SIGCHLDs

SIGCHLD is a signal in POSIX-compliant Unix-like systems sent to a process when one of its child processes changes state. A child may terminate, be terminated by a signal, stop due to a job-control signal, or continue after being stopped. The signal serves as notification that the parent may need to examine or reap the child’s exit status.

Delivery and handling: The signal is delivered to the parent process of the child. The default action is to ignore SIGCHLD, but the signal is generated regardless. When multiple children change state, the signal may be delivered multiple times. Some systems can suppress delivery of SIGCHLD when children stop by using flags such as SA_NOCLDSTOP.

Handling patterns: A typical approach is to install a handler for SIGCHLD and reap children with waitpid in a loop, using WNOHANG to avoid blocking. Alternatively, parents may poll for status changes. The status returned by wait or waitpid can be interpreted with macros such as WIFEXITED, WEXITSTATUS, WIFSIGNALED, WTERMSIG, WIFSTOPPED, WSTOPSIG, and on some systems WIFCONTINUED.

Importance for process management: Reaping children prevents zombie processes and frees system resources. Because signal delivery is asynchronous, robust programs often reap all available children in the handler. Programs should use sigaction rather than the older signal API for reliable behavior and consider race-condition mitigations when setting up handlers.

Notes and terminology: SIGCHLD is sometimes referred to in historical code as CLD. The plural notion SIGCHLDs refers to occurrences of the signal raised for different children rather than a separate signal type. This signal is central to how shells, daemons, and other parent processes manage child lifecycles.