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我有一個Python v3.4應用程序，它使用check_output()來調用一個叫做fork()的C++應用程序，原始進程退出並繼續子進程。看起來check_output()也在等待子進程，而不是在主進程返回時返回，並表示守護進程已成功啓動。調用守護進程時爲什麼不返回Python check_output（）？
我是否需要改變我如何在C++中調用fork()或者Python check_output()調用需要以某種方式被告知只等待父進程退出？我是否需要在C++中使用第二個fork（）作爲described here？
這裏被剝離下來的Python表現出的問題：
#! /usr/local/bin/python3
import logging
import argparse
from subprocess import CalledProcessError, check_output, STDOUT
ARGS = ["[email protected]:23021:"]
if __name__ == "__main__":
parser = argparse.ArgumentParser(
description="Try launching subprocess")
parser.add_argument("exec", type=str, help="the exec to run")
args = parser.parse_args()
logging.basicConfig(level=logging.INFO,
format='%(asctime)s - %(message)s',
datefmt='%Y-%m-%d %H:%M:%S')
cmd_list = [args.exec] + ARGS
logging.info(str(cmd_list))
try:
output = check_output(cmd_list,
stderr=STDOUT,
universal_newlines=True)
logging.info("Exec OK with output:")
logging.info(output)
except CalledProcessError as e:
logging.info("Exec Not OK with output:")
logging.info(str(e))
這裏是C++代碼來守護進程的C++應用程序：
void
daemonize()
{
// This routine backgrounds the process to run as a daemon.
// Returns to caller only if we are the child, otherwise exits normally.
if (getppid() == 1) {
return; // Leave if we're already a daemon
}
// Create the backgrounded child process.
const pid_t parent = getpid();
const pid_t pid = fork();
CSysParamAccess param;
const string progName(param.getProgramKindName());
::close(STDIN_FILENO);
::close(STDOUT_FILENO);
::close(STDERR_FILENO);
if (pid < 0) {
cerr << "Error: " << progName << " failed to fork server. Aborting."
<< endl; // inform the client of the failure
exit(appExit::failForkChild); // Error. No child created.
} else if (pid > 0) {
// We're in the parent. Optionally print the child's pid, then exit.
if (param.getDebug()) {
clog << "Successful fork. The Application server's (" << progName
<< ") pid is: " << pid << "(self) from parent " << parent << endl;
}
::close(STDIN_FILENO);
::close(STDOUT_FILENO);
::close(STDERR_FILENO);
exit(appExit::normal);
}
::close(STDIN_FILENO);
::close(STDOUT_FILENO);
::close(STDERR_FILENO);
// Here only in the child (daemon).
if (-1 == setsid()) { // Get a new process group
cerr << "Error: Failed to become session leader while daemonising - errno: "
<< errno;
exit(appExit::failForkChild); // Error. Child failed.
}
signal(SIGHUP, SIG_IGN); // Per example.
// Fork again, allowing the parent process to terminate.
const pid_t midParent = getpid();
const pid_t grandChildPid = fork();
if (grandChildPid < 0) {
cerr << "Error: Failed to fork while daemonising - errno: " << errno;
exit(appExit::failForkChild); // Error. GrandChild failed.
} else if (grandChildPid > 0) {
// We're in the parent. Optionally print the grandchild's pid, then exit.
if (param.getDebug()) {
clog << "Successful second fork. The Application server's (" << progName
<< ") pid is: " << grandChildPid << "(self) from parent "
<< midParent << endl;
}
::close(STDIN_FILENO);
::close(STDOUT_FILENO);
::close(STDERR_FILENO);
exit(appExit::normal);
}
// Here only in the grandchild (daemon).
appGlobalSetSignalHandlers();
// Set the current working directory to the root directory.
if (chdir("/") == -1) {
cerr <<
"Error: Failed to change working directory while daemonising - errno:"
<< errno;
exit(appExit::failForkChild); // Error. GrandChild failed.
}
// Set the user file creation mask to zero.
umask(0);
//close(STDIN_FILENO); // Cannot close due to assertion in transfer.cpp
// Theoretically, we would reopen stderr and stdout using the log file.
::close(STDIN_FILENO);
::close(STDOUT_FILENO);
::close(STDERR_FILENO);
// We only return here if we're the grandchild process, the Application
// server. The summoner exited in daemonize().
clog << "Application " << param.getProgramKindName()
<< " (" << appGlobalProgramName() << ") successfully started." << endl;
}
它與回聲時調用和失敗用我的C++應用程序：
> stuckfork.py echo
2016-02-05 10:17:34 - ['echo', '[email protected]:23021:']
2016-02-05 10:17:34 - Exec OK with output:
2016-02-05 10:17:34 - [email protected]:23021:
> stuckfork.py flumep
2016-02-05 10:17:53 - ['flumep', '[email protected]:23021:']
C-c Traceback (most recent call last):
File "/home/user/Bin/Bin/stuckfork.py", line 26, in <module>
universal_newlines=True)
File "/usr/local/lib/python3.4/subprocess.py", line 609, in check_output
output, unused_err = process.communicate(inputdata, timeout=timeout)
File "/usr/local/lib/python3.4/subprocess.py", line 947, in communicate
stdout = _eintr_retry_call(self.stdout.read)
File "/usr/local/lib/python3.4/subprocess.py", line 491, in _eintr_retry_call
return func(*args)
KeyboardInterrupt
>
我已經將問題縮小到我的一個C++靜態構造函數正在做的事情，導致啓動過程失效，這就是爲什麼Python仍在等待。現在劃分看哪一個。