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http://www.win.tue.nl/~aeb/linux/lk/lk-12.html
12.?Handling of asynchronous events
One wants to be notified of various events,like data that has become available,files that have changed,and signals that have been raised. FreeBSD has the nice?API. Let us discuss the Unix/Linux situation.
It is easy to wait for a single event. Usually one does a (blocking)?read(),and that is it.
Many mechanisms exist to wait for any of a set of events,or just to test whether anything interesting happened.
12.1?O_NONBLOCK
If the?open()?call that opened a file includes the O_NONBLOCK flag,the file is opened in non-blocking mode. Neither the?open()?nor any subsequent operations on the returned file descriptor will cause the calling process to wait.
A nonblocking open is useful (i) in order to obtain a file descriptor for subsequent use when no I/O is planned,e.g. for?ioctl()?calls to get or set properties of a device; especially on device files,an ordinary open might have unwanted side effects,such as a tape rewind etc. (ii) when reading from a pipe: the read will return immediately when no data is available; when writing to a pipe: the write will return immediately (without writing anything) when there are no readers.
O_NOACCESS
An obscure Linux feature is that one can open a file with the O_NOACCESS flag (defined as 3,where O_RDONLY is 0,O_WRONLY is 1 and O_RDWR is 2). In order to open a file with this mode,one needs both read and write permission. This had the same purpose: announce that no reading or writing was going to be done,and only a file descriptor for ioctl use was needed. (Used in LILO,fdformat,and a few similar utilities.)
People would love to have this facility also for directories,so that one could do a?fd = open(".",O_NOACCESS),go elsewhere,and return by?fchdir(fd). But an O_NOACCESS open fails on directories.
12.2?select
The?select()?mechanism was introduced in 4.2BSD. The prototype of this system call is
int select(int nfds,fd_set *restrict readfds,fd_set *restrict writefds,fd_set *restrict errorfds,struct timeval *restrict timeout);
It allows one to specify three sets of file descriptors (as bit masks) and a timeout. The call returns when the timeout expires or when one of the file descriptors inreadfds?has data available for reading,one of those in?writefds?has buffer space available for writing,or an error occurred for one of those in?errorfds. Upon return,the file descriptor sets and the timeout are rewritten to indicate which file descriptor has the stated condition,and how much time from the timeout is left. (Note that other Unix-type systems do not rewrite the timeout.)
There are two select system calls. The old one uses a parameter block,the new one uses five parameters. Otherwise they are equivalent.
12.3?pselect
The?pselect?system call was added in Linux 2.6.16 (and was present earlier elsewhere). With only?select()?it is difficult,almost impossible,to handle signals correctly. A signal handler itself cannot do very much: the main program is in some unknown state when the signal is delivered. The usual solution is to only raise a flag in the signal handler,and test that flag in the main program.
int gotsignal = 0;
void sighand(int x) {
gotsignal = 1;
}
int main() {
...
signal(SIGINT,sighand);
while (1) {
if (gotsignal) ...
select();
...
}
Now if one wants to wait for either a signal or some event on a file descriptor,then testing the flag and if it is not set calling?select()?has a race: maybe the signal arrived just after the flag was tested and just before select was called,and the program may hang in?select()?without reacting to the signal.
The call?pselect()?is designed to solve this problem. This function is just like?select()?but has prototype
int pselect(int nfds,const struct timespec *restrict timeout,const sigset_t *restrict sigmask);
with a sixth parameter?sigmask,and it does the equivalent of
sigset_t origmask;
sigprocmask(SIG_SETMASK,&sigmask,&origmask);
ready = select(nfds,&readfds,&writefds,&exceptfds,timeout);
sigprocmask(SIG_SETMASK,&origmask,NULL);
as an atomic action. Now one can block the signals of interest until the call of?pselect()?and have a?sigmask?that unblocks them. If a signal occurs,the call will return with?errno?set to EINTR.
This function uses a struct timespec (with nanoseconds) instead of a struct timeval (with microseconds),and does not update its value on return.
The self-pipe trick
Before the introduction of?pselect()?people resorted to obscure tricks to obtain the same effect. Famous is Daniel Bernstein's?: create a non-blocking pipe,and add a file descriptor for reading from this pipe to the?readfds?argument of?select(). In the signal handler,write a byte to the pipe. This works.
The system call
The pselect system call has a 7-parameter prototype (the 7th parameter being the size of the 6th?sigmask?parameter),but most architectures cannot handle 7-parameter system calls,so there is also a 6-parameter version where the 6th parameter is a pointer to a struct that has the last two parameters. Unlike the POSIX library routine,the system call does return the leftover part of the timeout.
This system call starts changing the signal mask,and ends restoring it. However,if it was interrupted by a signal,this signal should be delivered,while the signal mask might block it. This is solved by the recent?TIF_RESTORE_SIGMASK?mechanism in the kernel. When the pselect system call returns after being interrupted by a signal,it does not immediately restore the original signal mask,but first runs the user's signal handler,and first upon return from that the original signal mask is restored.
12.4?poll
The?poll()?system call is rather similar to?select(). The prototype is
struct pollfd {
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
};
int poll(struct pollfd *fds,nfds_t nfds,int timeout);
where the fields?events?amd?revents?are bitmasks indicating for what events?fd?should be watched,and what conditions actually occurred. The timeout is in milliseconds; a negative number means an infinite timeout.
ppoll
Just like?pselect?is a version of select that allows safe handling of signals,?ppoll?is such a version of?poll. The prototype is
int ppoll(struct pollfd *fds,const struct timespec *timeout,const sigset_t *sigmask);
12.5?epoll
When the number of file descriptors becomes very large,the?select()?and?poll()?mechanisms become inefficient. With N descriptors,O(N) information must be copied from user space to kernel and vice versa,and loops of length O(N) are needed to test the conditions.
