sivalptr

Sivalptr refers to the sival_ptr member of the POSIX sigval union, used in real-time signal delivery to pass a pointer value alongside a signal. The sigval type is defined to hold either an integer value or a pointer, with sival_ptr being the void pointer variant. This mechanism is commonly employed with the sigqueue API and related real-time signaling interfaces.

In practice, a program constructs a sigval value, assigns a pointer to sival_ptr, and delivers it with a signal to another process or thread. When the receiving side handles the signal and uses the SA_SIGINFO option, the signal handler can access the associated data through the si_value field, typically si_value.sival_ptr. The retrieved pointer can then be cast to the appropriate data type by the receiver.

Usage considerations include ensuring the pointed-to object remains valid for the duration of its use in the signal handler, as signals may interrupt execution at arbitrary points. Proper synchronization and memory management are important, since the lifecycle of the pointed-to object is not automatically tied to the signal delivery.

Portability notes: sival_ptr is defined by POSIX as part of the sigval union, and is widely supported on modern Unix-like systems (for example, Linux, BSD variants). Some non-POSIX or older implementations may have variations in the exact API or in how signals with values are handled. The complementary sival_int member exists for passing small integers, but sival_ptr is specifically for pointer values.

See also: sigval, sigqueue, SA_SIGINFO, siginfo_t, real-time signals.

In summary, sivalptr (sival_ptr) is the pointer-bearing portion of the sigval union used to transmit a data pointer alongside a signal, enabling callbacks or handlers to access user data associated with asynchronous signal delivery.