Manage signals and collect process state

A signal is first generated, may remain pending, and is eventually delivered or synchronously accepted according to its kind and the receiving state. For an ordinary standard signal, repeated generation while the same signal is already pending does not build an event counter. With that signal blocked throughout an interval, the pending indicator is either zero or one, even if a hundred requests arrive. This coalescing is appropriate for messages such as “something changed”; it is insufficient for “perform this action exactly one hundred times.” Use an explicit queue, counter with suitable synchronization, or another protocol when multiplicity matters.

The disposition describes the action for a signal and belongs to the process: default action, ignore, or an installed handler. Each thread has its own blocked-signal mask. A process-directed signal may go to an eligible thread that has it unblocked; a thread-directed signal targets a particular thread. Blocking in only the main thread therefore does not establish a process-wide delivery policy. A common controlled design blocks selected signals before creating worker threads, lets the workers inherit that mask, and dedicates one thread to synchronous acceptance. SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

An asynchronous handler can interrupt a thread while a library operation has partially updated internal state. Calling a convenient logging routine from the handler may re-enter the same library or encounter a lock held by the interrupted execution. Ordinary thread safety does not imply async-signal safety. Keep handler work within the documented safe interface set and use a minimal notification mechanism with a clearly defined protocol. A volatile sig_atomic_t flag can support a narrow flag-setting pattern in C, but it is not a general replacement for synchronization between threads or a lossless event counter. Preserve errno if the handler modifies it and the interrupted code depends on it.

Synchronous handling with sigwaitinfo or a suitable signalfd arrangement can move application logic into a normal control flow, provided the relevant signals are blocked consistently in the threads that should not receive handlers. Neither design makes delivery instantaneous. A blocked system call interrupted by a handler may restart or report EINTR depending on the interface and flags; check its specific contract. A loop must also track partial progress and its original deadline. Restarting a fresh full-duration timeout after every interruption can exceed the intended shutdown budget indefinitely, even though every individual call appears correct.

Fork creates a new process; in a multithreaded parent, only the calling thread exists in the child. The child inherits signal dispositions and the calling thread’s mask, while its pending signal set begins empty. Before exec in that child, restrictions on safe function calls matter because copied synchronization objects may describe locks whose owning threads no longer exist there. Exec then replaces the program image without creating a new process identity. Caught dispositions reset to defaults, ignored dispositions generally remain ignored, and the signal mask persists. A launcher must intentionally choose the mask and descriptor inheritance expected by the new program rather than assuming exec resets everything.

When a child terminates, ordinary waitable status can remain until the parent collects it with a wait-family operation. That zombie is not still running its former workload; it is retained accounting state. Treat SIGCHLD as a prompt to inspect available statuses, not as one receipt per exit. A supervisor commonly loops over waitpid with WNOHANG until no further eligible status is ready, handling errors and its chosen stop/continue options explicitly. This discussion assumes ordinary child reaping rather than explicit SIG_IGN or SA_NOCLDWAIT policies that change zombie behavior. During shutdown, request termination, stop admitting new work, maintain a deadline, and continue collecting children. Sending a termination request and observing a reaped status are different milestones.