服务器评测
信号常常被称为“软中断”，和“中断”类似，用来通知程序发生异步事件。对信号的处理一般来说有三种方式：忽略，终止进程以及使用信号处理函数。信号处理函数的方式是从一处执行流断开，转而去运行另外的一处代码（信号处理），当处理函数返回时，继续从断开的地方继续执行。
1、安装信号处理函数
在系统编程的层面上与信号的处理关系最直接相关的函数有两个，他们用来安装信号处理函数：
sighandler_t signal(int signum, sighandler_t handler);
int sigaction(int signum, const struct sigaction *act,，struct sigaction *oldact);
第一个函数signal比较简单，sighandler_t 是一个别名，其原型是 typedef void (*sighandler_t)(int)，他是一个函数指针，接受一个类型为int的参数(信号的编号)，返回void。例如要对SIGUSR1信号进行处理：
void handler(int sig)
{
//strsiganl 功能是把信号的编号转为信号说明的字符串
printf(“Rcv a signal:%s”,strsignal(sig));
}
int main()
{
signal(SIGUSR1,handler);
while(1)
;
}
（这段程序其实是有问题的，后面会说到）这段程序本来是一段死循环，但是对他发送SIGUSR1信号，程序会从while中“中断”转去执行handler中的代码。在shell中使用kill命令发送信号SIGUSR1 于是程序就答应出了一段这样的信息：Rcv a signal:User defined signal 1。signal()的用法几乎就是这么简单。但是由于可移植的原因，参与项目开发时，应该使用下面的这个函数。
sigaction()函数的参数中有两个结构体，其man手册原型如下：
struct sigaction {
void (*sa_handler)(int);
void (*sa_sigaction)(int, siginfo_t *, void *);
sigset_t sa_mask;
int sa_flags;
void (*sa_restorer)(void);
};
据我所知sa_handler和sa_sigaction其实是在一个union中，他们都是指向信号处理函数的指针。
sa_mask 是要屏蔽的信号，sa_flags 有多种选项。（关于这两点后文再细说）。从sigaction()原型中可以发现参数中有两个struct sigaction参数，其中act是要安装的信号处理，而oldact是用来带回原来的处理方式方便我们处理完信号后的恢复。如果不需要拿回之前的信号处理方式可以把第三个参数置为NULL，反之如果只想得到之前的处理方式而不像安装新的信号处理，可以把第二个参数置为NULL，这点用signal()是办不到的。用sigaction()改写上面的例子是这样的：
1 void handler(int sig)
2 {
3 printf(“Rcv a signal:%s”,strsignal(sig));
4 }
5
6 int main()
7 {
8 struct sigaction act;
9 sigemptyset(&act.sa_mask);
10 act.sa_handler = handler;
11 act.sa_flags = 0;
12 sigaction(SIGUSR1,&act,NULL);
13 while(1)
14 ;
15 }
2、信号阻塞、信号的未决
sigset_t 是一种将信号类型以为位掩码形式存在的数据类型（下文都称之为信号集），他是多种信号的集合（可以保证容纳所有的信号）。操作系统的PCB为每个进程都维护了一个这样的数据类型，并将其内所有的信号阻塞，使他们不可以实时到达进程。当信号屏蔽解除时他们才被传递到进程。在这之间的状态通常被称为未决（pending）。而在信号阻塞期间多次到来的信号，在信号屏蔽解除时只会被报告一次。
对sigset_t 处理有一系列函数，其中POSIX标准有5个
int sigemptyset(sigset_t *set);
int sigfillset(sigset_t *set);
int sigaddset(sigset_t *set, int signum);
int sigdelset(sigset_t *set, int signum);
int sigismember(const sigset_t *set, int signum);
这样的函数基本上看参数就能知道怎么用，不在赘述。
glibc中还实现了3个扩展的函数：
int sigisemptyset(sigset_t *set);
int sigorset(sigset_t *dest, sigset_t *left, sigset_t *right);
int sigandset(sigset_t *dest, sigset_t *left, sigset_t *right);
sigprocmask（）函数可以检测和更改信号屏蔽集。
每个进程都有一个用来描述哪些信号递送到进程时将被阻塞的信号集，该信号集中的所有信号在递送到进程后都将被阻塞。
int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);
| how | 说明 |
| SIG_BLOCK | 将set中的信号与原有的取并集，并更新进程的屏蔽字 |
| SIG_UNBLOCK | 解除原有的信号集中包含set中的信号，（set补集的交集） |
| SIG_SETMASK | 将进程的屏蔽字设置为set |
sigpending函数可以看到信号屏蔽期间那些信号来到过（不计次数的）。
更多详情见请继续阅读下一页的精彩内容： http://www.linuxidc.com/Linux/2015-07/120632p2.htm
Linux man手册关于signal的介绍
SIGNAL(7) Linux Programmer’s Manual SIGNAL(7)
NAME
signal – overview of signals
DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter “standard sig鈥�
nals”) and POSIX real-time signals.
