WUNTRACED

WUNTRACED is a flag used with POSIX wait-related system calls, notably waitpid and waitid. It requests that the call also report on children that have stopped due to a signal, in addition to those that have terminated. The flag is defined in the system header sys/wait.h and is combined with other options by using a bitwise OR when calling waitpid or waitid.

When WUNTRACED is set, a waitpid or waitid call will return for a child that has stopped (for example, due to a signal such as SIGSTOP or a debugger-related stop). The resulting status can be inspected with macros such as WIFSTOPPED to confirm the stop condition, and WSTOPSIG to determine the signal that caused the stop. If WUNTRACED is not set, the call typically returns only for terminated children. The WUNTRACED flag may be used together with WCONTINUED to also receive notifications when a stopped child continues.

Usage example in C:

- int status;

- pid_t p = waitpid(-1, &status, WUNTRACED);

- if (WIFSTOPPED(status)) { int sig = WSTOPSIG(status); /* handle stop */ }

Compatibility and notes:

- WUNTRACED is part of the POSIX wait family of options and is commonly available on Unix-like systems, including Linux, BSD variants, and macOS.

- It is used by debuggers, shells, and other tools that need visibility into not only terminated children but also those that have been stopped by signals.

In summary, WUNTRACED extends the information returned by waitpid and waitid to include stopped child processes, enabling more precise process control and monitoring in user-space programs.