SIGNAL(9) | Kernel Developer's Manual | SIGNAL(9) |
signal
, siginit
,
sigactsinit
, sigactsunshare
,
sigactsfree
, execsigs
,
sigaction1
, sigprocmask1
,
sigpending1
, sigsuspend1
,
sigaltstack1
, pgsignal
,
kpgsignal
, psignal
,
kpsignal
, issignal
,
postsig
, killproc
,
sigexit
, trapsignal
,
sendsig
, sigcode
,
sigtramp
—
#include <sys/signal.h>
#include <sys/signalvar.h>
void
siginit
(struct
proc *p);
void
sigactsinit
(struct
proc *pp, int
share);
void
sigactsunshare
(struct
proc *p);
void
sigactsfree
(struct
proc *p);
void
execsigs
(struct
proc *p);
int
sigaction1
(struct
lwp *l, int signum,
const struct sigaction
*nsa, struct sigaction
*osa, void *tramp,
int vers);
int
sigprocmask1
(struct
lwp *l, int how,
const sigset_t *nss,
sigset_t *oss);
void
sigpending1
(struct
lwp *l, sigset_t
*ss);
int
sigsuspend1
(struct
lwp *l, const sigset_t
*ss);
int
sigaltstack1
(struct
lwp *l, const struct
sigaltstack *nss, struct
sigaltstack *oss);
void
pgsignal
(struct
pgrp *pgrp, int
signum, int
checkctty);
void
kpgsignal
(struct
pgrp *pgrp, ksiginfo_t
*ks, void *data,
int checkctty);
void
psignal
(struct
proc *p, int
signum);
void
kpsignal
(struct
proc *p, ksiginfo_t
*ks, void
*data);
int
issignal
(struct
lwp *l);
void
postsig
(int
signum);
void
killproc
(struct
proc *p, const char
*why);
void
sigexit
(struct
lwp *l, int
signum);
void
trapsignal
(struct
lwp *l, const ksiginfo_t
*ks);
void
sendsig
(const
ksiginfo_t *ks, const
sigset_t *mask);
Signal numbers used throughout the kernel signal facilities should always be within the range of [1-NSIG].
Most of the kernel's signal infrastructure is implemented in machine-independent code. Machine-dependent code provides support for invoking a process's signal handler, restoring context when the signal handler returns, generating signals when hardware traps occur, triggering the delivery of signals when a process is about to return from the kernel to userspace.
The signal state for a process is contained in struct sigctx. This includes the list of signals with delivery pending, information about the signal handler stack, the signal mask, and the address of the signal trampoline.
The registered signal handlers for a process are recorded in struct sigacts. This structure may be shared by multiple processes.
The kernel's signal facilities are implemented by the following functions:
siginit
(p)This function initializes the signal state of proc0 to the system default. This signal state is then inherited by init(8) when it is started by the kernel.
sigactsinit
(pp,
share)This function creates an initial struct sigacts for the process pp. If the share argument is non-zero, then pp shares the struct sigacts by holding a reference. Otherwise, pp receives a new struct sigacts which is copied from the parent.
sigactsunshare
(p)This function causes the process p to no longer share its struct sigacts The current state of the signal actions is maintained in the new copy.
sigactsfree
(p)This function decrements the reference count on the struct sigacts of process p. If the reference count reaches zero, the struct sigacts is freed.
execsigs
(p)This function is used to reset the signal state of the process p to the system defaults when the process execs a new program image.
sigaction1
(l,
signum, nsa,
osa, tramp,
vers)This function implements the
sigaction(2) system
call. The tramp and vers
arguments provide support for userspace signal trampolines. Trampoline
version 0 is reserved for the legacy kernel-provided signal trampoline;
tramp must be NULL
in this
case. Otherwise, vers specifies the ABI of the
trampoline specified by tramp. The signal
trampoline ABI is machine-dependent, and must be coordinated with the
sendsig
() function.
sigprocmask1
(l,
how, nss,
oss)This function implements the sigprocmask(2) system call.
sigpending1
(l,
ss)This function implements the sigpending(2) system call.
sigsuspend1
(l,
ss)This function implements the sigsuspend(2) system call.
sigaltstack1
(l,
nss, oss)This function implements the sigaltstack(2) system call.
pgsignal
(pgrp,
signum, checkctty)This is a wrapper function for
kpgsignal
() which is described below.
kpgsignal
(pgrp,
ks, data,
checkctty)Schedule the signal ks->ksi_signo to
be delivered to all members of the process group
pgrp. If checkctty is
non-zero, the signal is only sent to processes which have a controlling
terminal. The data argument and the complete
signal scheduling semantics are described in the
kpsignal
() function below.
trapsignal
(l,
ks)Sends the signal ks->ksi_signo caused by a hardware trap to the current process.
psignal
(p,
signum)This is a wrapper function for
kpsignal
() which is described below.
kpsignal
(p,
ks, data)Schedule the signal ks->ksi_signo to
be delivered to the process p. The
data argument, if not
NULL
, points to the file descriptor data that
caused the signal to be generated in the SIGIO
case.