Signal Dispositions
Each signal has a current disposition, which determines how the process
behaves when it is delivered the signal.
The entries in the “Action” column of the tables below specify the
default disposition for each signal, as follows:
Term Default action is to terminate the process.
Ign Default action is to ignore the signal.
Core Default action is to terminate the process and dump core (see
core(5)).
Stop Default action is to stop the process.
Cont Default action is to continue the process if it is currently
stopped.
A process can change the disposition of a signal using sigaction(2) or
signal(2). (The latter is less portable when establishing a signal
handler; see signal(2) for details.) Using these system calls, a
process can elect one of the following behaviors to occur on delivery
of the signal: perform the default action; ignore the signal; or catch
the signal with a signal handler, a programmer-defined function that is
automatically invoked when the signal is delivered. (By default, the
signal handler is invoked on the normal process stack. It is possible
to arrange that the signal handler uses an alternate stack; see sigalt鈥�
stack(2) for a discussion of how to do this and when it might be use鈥�
ful.)
The signal disposition is a per-process attribute: in a multithreaded
application, the disposition of a particular signal is the same for all
threads.
A child created via fork(2) inherits a copy of its parent’s signal dis鈥�
positions. During an execve(2), the dispositions of handled signals
are reset to the default; the dispositions of ignored signals are left
unchanged.
Sending a Signal
The following system calls and library functions allow the caller to
send a signal:
raise(3) Sends a signal to the calling thread.
kill(2) Sends a signal to a specified process, to all members
of a specified process group, or to all processes on
the system.
killpg(2) Sends a signal to all of the members of a specified
process group.
pthread_kill(3) Sends a signal to a specified POSIX thread in the same
process as the caller.
tgkill(2) Sends a signal to a specified thread within a specific
process. (This is the system call used to implement
pthread_kill(3).)
sigqueue(3) Sends a real-time signal with accompanying data to a
specified process.
Waiting for a Signal to be Caught
The following system calls suspend execution of the calling process or
thread until a signal is caught (or an unhandled signal terminates the
process):
pause(2) Suspends execution until any signal is caught.
sigsuspend(2) Temporarily changes the signal mask (see below) and
suspends execution until one of the unmasked signals is
caught.
Synchronously Accepting a Signal
Rather than asynchronously catching a signal via a signal handler, it
is possible to synchronously accept the signal, that is, to block exe鈥�
cution until the signal is delivered, at which point the kernel returns
information about the signal to the caller. There are two general ways
to do this:
* sigwaitinfo(2), sigtimedwait(2), and sigwait(3) suspend execution
until one of the signals in a specified set is delivered. Each of
these calls returns information about the delivered signal.
* signalfd(2) returns a file descriptor that can be used to read infor鈥�
mation about signals that are delivered to the caller. Each read(2)
from this file descriptor blocks until one of the signals in the set
specified in the signalfd(2) call is delivered to the caller. The
buffer returned by read(2) contains a structure describing the sig鈥�
nal.
