сигналы (signals)
Использование в приложениях (Application usage)
On systems not supporting the XSI option, the si_pid and si_uid
members of siginfo_t are only required to be valid when si_code
is SI_USER or SI_QUEUE. On XSI-conforming systems, they are also
valid for all si_code values less than or equal to 0; however, it
is unspecified whether SI_USER and SI_QUEUE have values less than
or equal to zero, and therefore XSI applications should check
whether si_code has the value SI_USER or SI_QUEUE or is less than
or equal to 0 to tell whether si_pid and si_uid are valid.
