Copyright (c) 1980 The Regents of the University of California.
All rights reserved.

%sccs.include.redist.man%

@(#)sigvec.2 6.5 (Berkeley) 06/23/90

C 4 sigvec - software signal facilities

 #include <signal.h> 

 struct sigvec {  int (*sv_handler)();  int sv_mask;  int sv_flags;  }; 

 sigvec(sig, vec, ovec)  int sig;  struct sigvec *vec, *ovec; 

The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurence of a hardware interrupt: the signal is blocked from further occurrence, the current process context is saved, and a new one is built. A process may specify a handler to which a signal is delivered, or specify that a signal is to be blocked or ignored . A process may also specify that a default action is to be taken by the system when a signal occurs. Normally, signal handlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special "signal stack" .

All signals have the same priority . Signal routines execute with the signal that caused their invocation blocked , but other signals may yet occur. A global "signal mask" defines the set of signals currently blocked from delivery to a process. The signal mask for a process is initialized from that of its parent (normally 0). It may be changed with a sigblock (2) or sigsetmask (2) call, or when a signal is delivered to the process.

When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently blocked by the process then it is delivered to the process. When a signal is delivered, the current state of the process is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal handling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to resume in a different context, then it must arrange to restore the previous context itself.

When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a sigblock or sigsetmask call is made). This mask is formed by taking the current signal mask, adding the signal to be delivered, and or 'ing in the signal mask associated with the handler to be invoked.

Sigvec assigns a handler for a specific signal. If vec is non-zero, it specifies a handler routine and mask to be used when delivering the specified signal. Further, if the SV_ONSTACK bit is set in sv_flags, the system will deliver the signal to the process on a "signal stack" , specified with sigstack (2). If ovec is non-zero, the previous handling information for the signal is returned to the user.

The following is a list of all signals with names as in the include file < signal.h >:

SIGHUP 1 hangup
SIGINT 2 interrupt
SIGQUIT 3* quit
SIGILL 4* illegal instruction
SIGTRAP 5* trace trap
SIGIOT 6* IOT instruction
SIGEMT 7* EMT instruction
SIGFPE 8* floating point exception
SIGKILL 9 kill (cannot be caught, blocked, or ignored)
SIGBUS 10* bus error
SIGSEGV 11* segmentation violation
SIGSYS 12* bad argument to system call
SIGPIPE 13 write on a pipe with no one to read it
SIGALRM 14 alarm clock
SIGTERM 15 software termination signal
SIGURG 16\*b urgent condition present on socket
SIGSTOP 17\*d stop (cannot be caught, blocked, or ignored)
SIGTSTP 18\*d stop signal generated from keyboard
SIGCONT 19\*b continue after stop (cannot be blocked)
SIGCHLD 20\*b child status has changed
SIGTTIN 21\*d background read attempted from control terminal
SIGTTOU 22\*d background write attempted to control terminal
SIGIO 23\*b i/o is possible on a descriptor (see fcntl(2))
SIGXCPU 24 cpu time limit exceeded (see setrlimit(2))
SIGXFSZ 25 file size limit exceeded (see setrlimit(2))
SIGVTALRM 26 virtual time alarm (see setitimer(2))
SIGPROF 27 profiling timer alarm (see setitimer(2))
SIGWINCH 28\*b window size change
SIGUSR1 30 user defined signal 1
SIGUSR2 31 user defined signal 2

The starred signals in the list above cause a core image if not caught or ignored.

Once a signal handler is installed, it remains installed until another sigvec call is made, or an execve (2) is performed. The default action for a signal may be reinstated by setting sv_handler to SIG_DFL; this default is termination (with a core image for starred signals) except for signals marked with \*b or \*d. Signals marked with \*b are discarded if the action is SIG_DFL; signals marked with \*d cause the process to stop. If sv_handler is SIG_IGN the signal is subsequently ignored, and pending instances of the signal are discarded.

If a caught signal occurs during certain system calls, the call is normally restarted. The call can be forced to terminate prematurely with an EINTR error return by setting the SV_INTERRUPT bit in sv_flags. The affected system calls are read (2) or write (2) on a slow device (such as a terminal; but not a file) and during a wait (2).

After a fork (2) or vfork (2) the child inherits all signals, the signal mask, the signal stack, and the restart/interrupt flags.

Execve (2) resets all caught signals to default action and resets all signals to be caught on the user stack. Ignored signals remain ignored; the signal mask remains the same; signals that interrupt system calls continue to do so.

The mask specified in vec is not allowed to block SIGKILL, SIGSTOP, or SIGCONT. This is done silently by the system.

The SV_INTERRUPT flag is not available in 4.2BSD, hence it should not be used if backward compatibility is needed.

A 0 value indicated that the call succeeded. A -1 return value indicates an error occurred and errno is set to indicated the reason. Sigvec will fail and no new signal handler will be installed if one of the following occurs:

15 [EFAULT] Either vec or ovec points to memory that is not a valid part of the process address space.

15 [EINVAL] Sig is not a valid signal number.

15 [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

15 [EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).

kill(1), ptrace(2), kill(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), sigvec(2), setjmp(3), siginterrupt(3), tty(4) The handler routine can be declared:

handler(sig, code, scp) int sig, code; struct sigcontext *scp;

Here sig is the signal number, into which the hardware faults and traps are mapped as defined below. Code is a parameter that is either a constant as given below or, for compatibility mode faults, the code provided by the hardware (Compatibility mode faults are distinguished from the other SIGILL traps by having PSL_CM set in the psl). Scp is a pointer to the sigcontext structure (defined in < signal.h >), used to restore the context from before the signal.

The following defines the mapping of hardware traps to signals and codes. All of these symbols are defined in < signal.h >:

 Hardware condition Signal Code

Arithmetic traps:
 Integer overflow SIGFPE FPE_INTOVF_TRAP
 Integer division by zero SIGFPE FPE_INTDIV_TRAP
 Floating overflow trap SIGFPE FPE_FLTOVF_TRAP
 Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP
 Floating underflow trap SIGFPE FPE_FLTUND_TRAP
 Decimal overflow trap SIGFPE FPE_DECOVF_TRAP
 Subscript-range SIGFPE FPE_SUBRNG_TRAP
 Floating overflow fault SIGFPE FPE_FLTOVF_FAULT
 Floating divide by zero fault SIGFPE FPE_FLTDIV_FAULT
 Floating underflow fault SIGFPE FPE_FLTUND_FAULT
Length access control SIGSEGV
Protection violation SIGBUS
Reserved instruction SIGILL ILL_RESAD_FAULT
Customer-reserved instr. SIGEMT
Reserved operand SIGILL ILL_PRIVIN_FAULT
Reserved addressing SIGILL ILL_RESOP_FAULT
Trace pending SIGTRAP
Bpt instruction SIGTRAP
Compatibility-mode SIGILL hardware supplied code
Chme SIGSEGV
Chms SIGSEGV
Chmu SIGSEGV

This manual page is still confusing.