确定 python 子进程分段是否错误
问题描述
我正在编写一个对学生程序进行评分的程序，我相信您可以想象，它们有时会出现分段错误.我遇到的问题是，当学生编程分段错误时，没有迹象表明发生了什么.
I am writing a program that grades student programs, and as I am sure you can imagine, they sometimes segmentation fault. The problem I am having is that when the student programs segmentation fault, there is no indication that is what happened.
proc = subprocess.Popen(student_command,
stdout=subprocess.PIPE,
stderr=subprocess.PIPE)
self.stdout, self.stderr = proc.communicate()
self.returncode = proc.returncode
我从 subprocess 中提取 stderr、stdout 和返回码，但如果程序分段出错，stderr 为空，stdout 为空，返回码为-11.现在我可以查找 -11 退出代码并假设如果这是返回代码，则存在分段错误，但也没有什么可以阻止学生的代码将 -11 作为返回代码仅仅因为学生感觉想要返回-11.
I pick up the stderr, stdout, and the return code from the subprocess, but if the program segmentation faults, stderr is empty, stdout is empty, and the return code is -11. Now I could look for the -11 exit code and assume that if that is the return code there was a segmentation fault, but there is also nothing to prevent a student's code from having -11 as a return code just because the student felt like returning -11.
您如何判断子进程分段是否出错，而不是仅仅感觉返回 -11?我不太关心 stderr 和 stdout 中的内容，为此我看到了许多帖子，包括 this 处理拾取输出，但我不太在意输出，尽管获得 "分段错误"从 stderr 中取出字符串，但我真正需要的是一种明确说明子进程发生了什么的方法.
How do you tell if a subprocess segmentation faults, as opposed to just feeling like returning -11? I don't really care all that much about what is in stderr and stdout, and to that end have seen a number of posts including this that deal with picking up the output, but I don't care all that much about the output, although it would be nice to get the "Segmentation Fault" string out of stderr, but what I really need is a way to definitively tell what happened to the subprocess.
解决方案
事实上，在 UNIX 上，尝试返回 -11 的进程通常最终会返回一个正整数.这是因为 wait 系列函数的返回状态实际上是一组位域，其中一个域用于结束进程的信号，另一个域用于返回值.Python 从这些位域解码 wait 返回值.
Well, in fact, on UNIX, a process that attempts to return -11 will usually end up returning a positive integer instead. This is because the return status from the wait series of functions is actually a set of bitfields, with a field for the signal that ended the process and a separate field for the return value. Python decodes the wait return value from these bitfields.
在大多数系统上，这些字段是无符号的，大小为 8 位，因此您可能会看到如下内容:
On most systems, these fields are unsigned and 8 bits in size, so you will probably see something like this:
>>> import subprocess
>>> subprocess.Popen(['python','-c','import os; os.kill(os.getpid(),11)']).wait()
-11
>>> subprocess.Popen(['python','-c','exit(-11)']).wait()
245
在前一种情况下，进程segfaults"(通过使用 SIGSEGV 杀死自己)，因此 wait 返回 -11.在后一种情况下，进程以 -11 的返回码退出，结果 wait 值为 245 (256-11).因此，您可以放心，来自 wait 的任何负返回值都必须代表致命信号，而不是正常返回.但请注意，这些进程可能会杀死自己以伪造致命错误.
In the former case, the process "segfaults" (by killing itself with SIGSEGV), and so wait returns -11. In the latter case, the process exits with a return code of -11, and the resulting wait value is 245 (256-11). You can therefore rest assured that any negative return value from wait must represent a fatal signal, as opposed to a normal return. Note, though, that processes may kill themselves to fake a fatal error.
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