sigvec(2) System Calls Manual sigvec(2)
Name
sigvec - software signal facilities
Syntax
#include <signal.h>
struct sigvec {
void (*sv_handler)();
sigset_t sv_mask;
int sv_flags;
};
sigvec(sig, vec, ovec)
int sig;
struct sigvec *vec, *ovec;
Description
The system defines a set of signals that can be delivered to a process. Signal delivery resembles the occurrence 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 can specify a han-
dler to which a signal is delivered, or specify that a signal is to be blocked or ignored . A process can 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 can
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 can
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 can be changed with a or 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 cur-
rently blocked by the process, 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 later), and the signal handler is invoked. The call to the handler is arranged so that, if the
signal handling routine returns normally, the process resumes execution in the context from before the signal's delivery. If the process
wishes to resume in a different context, 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's signal handler (or until a or
call is made). This mask is formed by taking the current signal mask, adding the signal to be delivered, and ORing in the signal mask
associated with the handler to be invoked.
The System call assigns a handler for a specific signal. If vec is nonzero, it specifies a handler routine and mask to be used when deliv-
ering the specified signal. Further, if the SV_ONSTACK bit is set in sv_flags, the system delivers the signal to the process on a signal
stack, specified with If ovec is nonzero, 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
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. Urgent condition present on socket
SIGSTOP 17+ Stop (cannot be caught, blocked, or ignored)
SIGTSTP 18+ Stop signal generated from keyboard
SIGCONT 19. Continue after stop (cannot be blocked)
SIGCHLD 20. Child status has changed
SIGTTIN 21+ Background read attempted from control terminal
SIGTTOU 22+ Background write attempted to control terminal
SIGIO 23. 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. Window size change
SIGLOST 29 Lock not reclaimed after server recovery
SIGUSR1 30 User-defined signal 1
SIGUSR2 31 User-defined signal 2
SIGCLD System V name for SIGCHLD
SIGABRT X/OPEN name for SIGIOT
The signals marked with asterisks (*) in this list cause a core image if not caught or ignored. Explanations of the meaning of the periods
(.) and plus signs (+) are included in the following paragraph.
Once a signal handler is installed, it remains installed until another call is made or an is performed. The default action for a signal
can be reinstated by setting sv_handler to SIG_DFL. This default is termination (with a core image for signals marked with asterisks (*)),
except for signals marked with periods (.) or plus signs (+). Signals marked with periods (.) are discarded if the action is SIG_DFL. Sig-
nals marked with plus signs (+) 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 or on a slow device (such as a ter-
minal, but not a file), and
After a or the child inherits all signals, the signal mask, the signal stack, and the restart/interrupt flags.
The system call 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 ULTRIX 2.0 or earlier versions. Therefore, it should not be used if backward compatibility is
needed.
Return Values
A zero (0) value indicates that the call succeeded. A -1 return value indicates an error occurred, and errno is set to indicated the rea-
son.
Diagnostics
The system call fails and no new signal handler is installed, if one of the following occurs:
[EFAULT] Either vec or ovec points to memory that is not a valid part of the process address space.
[EINVAL] Sig is not a valid signal number.
[EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.
[EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).
Notes
The handler routine can be declared:
void handler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
Here sig is the signal number. MIPS hardware exceptions are mapped to specific signals as defined by the following table. Code is a
parameter that is either a constant or zero. The scp is a pointer to the sigcontext structure (defined in that is the context at the time
of the signal and is used to restore the context, if the signal handler returns.
The following defines the mapping of MIPS hardware exceptions to signals and codes. All of these symbols are defined in either <signal.h>
or <mips/cpu.h>:
Hardware exception Signal Code
Integer overflow SIGFPE EXC_OV
Segmentation violation SIGSEGV SEXC_SEGV
Illegal instruction SIGILL EXC_II
Coprocessor unusable SIGILL SEXC_CPU
Data bus error SIGBUS EXC_DBE
Instruction bus error SIGBUS EXC_IBE
Read address error SIGBUS EXC_RADE
Write address error SIGBUS EXC_WADE
User breakpoint (used by debuggers) SIGTRAP BRK_USERBP
Kernel breakpoint (used by prom) SIGTRAP BRK_KERNELBP
Taken branch delay emulation SIGTRAP BRK_BD_TAKEN
Not taken branch delay emulation SIGTRAP BRK_BD_NOTTAKEN
User single step (used by debuggers) SIGTRAP BRK_SSTEPBP
Overflow check SIGTRAP BRK_OVERFLOW
Divide by zero check SIGTRAP BRK_DIVZERO
Range error check SIGTRAP BRK_RANGE
When a signal handler is reached, the program counter in the signal context structure (sc_pc) points at the instruction that caused the
exception, as modified by the branch delay bit in the cause register. The cause register at the time of the exception is also saved in the
sigcontext structure (sc_cause). If the instruction that caused the exception is at a valid user address, it can be retrieved with the
following code sequence:
if(scp->sc_cause & CAUSE_BD){
branch_instruction = *(unsigned long *)(scp->sc_pc);
exception_instruction = *(unsigned long *)(scp->sc_pc + 4);
}
else
exception_instruction = *(unsigned long *)(scp->sc_pc);
CAUSE_BD is defined in
The signal handler can fix the cause of the exception and re-execute the instruction, emulate the instruction and then step over it, or
perform some nonlocal redirection, such as a or an
If corrective action is performed in the signal handler and the instruction that caused the exception would then execute without a further
exception, the signal handler simply returns and re-executes the instruction (even when the branch delay bit is set).
If execution is to continue after stepping over the instruction that caused the exception, the program counter must be advanced. If the
branch delay bit is set, the program counter is set to the target of the branch. Otherwise, it is incremented by four. This can be done
with the following code sequence:
if(scp->sc_cause & CAUSE_BD)
emulate_branch(scp, branch_instruction);
else
scp->sc_pc += 4;
Emulate_branch() modifies the program counter value in the sigcontext structure to the target of the branch instruction. See for more
details.
For SIGFPE's generated by floating-point instructions (code == 0) the floating-point control and status register at the time of the excep-
tion is also saved in the sigcontext structure (sc_fpc_csr). This register has the information on which exceptions have occurred. When a
signal handler is entered, the register contains the value at the time of the exception but with the exceptions bits cleared. On a return
from the signal handler, the exception bits in the floating-point control and status register are also cleared so that another SIGFPE will
not occur (all other bits are restored from sc_fpc_csr).
For SIGSEGV and SIGBUS errors, the faulting virtual address is saved in sc_badvaddr in the signal context structure.
The SIGTRAPs caused by instructions noted in the previous table and all other yet to be defined instructions fill the code parameter with
the first argument to the instruction (bits 25-16 of the instruction).
See Also
kill(1), kill(2), ptrace(2), sigblock(2), sigpause(2), sigsetmask(2), sigstack(2), setjmp(3), siginterrupt(3), tty(4)
RISC sigvec(2)