Signal Mask and Pending Signals
A signal may be blocked, which means that it will not be delivered
until it is later unblocked. Between the time when it is generated and
when it is delivered a signal is said to be pending.
Each thread in a process has an independent signal mask, which indi鈥�
cates the set of signals that the thread is currently blocking. A
thread can manipulate its signal mask using pthread_sigmask(3). In a
traditional single-threaded application, sigprocmask(2) can be used to
manipulate the signal mask.
A child created via fork(2) inherits a copy of its parent’s signal
mask; the signal mask is preserved across execve(2).
A signal may be generated (and thus pending) for a process as a whole
(e.g., when sent using kill(2)) or for a specific thread (e.g., certain
signals, such as SIGSEGV and SIGFPE, generated as a consequence of exe鈥�
cuting a specific machine-language instruction are thread directed, as
are signals targeted at a specific thread using pthread_kill(3)). A
process-directed signal may be delivered to any one of the threads that
does not currently have the signal blocked. If more than one of the
threads has the signal unblocked, then the kernel chooses an arbitrary
thread to which to deliver the signal.
A thread can obtain the set of signals that it currently has pending
using sigpending(2). This set will consist of the union of the set of
pending process-directed signals and the set of signals pending for the
calling thread.
A child created via fork(2) initially has an empty pending signal set;
the pending signal set is preserved across an execve(2).
Standard Signals
Linux supports the standard signals listed below. Several signal num鈥�
bers are architecture-dependent, as indicated in the “Value” column.
(Where three values are given, the first one is usually valid for alpha
and sparc, the middle one for ix86, ia64, ppc, s390, arm and sh, and
the last one for mips. A – denotes that a signal is absent on the cor鈥�
responding architecture.)
First the signals described in the original POSIX.1-1990 standard.
Signal Value Action Comment
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SIGHUP 1 Term Hangup detected on controlling terminal
or death of controlling process
SIGINT 2 Term Interrupt from keyboard
SIGQUIT 3 Core Quit from keyboard
SIGILL 4 Core Illegal Instruction
SIGABRT 6 Core Abort signal from abort(3)
SIGFPE 8 Core Floating point exception
SIGKILL 9 Term Kill signal
SIGSEGV 11 Core Invalid memory reference
SIGPIPE 13 Term Broken pipe: write to pipe with no
readers
SIGALRM 14 Term Timer signal from alarm(2)
SIGTERM 15 Term Termination signal
SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGCHLD 20,17,18 Ign Child stopped or terminated
SIGCONT 19,18,25 Cont Continue if stopped
SIGSTOP 17,19,23 Stop Stop process
SIGTSTP 18,20,24 Stop Stop typed at tty
SIGTTIN 21,21,26 Stop tty input for background process
SIGTTOU 22,22,27 Stop tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
Next the signals not in the POSIX.1-1990 standard but described in
SUSv2 and POSIX.1-2001.
Signal Value Action Comment
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SIGBUS 10,7,10 Core Bus error (bad memory access)
SIGPOLL Term Pollable event (Sys V).
Synonym for SIGIO
SIGPROF 27,27,29 Term Profiling timer expired
SIGSYS 12,31,12 Core Bad argument to routine (SVr4)
SIGTRAP 5 Core Trace/breakpoint trap
SIGURG 16,23,21 Ign Urgent condition on socket (4.2BSD)
SIGVTALRM 26,26,28 Term Virtual alarm clock (4.2BSD)
SIGXCPU 24,24,30 Core CPU time limit exceeded (4.2BSD)
SIGXFSZ 25,25,31 Core File size limit exceeded (4.2BSD)
Up to and including Linux 2.2, the default behavior for SIGSYS, SIGX鈥�
CPU, SIGXFSZ, and (on architectures other than SPARC and MIPS) SIGBUS
was to terminate the process (without a core dump). (On some other
UNIX systems the default action for SIGXCPU and SIGXFSZ is to terminate
the process without a core dump.) Linux 2.4 conforms to the
POSIX.1-2001 requirements for these signals, terminating the process
with a core dump.
