Linux中signal函数说明：
NAME
signal - ANSI C signal handling
SYNOPSIS
#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
DESCRIPTION
The behavior of signal() varies across Unix versions, and has also var-
ied historically across different versions of Linux. Avoid its use:
use sigaction(2) instead. See Portability below.
signal() sets the disposition of the signal signum to handler, which is
either SIG_IGN, SIG_DFL, or the address of a programmer-defined func-
tion (a "signal handler").
If the signal signum is delivered to the process, then one of the fol-
lowing happens:
* If the disposition is set to SIG_IGN, then the signal is ignored.
* If the disposition is set to SIG_DFL, then the default action asso-
ciated with the signal (see signal(7)) occurs.
* If the disposition is set to a function, then first either the dis-
position is reset to SIG_DFL, or the signal is blocked (see Porta-
bility below), and then handler is called with argument signum. If
invocation of the handler caused the signal to be blocked, then the
signal is unblocked upon return from the handler.
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
RETURN VALUE
signal() returns the previous value of the signal handler, or SIG_ERR
on error.
ERRORS
EINVAL signum is invalid.
使用示例：
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void ouch(int sig)
{
printf("I got signal %d\n", sig);
// (void) signal(SIGINT, SIG_DFL);
//(void) signal(SIGINT, ouch);
}
int main()
{
(void) signal(SIGINT, ouch);
while(1)
{
printf("hello world...\n");
sleep(1);
}
}
实际运用中，需要对不同到signal设定不同的到信号处理函数，SIG_IGN忽略/SIG_DFL默认，这俩宏也可以作为信号处理函数。同时SIGSTOP/SIGKILL这俩信号无法捕获和忽略。注意，经过实验发现，signal函数也会堵塞当前正在处理的signal，但是没有办法阻塞其它signal，比如正在处理SIG_INT，再来一个SIG_INT则会堵塞，但是来SIG_QUIT则会被其中断，如果SIG_QUIT有处理，则需要等待SIG_QUIT处理完了，SIG_INT才会接着刚才处理。
Linux中sigaction函数说明：
NAME
sigaction - examine and change a signal action
SYNOPSIS
#include <signal.h>
int sigaction(int signum, const struct sigaction *act,
struct sigaction *oldact);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
sigaction(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
DESCRIPTION
The sigaction() system call is used to change the action taken by a
process on receipt of a specific signal. (See signal(7) for an
overview of signals.)
signum specifies the signal and can be any valid signal except SIGKILL
and SIGSTOP.
If act is non-null, the new action for signal signum is installed from
act. If oldact is non-null, the previous action is saved in oldact.
The sigaction structure is defined as something like:
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
};
On some architectures a union is involved: do not assign to both
sa_handler and sa_sigaction.
The sa_restorer element is obsolete and should not be used. POSIX does
not specify a sa_restorer element.
sa_handler specifies the action to be associated with signum and may be
SIG_DFL for the default action, SIG_IGN to ignore this signal, or a
pointer to a signal handling function. This function receives the sig-
nal number as its only argument.
If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of
sa_handler) specifies the signal-handling function for signum. This
function receives the signal number as its first argument, a pointer to
a siginfo_t as its second argument and a pointer to a ucontext_t (cast
to void *) as its third argument.
sa_mask specifies a mask of signals which should be blocked (i.e.,
added to the signal mask of the thread in which the signal handler is
invoked) during execution of the signal handler. In addition, the sig-
nal which triggered the handler will be blocked, unless the SA_NODEFER
flag is used.
sa_flags specifies a set of flags which modify the behavior of the sig-
nal. It is formed by the bitwise OR of zero or more of the following:
SA_NOCLDSTOP
If signum is SIGCHLD, do not receive notification when child
processes stop (i.e., when they receive one of SIGSTOP,
SIGTSTP, SIGTTIN or SIGTTOU) or resume (i.e., they receive
SIGCONT) (see wait(2)). This flag is only meaningful when
establishing a handler for SIGCHLD.
SA_NOCLDWAIT (Since Linux 2.6)
If signum is SIGCHLD, do not transform children into zombies
when they terminate. See also waitpid(2). This flag is
only meaningful when establishing a handler for SIGCHLD, or
when setting that signal’s disposition to SIG_DFL.
If the SA_NOCLDWAIT flag is set when establishing a handler
for SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD
signal is generated when a child process terminates. On
Linux, a SIGCHLD signal is generated in this case; on some
other implementations, it is not.
SA_NODEFER
Do not prevent the signal from being received from within
its own signal handler. This flag is only meaningful when
establishing a signal handler. SA_NOMASK is an obsolete,
non-standard synonym for this flag.
SA_ONSTACK
Call the signal handler on an alternate signal stack pro-
vided by sigaltstack(2). If an alternate stack is not
available, the default stack will be used. This flag is
only meaningful when establishing a signal handler.
SA_RESETHAND
Restore the signal action to the default state once the sig-
nal handler has been called. This flag is only meaningful
when establishing a signal handler. SA_ONESHOT is an obso-
lete, non-standard synonym for this flag.
SA_RESTART
Provide behavior compatible with BSD signal semantics by
making certain system calls restartable across signals.
This flag is only meaningful when establishing a signal han-
dler. See signal(7) for a discussion of system call
restarting.
SA_SIGINFO (since Linux 2.2)
The signal handler takes 3 arguments, not one. In this
case, sa_sigaction should be set instead of sa_handler.
This flag is only meaningful when establishing a signal han-
dler.