With a few exceptions noted below, the target process signal
disposition is updated and is marked as runnable, so further handling of
the signal is done in the context of the target process after a context
switch; see issignal
() below. Note that
kpsignal
() does not by itself cause a context
switch to happen.
The target process is not marked as runnable in the following cases:
If the target process is being traced,
kpsignal
() behaves as if the target process were
taking the default action for signum. This allows
the tracing process to be notified of the signal.
issignal
(l)This function determines which signal, if any, is to be posted to the current process. A signal is to be posted if:
Signals which cause the process to be stopped are handled
within issignal
() directly.
issignal
() should be called by
machine-dependent code when returning to userspace from a system call or
other trap or interrupt by using the following code:
while (signum = CURSIG(curproc)) postsig(signum);
postsig
(signum)The postsig
() function is used to
invoke the action for the signal signum in the
current process. If the default action of a signal is to terminate the
process, and the signal does not have a registered handler, the process
exits using sigexit
(), dumping a core image if
necessary.
killproc
(p,
why)This function sends a SIGKILL signal to the specified process. The message provided by why is sent to the system log and is also displayed on the process's controlling terminal.
sigexit
(l,
signum)This function forces the current process to exit with the signal signum, generating a core file if appropriate. No checks are made for masked or caught signals; the process always exits.
sendsig
(ks,
mask)This function is provided by machine-dependent code, and is
used to invoke a signal handler for the current process.
sendsig
() must prepare the registers and stack
of the current process to invoke the signal handler stored in the
process's struct sigacts. This may include
switching to an alternate signal stack specified by the process. The
previous register, stack, and signal state are stored in a
ucontext_t, which is then copied out to the user's
stack.
The registers and stack must be set up to invoke the signal handler as follows:
(*handler)(int signum, siginfo_t *info, void *ctx)
where signum is the signal number,
info contains additional signal specific
information when SA_SIGINFO
is specified when
setting up the signal handler. ctx is the pointer
to ucontext_t on the user's stack. The registers
and stack must also arrange for the signal handler to return to the
signal trampoline. The trampoline is then used to return to the code
which was executing when the signal was delivered using the
setcontext(2) system
call.
For performance reasons, it is recommended that
sendsig
() arrange for the signal handler to be
invoked directly on architectures where it is convenient to do so. In
this case, the trampoline is used only for the signal return path. If it
is not feasible to directly invoke the signal handler, the trampoline is
also used to invoke the handler, performing any final set up that was
not possible for sendsig
() to perform.
sendsig
() must invoke the signal
trampoline with the correct ABI. The ABI of the signal trampoline is
specified on a per-signal basis in the sigacts
()
structure for the process. Trampoline version 0 is reserved for the
legacy kernel-provided, on-stack signal trampoline. All other trampoline
versions indicate a specific trampoline ABI. This ABI is coordinated
with machine-dependent code in the system C library.
In traditional UNIX systems, the signal trampoline, also referred to as the “sigcode”, is provided by the kernel and copied to the top of the user's stack when a new process is created or a new program image is exec'd. Starting in NetBSD 2.0, the signal trampoline is provided by the system C library. This allows for more flexibility when the signal facility is extended, makes dealing with signals easier in debuggers, such as gdb(1), and may also enhance system security by allowing the kernel to disallow execution of code on the stack.
The signal trampoline is specified on a per-signal basis. The correct trampoline is selected automatically by the C library when a signal handler is registered by a process.
Signal trampolines have a special naming convention which enables debuggers to determine the characteristics of the signal handler and its arguments. Trampoline functions are named like so:
__sigtramp_<flavor>_<version>
where:
void (*handler)(int signum, int code, struct sigcontext *scp);
void (*handler)(int signum, siginfo_t *si, void *uc);
Note: sigcontext style signal handlers are deprecated, and retained only for compatibility with older binaries.
sendsig
() function. The trampoline version needs
to be unique even across different trampoline flavors, in order to
simplify trampoline selection in the kernel.The following is an example if a signal trampoline name which indicates that the trampoline is used for traditional BSD-style signal handlers and implements version 1 of the signal trampoline ABI:
__sigtramp_sigcontext_1
The current signal trampoline is:
__sigtramp_siginfo_2
April 29, 2010 | NetBSD 9.0 |