SIGINFO(3) FreeBSD Library Functions Manual SIGINFO(3)


siginfosignal generation information


#include < signal.h>


A process may request signal information when it is catching a signal. The information specifies why the system generated that signal. To request signal information in a signal handler, the user can set SA_SIGINFO in sa_flags before sigaction(2) is called, otherwise the user can use sigwaitinfo(2) and sigtimedwait(2) to get signal information. In either case, the system returns the information in a structure of type siginfo_t, which includes the following information:
Type Member Description
int si_signo signal number
int si_errno error number
int si_code signal code
union sigval si_value signal value
pid_t si_pid sending process ID
uid_t si_uid sending process's real user ID
void *si_addr address of faulting instruction
int si_status exit value or signal
long si_band band event for SIGPOLL
int si_trapno machine trap code
int si_timerid POSIX timer ID
int si_overrun POSIX timer overrun count
int si_mqd POSIX message queue ID

The si_signo member contains the signal number.

The si_errno member contains an error number defined in the file < errno.h>.

The si_code member contains a code which describes the cause of the signal. The macros specified in the Code column of the following table are defined for use as values of si_code that are signal-specific or non-signal-specific reasons why the signal was generated:

Signal Code Reason
SIGILL ILL_ILLOPC illegal opcode
ILL_ILLOPN illegal operand
ILL_ILLADR illegal addressing mode
ILL_ILLTRP illegal trap
ILL_PRVOPC illegal privileged opcode
ILL_PRVREG illegal privileged register
ILL_COPROC coprocessor error
ILL_BADSTK internal stack error
SIGFPE FPE_INTDIV integer divide by zero
FPE_INTOVF integer overflow
FPE_FLTDIV floating-point divide by zero
FPE_FLTOVF floating-point overflow
FPE_FLTUND floating-point underflow
FPE_FLTRES floating-point inexact result
FPE_FLTINV invalid floating-point operation
FPE_FLTSUB subscript out of range
SIGSEGV SEGV_MAPERR address not mapped to object
SEGV_ACCERR invalid permissions for mapped object
SIGBUS BUS_ADRALN invalid address alignment
BUS_ADRERR nonexistent physical address
BUS_OBJERR object-specific hardware error
SIGTRAP TRAP_BRKPT process breakpoint
TRAP_TRACE process trace trap
TRAP_DTRACE DTrace induced trap
SIGCHLD CLD_EXITED child has exited
CLD_KILLED child has terminated abnormally and did not create a core file
CLD_DUMPED child has terminated abnormally and created a core file
CLD_TRAPPED traced child has trapped
CLD_STOPPED child has stopped
CLD_CONTINUED stopped child has continued
SIGPOLL POLL_IN data input available
POLL_OUT output buffers available
POLL_MSG input message available
POLL_ERR I/O error
POLL_PRI high priority input available
POLL_HUP device disconnected
Any SI_NOINFO Only the si_signo member is meaningful; the value of all other members is unspecified.
SI_USER signal sent by kill(2)
SI_QUEUE signal sent by sigqueue(2)
SI_TIMER signal generated by expiration of a timer set by timer_settime(2)
SI_ASYNCIO signal generated by completion of an asynchronous I/O request
SI_MESGQ signal generated by arrival of a message on an empty message queue
SI_KERNEL signal generated by miscellaneous parts of the kernel
SI_LWP signal sent by pthread_kill(3)

In addition, the following signal-specific information is available:

Signal Member Value
SIGILL si_addr address of faulting instruction
si_trapno machine dependent of trap code
SIGFPE si_addr address of faulting instruction
si_trapno machine dependent of trap code
SIGSEGV si_addr address of faulting memory reference
si_trapno machine dependent of trap code
SIGBUS si_addr address of faulting instruction
si_trapno machine dependent of trap code
SIGCHLD si_pid child process ID
si_status exit value or signal; if si_code is equal to CLD_EXITED, then it is equal to the exit value of the child process, otherwise, it is equal to a signal that caused the child process to change state.
si_uid real user ID of the process that sent the signal
SIGPOLL si_band band event for POLL_IN, POLL_OUT, or POLL_MSG

Finally, the following code-specific information is available:

Code Member Value
SI_USER si_pid the process ID that sent the signal
si_uid real user ID of the process that sent the signal
SI_QUEUE si_value the value passed to sigqueue(2) system call
si_pid the process ID that sent the signal
si_uid real user ID of the process that sent the signal
SI_TIMER si_value the value passed to timer_create(2) system call
si_timerid the timer ID returned by timer_create(2) system call
si_overrun timer overrun count corresponding to the signal
si_errno If timer overrun will be { DELAYTIMER_MAX}, an error code defined in < errno.h> is set
SI_ASYNCIO si_value the value passed to aio system calls
SI_MESGQ si_value the value passed to mq_notify(2) system call
si_mqd the ID of the message queue which generated the signal
SI_LWP si_pid the process ID that sent the signal
si_uid real user ID of the process that sent the signal


Currently, the kernel never generates the SIGPOLL signal. SIGCHLD signal is queued when a process changed its status or exited. POSIX Realtime Extensions like aio, timer, and message queue also queue signals. Signals with code SI_USER, SI_KERNEL or SI_LWP are only queued if there are sufficient resources; otherwise, SI_NOINFO results. For some hardware architectures, the exact value of si_addr might not be available.


The siginfo_t type conforms to IEEE Std 1003.1-2004 (“POSIX.1”).


Full support for POSIX signal information first appeared in FreeBSD 7.0. The codes SI_USER and SI_KERNEL can be generated as of FreeBSD 8.1. The code SI_LWP can be generated as of FreeBSD 9.0.


This manual page was written by David Xu <davidxu@FreeBSD.org>.
September 14, 2012 FreeBSD