The siginfo_t argument to sa_sigaction is a struct with the following
elements:
siginfo_t {
int si_signo;
int si_errno;
int si_code;
int si_trapno;
pid_t si_pid;
uid_t si_uid;
int si_status;
clock_t si_utime;
clock_t si_stime;
sigval_t si_value;
int si_int;
void *si_ptr;
int si_overrun;
int si_timerid;
void *si_addr;
int si_band;
}
si_signo, si_errno and si_code are defined for all signals. (si_errno
is generally unused on Linux.) The rest of the struct may be a union,
so that one should only read the fields that are meaningful for the
given signal:
* POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.
* POSIX.1b timers (since Linux 2.6) fill in si_overrun and si_timerid.
The si_timerid field is an internal ID used by the kernel to identify
the timer; it is not the same as the timer ID returned by timer_cre-
ate(2).
* SIGCHLD fills in si_status, si_utime and si_stime. The si_utime and
si_stime fields do not include the times used by waited for children
(unlike getrusage(2) and time(2). In kernels up to 2.6, and since
2.6.27, these fields report CPU time in units of
sysconf(_SC_CLK_TCK). In 2.6 kernels before 2.6.27, a bug meant that
these fields reported time in units of the (configurable) system
jiffy (see time(7)).
* si_int and si_ptr are specified by the sender of the POSIX.1b signal.
See sigqueue(2) for more details.
* SIGILL, SIGFPE, SIGSEGV, and SIGBUS fill in si_addr with the address
of the fault. SIGPOLL fills in si_band and si_fd.
si_code is a value (not a bit mask) indicating why this signal was
sent. The following list shows the values which can be placed in
si_code for any signal, along with reason that the signal was gener-
ated.
SI_USER kill(2) or raise(3)
SI_KERNEL Sent by the kernel.
SI_QUEUE sigqueue(2)
SI_TIMER POSIX timer expired
SI_MESGQ POSIX message queue state changed (since Linux
2.6.6); see mq_notify(3)
SI_ASYNCIO AIO completed
SI_SIGIO queued SIGIO
SI_TKILL tkill(2) or tgkill(2) (since Linux 2.4.19)
The following values can be placed in si_code for a SIGILL signal:
ILL_ILLOPC illegal opcode
ILL_ILLOPN illegal operand
ILL_ILLADR illegal addressing mode
ILL_ILLTRP illegal trap
ILL_PRVOPC privileged opcode
ILL_PRVREG privileged register
ILL_COPROC coprocessor error
ILL_BADSTK internal stack error
The following values can be placed in si_code for a SIGFPE signal:
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 floating-point invalid operation
FPE_FLTSUB subscript out of range
The following values can be placed in si_code for a SIGSEGV signal:
SEGV_MAPERR address not mapped to object
SEGV_ACCERR invalid permissions for mapped object
The following values can be placed in si_code for a SIGBUS signal:
BUS_ADRALN invalid address alignment
BUS_ADRERR nonexistent physical address
BUS_OBJERR object-specific hardware error
The following values can be placed in si_code for a SIGTRAP signal:
TRAP_BRKPT process breakpoint
TRAP_TRACE process trace trap
The following values can be placed in si_code for a SIGCHLD signal:
CLD_EXITED child has exited
CLD_KILLED child was killed
CLD_DUMPED child terminated abnormally
CLD_TRAPPED traced child has trapped
CLD_STOPPED child has stopped
CLD_CONTINUED stopped child has continued (since Linux 2.6.9)
The following values can be placed in si_code for a SIGPOLL signal:
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
RETURN VALUE
sigaction() returns 0 on success and -1 on error.
ERRORS
EFAULT act or oldact points to memory which is not a valid part of the
process address space.
EINVAL An invalid signal was specified. This will also be generated if
an attempt is made to change the action for SIGKILL or SIGSTOP,
which cannot be caught or ignored.
使用示例：
#include <signal.h>
#include <stdio.h>
#include <unistd.h>
void ouch(int sig)
{
printf("oh, got a signal %d\n", sig);
int i = 0;
for (i = 0; i < 5; i++)
{
printf("signal func %d\n", i);
sleep(1);
}
}
int main()
{
struct sigaction act;
act.sa_handler = ouch;
sigemptyset(&act.sa_mask);
sigaddset(&act.sa_mask, SIGQUIT);
// act.sa_flags = SA_RESETHAND;
// act.sa_flags = SA_NODEFER;
act.sa_flags = 0;
sigaction(SIGINT, &act, 0);
struct sigaction act_2;
act_2.sa_handler = ouch;
sigemptyset(&act_2.sa_mask);
act.sa_flags = 0;
sigaction(SIGQUIT, &act_2, 0);
while(1)
{
sleep(1);
}
return;
} 1. 阻塞，sigaction函数有阻塞的功能，比如SIGINT信号来了，进入信号处理函数，默认情况下，在信号处理函数未完成之前，如果又来了一个SIGINT信号，其将被阻塞，只有信号处理函数处理完毕，才会对后来的SIGINT再进行处理，同时后续无论来多少个SIGINT，仅处理一个SIGINT，sigaction会对后续SIGINT进行排队合并处理。
2. sa_mask，信号屏蔽集，可以通过函数sigemptyset/sigaddset等来清空和增加需要屏蔽的信号，上面代码中，对信号SIGINT处理时，如果来信号SIGQUIT，其将被屏蔽，但是如果在处理SIGQUIT，来了SIGINT，则首先处理SIGINT，然后接着处理SIGQUIT。
3. sa_flags如果取值为0，则表示默认行为。还可以取如下俩值，但是我没觉得这俩值有啥用。
SA_NODEFER，如果设置来该标志，则不进行当前处理信号到阻塞
SA_RESETHAND，如果设置来该标志，则处理完当前信号后，将信号处理函数设置为SIG_DFL行为
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