Next various other signals.
Signal Value Action Comment
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SIGIOT 6 Core IOT trap. A synonym for SIGABRT
SIGEMT 7,-,7 Term
SIGSTKFLT -,16,- Term Stack fault on coprocessor (unused)
SIGIO 23,29,22 Term I/O now possible (4.2BSD)
SIGCLD -,-,18 Ign A synonym for SIGCHLD
SIGPWR 29,30,19 Term Power failure (System V)
SIGINFO 29,-,- A synonym for SIGPWR
SIGLOST -,-,- Term File lock lost
SIGWINCH 28,28,20 Ign Window resize signal (4.3BSD, Sun)
SIGUNUSED -,31,- Core Synonymous with SIGSYS
(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)
SIGEMT is not specified in POSIX.1-2001, but nevertheless appears on
most other UNIX systems, where its default action is typically to ter鈥�
minate the process with a core dump.
SIGPWR (which is not specified in POSIX.1-2001) is typically ignored by
default on those other UNIX systems where it appears.
SIGIO (which is not specified in POSIX.1-2001) is ignored by default on
several other UNIX systems.
Where defined, SIGUNUSED is synonymous with SIGSYS on most architec鈥�
tures.
Real-time Signals
Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001). The range of
supported real-time signals is defined by the macros SIGRTMIN and
SIGRTMAX. POSIX.1-2001 requires that an implementation support at
least _POSIX_RTSIG_MAX (8) real-time signals.
The Linux kernel supports a range of 32 different real-time signals,
numbered 33 to 64. However, the glibc POSIX threads implementation
internally uses two (for NPTL) or three (for LinuxThreads) real-time
signals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably
(to 34 or 35). Because the range of available real-time signals varies
according to the glibc threading implementation (and this variation can
occur at run time according to the available kernel and glibc), and
indeed the range of real-time signals varies across UNIX systems, pro鈥�
grams should never refer to real-time signals using hard-coded numbers,
but instead should always refer to real-time signals using the notation
SIGRTMIN+n, and include suitable (run-time) checks that SIGRTMIN+n does
not exceed SIGRTMAX.
Unlike standard signals, real-time signals have no predefined meanings:
the entire set of real-time signals can be used for application-defined
purposes.
The default action for an unhandled real-time signal is to terminate
the receiving process.
Real-time signals are distinguished by the following:
1. Multiple instances of real-time signals can be queued. By con鈥�
trast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is
queued.
2. If the signal is sent using sigqueue(3), an accompanying value
(either an integer or a pointer) can be sent with the signal. If
the receiving process establishes a handler for this signal using
the SA_SIGINFO flag to sigaction(2) then it can obtain this data
via the si_value field of the siginfo_t structure passed as the
second argument to the handler. Furthermore, the si_pid and si_uid
fields of this structure can be used to obtain the PID and real
user ID of the process sending the signal.
3. Real-time signals are delivered in a guaranteed order. Multiple
real-time signals of the same type are delivered in the order they
were sent. If different real-time signals are sent to a process,
they are delivered starting with the lowest-numbered signal.
(I.e., low-numbered signals have highest priority.) By contrast,
if multiple standard signals are pending for a process, the order
in which they are delivered is unspecified.
If both standard and real-time signals are pending for a process, POSIX
leaves it unspecified which is delivered first. Linux, like many other
implementations, gives priority to standard signals in this case.
According to POSIX, an implementation should permit at least
_POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process.
