siginfo − signal generation information


#include <siginfo.h>


If a process is catching a signal, it might request information that tells why the system generated that signal. See sigaction(2). If a process is monitoring its children, it might receive information that tells why a child changed state. See waitid(2). In either case, the system returns the information in a structure of type siginfo_t, which includes the following information:

int            si_signo        /* signal number */
int            si_errno        /* error number */
int            si_code         /* signal code */
union sigval   si_value        /* signal value */

si_signo contains the system-generated signal number. For the waitid(2) function, si_signo is always SIGCHLD.

If si_errno is non-zero, it contains an error number associated with this signal, as defined in <errno.h>.

si_code contains a code identifying the cause of the signal.

If the value of the si_code member is SI_NOINFO, only the si_signo member of siginfo_t is meaningful, and the value of all other members is unspecified.

User Signals
If the value of si_code is less than or equal to 0, then the signal was generated by a user process (see kill(2), _lwp_kill(2), sigqueue(3RT), sigsend(2), abort(3C), and raise(3C)) and the siginfo structure contains the following additional information:

typedef   long pid_t   si_pid   /* sending process ID */
typedef   long uid_t   si_uid   /* sending user ID */

If the signal was generated by a user process, the following values are defined for si_code:

The implementation sets si_code to SI_USER if the signal was sent by kill(2), sigsend(2), raise(3C) or abort(3C).


The signal was sent by _lwp_kill(2).


The signal was sent by sigqueue(3RT).


The signal was generated by the expiration of a timer created by timer_settime(3RT).


The signal was generated by the completion of an asynchronous I/O request.


The signal was generated by the arrival of a message on an empty message queue. See mq_notify(3RT).

si_value contains the application specified value, which is passed to the application’s signal-catching function at the time of the signal delivery if si_code is any of SI_QUEUE, SI_TIMER, SI_ASYNCHIO, or SI_MESGQ.

System Signals
Non-user generated signals can arise for a number of reasons. For all of these cases, si_code contains a positive value reflecting the reason why the system generated the signal:

Image /var/www/mancx/application/src/../www/___/img/man3/man3/siginfo1.png

Signals can also be generated from the resource control subsystem. Where these signals do not already possess kernel-level siginfo codes, the siginfo si_code will be filled with SI_RCTL to indicate a kernel-generated signal from an established resource control value.

Image /var/www/mancx/application/src/../www/___/img/man3/man3/siginfo2.png

The uncatchable signals SIGSTOP and SIGKILL have undefined siginfo codes.

Signals sent with a siginfo code of SI_RCTL contain code-dependent information for kernel-generated signals:

Image /var/www/mancx/application/src/../www/___/img/man3/man3/siginfo3.png

In addition, the following signal-dependent information is available for kernel-generated signals:

Image /var/www/mancx/application/src/../www/___/img/man3/man3/siginfo4.png


_lwp_kill(2), kill(2), setrctl(2), sigaction(2), sigsend(2), waitid(2), abort(3C), aio_read(3RT), mq_notify(3RT), raise(3C), signal(3HEAD), sigqueue(3RT), timer_create(3RT), timer_settime(3RT)


For SIGCHLD signals, if si_code is equal to CLD_EXITED, then si_status is equal to the exit value of the process; otherwise, it is equal to the signal that caused the process to change state. For some implementations, the exact value of si_addr might not be available; in that case, si_addr is guaranteed to be on the same page as the faulting instruction or memory reference.