Solaris introduced the?/dev/poll?mechanism (see?poll(7d)?on Solaris),where the idea is that one does the copy from user space to kernel only once (by writing an array of struct pollfd's to?/dev/poll) and gets only interesting information back (via an ioctl on this device that copies the interesting struct pollfd back to userspace).
Linux tries something similar using the three system calls?epoll_create,?epoll_ctl,?epoll_wait?(added in 2.5.44,see?epoll(7)). Benchmarks seem to indicate that the performance is comparable to that of select and poll until one has thousands of descriptors,only a small fraction of which is ready. (And then epoll is clearly better.) In most tests,the FreeBSD kqueue wins.
For a discussion of these and several other mechanisms,especially for the context of web servers,see?.
epoll_pwait
Just like?pselect?and?ppoll?are versions of?select?and?poll,there is (since 2.6.19) a?epoll_pwait?version of?epoll_wait?that includes a signal mask.
12.6?dnotify
The above was about notification about file descriptors that become ready for I/O. A different type of notification is that about file system events. In 2.4.0-test9 the?dnotify?feature was introduced. Today it is obsoleted by?inotify?(see below). See?Documentation/dnotify.txt?and?fs/dnotify.c.
The idea was that one could register interest in changes in a directory?dir?using?fd = open(dir,O_RDONLY)?followed by?fcntl(fd,F_NOTIFY,...). Notification occurs via delivery of a signal.
/* dnotify demo,basically from Documentation/dnotify.txt */
#define _GNU_SOURCE
#include
#include
#include
#include
static volatile int dir_fd;
/ A very weak interface: we report that something changed,but
the only info is in which directory,but not what the change is. /
static void handler(int sig,siginfo_t si,void data) {
dir_fd = si->si_fd;
}
int main(void) {
struct sigaction act;
int fd;
act.sa_sigaction = handler;
sigemptyset(&act.sa_mask);
act.sa_flags = SA_SIGINFO;
sigaction(SIGRTMIN + 1,&act,NULL);
fd = open(".",O_RDONLY);
fcntl(fd,F_SETSIG,SIGRTMIN + 1);
fcntl(fd,DN_MODIFY|DN_CREATE|DN_DELETE|DN_RENAME|DN_MULTISHOT);
while (1) {
pause();
printf("Got some event on fd=%dn",dir_fd);
}
}
There are many problems with this interface. It can only watch directories. If one wants to watch many directories,it takes many file descriptors. Moreover,the open file pins the filesystem so that it cannot be unmounted. When something happens it is unknown what,and a?stat()?on all files of interest is needed. The communication mechanism,signals,is unfortunate. Dnotify is obsolete now.
12.7?inotify
(Since 2.6.13.) Inotify is implemented using three new system calls and the usual?read(),?poll(),?close()?calls:
int inotify_init(void);
int inotify_add_watch (int fd,const char *pathname,int mask);
int inotify_rm_watch (int fd,int wd);
The first returns a file descriptor:?fd = inotify_init(). The second tells what to watch,and what to watch for,and returns a watch descriptor:?wd = inotify_add_watch(fd,"/home/aeb",IN_CREATE | IN_DELETE). The file descriptor?fd?can be used in a?read()?call,and then returns an array of struct inotify_event's. One can use?select()?and?poll()on it. A watch is removed by?inotify_rm_watch(fd,wd). The inotify instance is closed by?close(fd).
An inotify_event is defined by
struct inotify_event {
int wd; /* Watch descriptor */
uint32_t mask; /* Mask of events */
uint32_t cookie; /* Unique cookie associating related
events (for rename(2)) */
uint32_t len; /* Size of 'name' field */
char name[]; /* Optional null-terminated name */
};
The?name?field defines the file involved,when one is watching a directory.
There is a?/proc?interface with settable limits:
% ls /proc/sys/fs/inotify/
max_queued_events max_user_instances max_user_watches
% cat $_/*
16384
128
8192
Applications are?,??and?.
/* inotify demo,mimicking the above dnotify one */
#include
#include
#include
#include
#include
define BUFSZ 16384
static void errexit(char *s) {
fprintf(stderr,"%sn",s);
exit(1);
}
int main(void) {
int ifd,wd,i,n;
char buf[BUFSZ];
ifd = inotify_init();
if (ifd < 0)
errexit("cannot obtain an inotify instance");
wd = inotify_add_watch(ifd,".",IN_MODIFY|IN_CREATE|IN_DELETE);
if (wd < 0)
errexit("cannot add inotify watch");
while (1) {
n = read(ifd,buf,sizeof(buf));
if (n <= 0)
errexit("read problem");
i = 0;
while (i < n) {
struct inotify_event *ev;
ev = (struct inotify_event *) &buf[i];
if (ev->len)
printf("file %s %sn",ev->name,(ev->mask & IN_CREATE) ? "created" :
(ev->mask & IN_DELETE) ? "deleted" :
"modified");
else
printf("unexpected event - wd=%d mask=%dn",ev->wd,ev->mask);
i += sizeof(struct inotify_event) + ev->len;
}
printf("---n");
}
} （编辑：李大同）
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