However, Linux does things differently. In kernels up to and including
2.6.7, Linux imposes a system-wide limit on the number of queued real-
time signals for all processes. This limit can be viewed and (with
privilege) changed via the /proc/sys/kernel/rtsig-max file. A related
file, /proc/sys/kernel/rtsig-nr, can be used to find out how many real-
time signals are currently queued. In Linux 2.6.8, these /proc inter鈥�
faces were replaced by the RLIMIT_SIGPENDING resource limit, which
specifies a per-user limit for queued signals; see setrlimit(2) for
further details.
Async-signal-safe functions
A signal handler function must be very careful, since processing else鈥�
where may be interrupted at some arbitrary point in the execution of
the program. POSIX has the concept of “safe function”. If a signal
interrupts the execution of an unsafe function, and handler calls an
unsafe function, then the behavior of the program is undefined.
POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2)
requires an implementation to guarantee that the following functions
can be safely called inside a signal handler:
_Exit()
_exit()
abort()
accept()
access()
aio_error()
aio_return()
aio_suspend()
alarm()
bind()
cfgetispeed()
cfgetospeed()
cfsetispeed()
cfsetospeed()
chdir()
chmod()
chown()
clock_gettime()
close()
connect()
creat()
dup()
dup2()
execle()
execve()
fchmod()
fchown()
fcntl()
fdatasync()
fork()
fpathconf()
fstat()
fsync()
ftruncate()
getegid()
geteuid()
getgid()
getgroups()
getpeername()
getpgrp()
getpid()
getppid()
getsockname()
getsockopt()
getuid()
kill()
link()
listen()
lseek()
lstat()
mkdir()
mkfifo()
open()
pathconf()
pause()
pipe()
poll()
posix_trace_event()
pselect()
raise()
read()
readlink()
recv()
recvfrom()
recvmsg()
rename()
rmdir()
select()
sem_post()
send()
sendmsg()
sendto()
setgid()
setpgid()
setsid()
setsockopt()
setuid()
shutdown()
sigaction()
sigaddset()
sigdelset()
sigemptyset()
sigfillset()
sigismember()
signal()
sigpause()
sigpending()
sigprocmask()
sigqueue()
sigset()
sigsuspend()
sleep()
sockatmark()
socket()
socketpair()
stat()
symlink()
sysconf()
tcdrain()
tcflow()
tcflush()
tcgetattr()
tcgetpgrp()
tcsendbreak()
tcsetattr()
tcsetpgrp()
time()
timer_getoverrun()
timer_gettime()
timer_settime()
times()
umask()
uname()
unlink()
utime()
wait()
waitpid()
write()
POSIX.1-2008 removes fpathconf(), pathconf(), and sysconf() from the
above list, and adds the following functions:
execl()
execv()
faccessat()
fchmodat()
fchownat()
fexecve()
fstatat()
futimens()
linkat()
mkdirat()
mkfifoat()
mknod()
mknodat()
openat()
readlinkat()
renameat()
symlinkat()
unlinkat()
utimensat()
utimes()
Interruption of System Calls and Library Functions by Signal Handlers
If a signal handler is invoked while a system call or library function
call is blocked, then either:
* the call is automatically restarted after the signal handler returns;
or
* the call fails with the error EINTR.
Which of these two behaviors occurs depends on the interface and
whether or not the signal handler was established using the SA_RESTART
flag (see sigaction(2)). The details vary across UNIX systems; below,
the details for Linux.
If a blocked call to one of the following interfaces is interrupted by
a signal handler, then the call will be automatically restarted after
the signal handler returns if the SA_RESTART flag was used; otherwise
the call will fail with the error EINTR:
* read(2), readv(2), write(2), writev(2), and ioctl(2) calls on
“slow” devices. A “slow” device is one where the I/O call may
block for an indefinite time, for example, a terminal, pipe, or
socket. (A disk is not a slow device according to this defini鈥�
tion.) If an I/O call on a slow device has already transferred
some data by the time it is interrupted by a signal handler, then
the call will return a success status (normally, the number of
bytes transferred).
* open(2), if it can block (e.g., when opening a FIFO; see
fifo(7)).
