The following values apply only to long-running child processes that have been invoked by a long-running process through a synchronous interface.

下面这些值只用在长期运行的子过程，它们通过异步接口被长期运行的流程调用。

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ECHILD -- The wait() or waitpid() function tried to wait for a child process to exit, but all children have already exited.

ECHILD：wait() 或 waitpid() 函数尝试等待退出子进程，但所有子项都已经退出。

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