1. Interrupted system call
2. Signals are not queued
3. The child process exits
Because the signal is not queued, when multiple child processes exit at the same time, the signal processing function may only be executed once (because during the execution of the processing function, several other exit signals arrive, but they cannot be queued and discarded), so only recycle at this time If the resources of one child process are removed, several other child processes have become zombie processes
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