

嗯，还有很多其他信号会导致它退出，当然包括SIGKILL，你将无能为力。基本上来自man 7 signal 的以下内容中的任何内容，其中Action 是Term 或Core（尽管后者至少会留下核心转储）：

   First the signals described in the original POSIX.1-1990 standard.

   Signal     Value     Action   Comment
   -------------------------------------------------------------------------
   SIGHUP        1       Term    Hangup detected on controlling terminal
                                 or death of controlling process
   SIGINT        2       Term    Interrupt from keyboard
   SIGQUIT       3       Core    Quit from keyboard
   SIGILL        4       Core    Illegal Instruction

   SIGABRT       6       Core    Abort signal from abort(3)
   SIGFPE        8       Core    Floating point exception
   SIGKILL       9       Term    Kill signal
   SIGSEGV      11       Core    Invalid memory reference
   SIGPIPE      13       Term    Broken pipe: write to pipe with no readers
   SIGALRM      14       Term    Timer signal from alarm(2)
   SIGTERM      15       Term    Termination signal
   SIGUSR1   30,10,16    Term    User-defined signal 1
   SIGUSR2   31,12,17    Term    User-defined signal 2
   SIGCHLD   20,17,18    Ign     Child stopped or terminated
   SIGCONT   19,18,25    Cont    Continue if stopped
   SIGSTOP   17,19,23    Stop    Stop process
   SIGTSTP   18,20,24    Stop    Stop typed at tty
   SIGTTIN   21,21,26    Stop    tty input for background process
   SIGTTOU   22,22,27    Stop    tty output for background process

   The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

   Next the signals not in the POSIX.1-1990 standard but described in SUSv2 and POSIX.1-2001.

   Signal       Value     Action   Comment
   -------------------------------------------------------------------------
   SIGBUS      10,7,10     Core    Bus error (bad memory access)
   SIGPOLL                 Term    Pollable event (Sys V). Synonym of SIGIO
   SIGPROF     27,27,29    Term    Profiling timer expired
   SIGSYS      12,-,12     Core    Bad argument to routine (SVr4)
   SIGTRAP        5        Core    Trace/breakpoint trap
   SIGURG      16,23,21    Ign     Urgent condition on socket (4.2BSD)
   SIGVTALRM   26,26,28    Term    Virtual alarm clock (4.2BSD)
   SIGXCPU     24,24,30    Core    CPU time limit exceeded (4.2BSD)
   SIGXFSZ     25,25,31    Core    File size limit exceeded (4.2BSD)

   Up to and including Linux 2.2, the default behaviour for SIGSYS, SIGXCPU, SIGXFSZ, and (on architectures other than SPARC
   and  MIPS) SIGBUS was to terminate the process (without a core dump).  (On some other Unices the default action for SIGX-
   CPU and SIGXFSZ is to terminate the process without a core dump.)  Linux 2.4 conforms to  the  POSIX.1-2001  requirements
   for these signals, terminating the process with a core dump.

   Next various other signals.

   Signal       Value     Action   Comment
   --------------------------------------------------------------------
   SIGIOT         6        Core    IOT trap. A synonym for SIGABRT
   SIGEMT       7,-,7      Term
   SIGSTKFLT    -,16,-     Term    Stack fault on coprocessor (unused)
   SIGIO       23,29,22    Term    I/O now possible (4.2BSD)
   SIGCLD       -,-,18     Ign     A synonym for SIGCHLD
   SIGPWR      29,30,19    Term    Power failure (System V)
   SIGINFO      29,-,-             A synonym for SIGPWR
   SIGLOST      -,-,-      Term    File lock lost
   SIGWINCH    28,28,20    Ign     Window resize signal (4.3BSD, Sun)
   SIGUNUSED    -,31,-     Term    Unused signal (will be SIGSYS)

Answer 5

shell 包装器可以捕获您的守护程序的退出状态。以下是它的工作原理：

$ ./waitstatus true
pid 1512: exit status 0 (success)

$ ./waitstatus false
pid 1514: exit status 1 (abnormal)

$ ./waitstatus perl -e 'exit 21'
pid 1518: exit status 21 (abnormal)

$ ./waitstatus perl -e 'kill TERM => $$'
pid 1520: terminated on signal 15

$ ./waitstatus no-such-command
pid 1522: command not found: no-such-command

$ ./waitstatus /sbin/EACCES.contrived
pid 1524: command not executable: /sbin/EACCES.contrived

... 以下是它的实现方式：

$ cat ./waitstatus
#! /bin/bash

"$@" &
PID=$!

wait $PID
STATUS=$?

if   [ $STATUS -gt 128 ]; then
  MSG="terminated on signal $(( $STATUS - 128 ))";
else
  case $STATUS in
    0)
      MSG="exit status 0 (success)"
      ;;
    127)
      MSG="command not found: $1"
      ;;
    126)
      MSG="command not executable: $1"
      ;;
    *)
      MSG="exit status $STATUS (abnormal)"
      ;;
  esac
fi

echo "pid $PID: $MSG"
exit $STATUS

您可能希望将最后一行 echo 更改为调用系统的 logger 命令，例如，将状态消息定向到 syslog。