* wait(2), wait3(2), wait4(2), waitid(2), and waitpid(2).
* Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2),
recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a timeout
has been set on the socket (see below).
* File locking interfaces: flock(2) and fcntl(2) F_SETLKW.
* POSIX message queue interfaces: mq_receive(3), mq_time鈥�
dreceive(3), mq_send(3), and mq_timedsend(3).
* futex(2) FUTEX_WAIT (since Linux 2.6.22; beforehand, always
failed with EINTR).
* POSIX semaphore interfaces: sem_wait(3) and sem_timedwait(3)
(since Linux 2.6.22; beforehand, always failed with EINTR).
The following interfaces are never restarted after being interrupted by
a signal handler, regardless of the use of SA_RESTART; they always fail
with the error EINTR when interrupted by a signal handler:
* Socket interfaces, when a timeout has been set on the socket
using setsockopt(2): accept(2), recv(2), recvfrom(2), and
recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; con鈥�
nect(2), send(2), sendto(2), and sendmsg(2), if a send timeout
(SO_SNDTIMEO) has been set.
* Interfaces used to wait for signals: pause(2), sigsuspend(2),
sigtimedwait(2), and sigwaitinfo(2).
* File descriptor multiplexing interfaces: epoll_wait(2),
epoll_pwait(2), poll(2), ppoll(2), select(2), and pselect(2).
* System V IPC interfaces: msgrcv(2), msgsnd(2), semop(2), and sem鈥�
timedop(2).
* Sleep interfaces: clock_nanosleep(2), nanosleep(2), and
usleep(3).
* read(2) from an inotify(7) file descriptor.
* io_getevents(2).
The sleep(3) function is also never restarted if interrupted by a han鈥�
dler, but gives a success return: the number of seconds remaining to
sleep.
Interruption of System Calls and Library Functions by Stop Signals
On Linux, even in the absence of signal handlers, certain blocking
interfaces can fail with the error EINTR after the process is stopped
by one of the stop signals and then resumed via SIGCONT. This behavior
is not sanctioned by POSIX.1, and doesn’t occur on other systems.
The Linux interfaces that display this behavior are:
* Socket interfaces, when a timeout has been set on the socket
using setsockopt(2): accept(2), recv(2), recvfrom(2), and
recvmsg(2), if a receive timeout (SO_RCVTIMEO) has been set; con鈥�
nect(2), send(2), sendto(2), and sendmsg(2), if a send timeout
(SO_SNDTIMEO) has been set.
* epoll_wait(2), epoll_pwait(2).
* semop(2), semtimedop(2).
* sigtimedwait(2), sigwaitinfo(2).
* read(2) from an inotify(7) file descriptor.
* Linux 2.6.21 and earlier: futex(2) FUTEX_WAIT, sem_timedwait(3),
sem_wait(3).
* Linux 2.6.8 and earlier: msgrcv(2), msgsnd(2).
* Linux 2.4 and earlier: nanosleep(2).
CONFORMING TO
POSIX.1, except as noted.
BUGS
SIGIO and SIGLOST have the same value. The latter is commented out in
the kernel source, but the build process of some software still thinks
that signal 29 is SIGLOST.
SEE ALSO
kill(1), getrlimit(2), kill(2), killpg(2), rt_sigqueueinfo(2),
setitimer(2), setrlimit(2), sgetmask(2), sigaction(2), sigaltstack(2),
signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2),
sigwaitinfo(2), abort(3), bsd_signal(3), longjmp(3), raise(3),
pthread_sigqueue(3), sigqueue(3), sigset(3), sigsetops(3), sigvec(3),
sigwait(3), strsignal(3), sysv_signal(3), core(5), proc(5),
pthreads(7), sigevent(7)
COLOPHON
This page is part of release 3.35 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://man7.org/linux/man-pages/.
Linux 2011-09-18 SIGNAL(7)
本文永久更新链接地址：http://www.linuxidc.com/Linux/2015-07/120632.htm
