signal.3

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)signal.3	2.1	3/10/87
.TH signal 3 RISC
.SH Name
signal \- simplified software signal facilities
.SH Syntax
.nf
.B #include <signal.h>
.PP
.B (*signal(sig, func))()
.B void (*func)();
.fi
.SH Description
.NXR "signal subroutine (standard C)"
.NXR "signal" "changing action"
.NXA "sigvec system call" "signal subroutine (standard C)"
The
.PN signal
subroutine
is a simplified interface to the more general sigvec(2)
facility.
.PP
A signal
is generated by some abnormal event,
initiated by a user at a terminal (quit, interrupt, stop),
by a program error (bus error, etc.),
by request of another program (kill),
or when a process is stopped because it wishes to access
its control terminal while in the background.  For further
information, see 
.MS tty 4 .
Signals are optionally generated
when a process resumes after being stopped,
when the status of child process changes,
or when input is ready at the control terminal.
Most signals cause termination of the receiving process if no action
is taken; some signals instead cause the process receiving them
to be stopped, or are simply discarded if the process has not
requested otherwise.
Except for the SIGKILL and SIGSTOP
signals, the
.PN signal
call allows signals either to be ignored
or to cause an interrupt to a specified location.
The following is a list of all signals with
names as in the include file 
.br
< signal.h >:
.PP
.NXR "signal subroutine (standard C)" "signal list"
.nf
.ta \w'SIGVTALRM 'u +\w'15*  'u
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 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\(bu	Urgent condition present on socket
SIGSTOP	17+	Stop (cannot be caught or ignored)
SIGTSTP	18+	Stop signal generated from keyboard
SIGCONT	19\(bu	Continue after stop
SIGCHLD	20\(bu	Child status has changed
SIGTTIN	21+	Background read attempted from control terminal
SIGTTOU	22+	Background write attempted to control terminal
SIGIO	23\(bu	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\(bu	Window size change
SIGLOST	29	lock not reclaimed after server recovery
SIGUSR1	30	User defined signal
SIGUSR2	31	User defined signal
SIGCLD		System V name for SIGCHLD
SIGABRT		X/OPEN name for SIGIOT
.fi
.PP
The starred signals in the list above cause a core image
if not caught or ignored.
.PP
If
.I func
is SIG_DFL, the default action
for signal
.I sig
is reinstated; this default is termination
(with a core image for starred signals)
except for signals marked with \(bu or +.
Signals marked with \(bu are discarded if the action
is SIG_DFL; signals marked
with + cause the process to stop.
If
.I func
is SIG_IGN the signal is subsequently ignored
and pending instances of the signal are discarded.
Otherwise, when the signal occurs
further occurrences of the signal are
automatically blocked and
.I func
is called.
.PP
A return from the function unblocks
the handled signal and
continues the process at the point it was interrupted.
Unlike previous signal facilities, the handler \fIfunc\fP
remains installed after a signal has been delivered.
.PP
If a caught signal occurs
during certain system calls, causing
the call to terminate prematurely, the call
is automatically restarted.
In particular this can occur
during a
.I read
or 
.MS write 2
on a slow device (such as a terminal; but not a file)
and during a 
.MS wait 2 .
.PP
The value of
.PN signal
is the previous (or initial)
value of
.I func
for the particular signal.
.PP
After a 
.MS fork 2
or 
.MS vfork 2
the child inherits
all signals.
The
.MS execve 2
system call
resets all caught signals to the default action;
ignored signals remain ignored.
.SH Environment
When your program is compiled using the System V environment the handler
function does NOT remain installed after the signal has been delivered.
.PP
Also, when a signal which is to be caught occurs during a
read,  write, or ioctl to a slow device (like
a terminal, but not a file); or during a  pause;  or
wait that does not return immediately,  the  signal  handler  function 
is executed,
and then the interrupted  system  call  may
return  a -1 to the calling process with errno set
to EINTR.
.SH Notes 
The handler routine can be declared as follows:
.PP
    \f(CWhandler(sig, code, scp)
    int sig, code;
    struct sigcontext *scp;\fR
.PP
Here
.I sig
is the signal number.
The MIPS hardware exceptions are mapped to specific signals as defined by
the table below.
The parameter
.I code
is either a constant as given below or zero.
The parameter
.I scp
is a pointer to the
.I sigcontext
structure (defined in
.RI < signal.h >),
that is the context at the time of the signal and is
used to restore the context if the signal handler returns.
.PP
The following defines the mapping of MIPS hardware exceptions to signals
and codes.  All of these symbols are defined in either
.RI < signal.h >
or
.RI < mips/cpu.h >:
.PP
.ta \w'     User Single Step (used by debuggers)   'u +\w'15*  'u +8n
.nf
Hardware exception	Signal	Code
.sp +.5
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
.fi
.PP
When a signal handler is reached, the program counter in the signal context
structure
.RI ( sc_pc )
points at the instruction that caused the exception as modified by the
.I "branch delay"
bit in the
.I cause
register.
The
.I cause
register at the time of the exception is also saved in the sigcontext
structure
.RI ( sc_cause ).
If the instruction that caused the exception is at a valid user address it
can be retrieved with the following code sequence:
.PP
    \f(CWif(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);\fR
.PP
Where CAUSE_BD is defined in
.RI < mips/cpu.h >.
.PP
The signal handler may fix the cause of the exception and re-execute the
instruction, emulate the instruction and then step over it or perform some
non-local goto such as a
.I longjump()
or an
.I exit().
.PP
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
.I "branch delay"
bit is set).
.PP
If execution is to continue after stepping over the instruction that caused the
exception the program counter must be advanced.  If the
.I "branch delay"
bit is set the program counter is set to the target of the branch else it is
incremented by 4.
.PP
This can be done with the following code sequence:
.PP
    \f(CWif(scp->sc_cause & CAUSE_BD)
        emulate_branch(scp, branch_instruction);
    else
        scp->sc_pc += 4;\fR
.PP
.I Emulate_branch()
modifies the program counter value in the sigcontext structure to the target
of the branch instruction.  See
.I emulate_branch(3)
for more details.
.PP
For SIGFPE's generated by floating-point instructions
.RI ( code
== 0) the
.I "floating-point control and status"
register at the time of the exception is also saved in the sigcontext structure
.RI ( 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
.I "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 does not occur (all other bits are restored from
.IR sc_fpc_csr ).
.PP
For SIGSEGV and SIGBUS errors the faulting virtual address is saved in
.I sc_badvaddr
in the signal context structure.
.PP
The SIGTRAP's caused by
.B break
instructions noted in the above table and all other yet to be defined
.B break
instructions fill the
.I code
parameter with the first argument to the
.B break
instruction (bits 25-16 of the instruction).
.SH Return Values
.NXR "signal subroutine (standard C)" "return value"
The previous action is returned on a successful call.
Otherwise, \-1 is returned and 
.I errno
is set to indicate the error.
.SH Diagnostics
The
.PN signal
subroutine fails and action is not taken if one of the 
following occurs:
.TP 15
[EINVAL]
The
.I sig
is not a valid signal number.
.TP 15
[EINVAL]
An attempt is made to ignore or supply a handler for SIGKILL
or SIGSTOP.
.SH See Also
kill(1), kill(2), ptrace(2), sigblock(2), sigpause(2),
sigsetmask(2), sigstack(2), sigvec(2), setjmp(3), tty(4)