IO复用之poll与ppoll

来源：互联网发布：华发集团怎么样知乎编辑：程序博客网时间：2024/05/21 10:20

poll实现了与select类似的功能，但是，它突破了select最大描述字的限制。并且，不用遍历所有的描述字，所以，相对select有着较高的效率。

下面给出poll在linux下的解释信息。

NAME
poll, ppoll - wait for some event on a file descriptor

SYNOPSIS
#include <poll.h>

int poll(struct pollfd *fds, nfds_t nfds, int timeout);

#define _GNU_SOURCE
#include <poll.h>

int ppoll(struct pollfd *fds, nfds_t nfds,
const struct timespec *timeout, const sigset_t *sigmask);

DESCRIPTION
poll() performs a similar task to select(2): it waits for one of a set of file descriptors to
become ready to perform I/O.

The set of file descriptors to be monitored is specified in the fds argument, which is an array of
nfds structures of the following form:

           struct pollfd {               int   fd;         /* file descriptor */               short events;     /* requested events */               short revents;    /* returned events */           };

The field fd contains a file descriptor for an open file.

The field events is an input parameter, a bitmask specifying the events the application is inter-
ested in.

       The field revents is an output parameter, filled by the kernel with the events that actually
       occurred.   The bits returned in revents can include any of those specified in events, or one of
       the values POLLERR, POLLHUP, or POLLNVAL. (These three bits are meaningless in the events field,
       and will be set in the revents field whenever the corresponding condition is true.)

If none of the events requested (and no error) has occurred for any of the file descriptors, then
poll() blocks until one of the events occurs.

The timeout argument specifies an upper limit on the time for which poll() will block, in mil-
liseconds. Specifying a negative value in timeout means an infinite timeout.

The bits that may be set/returned in events and revents are defined in <poll.h>:

POLLIN There is data to read.

              POLLPRI
                     There is urgent data to read (e.g., out-of-band data on TCP socket; pseudo-terminal
                     master in packet mode has seen state change in slave).

POLLOUT
Writing now will not block.

              POLLRDHUP (since Linux 2.6.17)
                     Stream socket peer closed connection, or shut down writing half of connection.   The
                     _GNU_SOURCE feature test macro must be defined in order to obtain this definition.

POLLERR
Error condition (output only).

POLLHUP
Hang up (output only).

POLLNVAL
Invalid request: fd not open (output only).

When compiling with _XOPEN_SOURCE defined, one also has the following, which convey no further
information beyond the bits listed above:

POLLRDNORM
Equivalent to POLLIN.

POLLRDBAND
Priority band data can be read (generally unused on Linux).

POLLWRNORM
Equivalent to POLLOUT.

POLLWRBAND
Priority data may be written.

Linux also knows about, but does not use POLLMSG.

   ppoll()
       The relationship between poll() and ppoll() is analogous to the relationship between select() and
       pselect(): like pselect(), ppoll() allows an application to safely wait until either a file
       descriptor becomes ready or until a signal is caught.

Other than the difference in the timeout argument, the following ppoll() call:

           ready = ppoll(&fds, nfds, timeout, &sigmask);

is equivalent to atomically executing the following calls:

           sigset_t origmask;           sigprocmask(SIG_SETMASK, &sigmask, &origmask);           ready = ppoll(&fds, nfds, timeout);           sigprocmask(SIG_SETMASK, &origmask, NULL);

See the description of pselect(2) for an explanation of why ppoll() is necessary.

The timeout argument specifies an upper limit on the amount of time that ppoll() will block. This
argument is a pointer to a structure of the following form:

       struct timespec {             long    tv_sec;         /* seconds */             long    tv_nsec;        /* nanoseconds */         };

If timeout is specified as NULL, then ppoll() can block indefinitely.

RETURN VALUE
       On success, a positive number is returned; this is the number of structures which have non-zero
       revents fields (in other words, those descriptors with events or errors reported). A value of 0
       indicates that the call timed out and no file descriptors were ready. On error, -1 is returned,
       and errno is set appropriately.

ERRORS
EBADF An invalid file descriptor was given in one of the sets.

EFAULT The array given as argument was not contained in the calling program’s address space.

EINTR A signal occurred before any requested event.

EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

ENOMEM There was no space to allocate file descriptor tables.

LINUX NOTES
       The Linux ppoll() system call modifies its timeout argument. However, the glibc wrapper function
       hides this behaviour by using a local variable for the timeout argument that is passed to the sys-
       revents fields (in other words, those descriptors with events or errors reported). A value of 0
       indicates that the call timed out and no file descriptors were ready. On error, -1 is returned,
       and errno is set appropriately.

ERRORS
EBADF An invalid file descriptor was given in one of the sets.

EFAULT The array given as argument was not contained in the calling program’s address space.

EINTR A signal occurred before any requested event.

EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

ENOMEM There was no space to allocate file descriptor tables.

每一个pollfd结构体指定了一个被监视的文件描述符，可以传递多个结构体，指示poll()监视多个文件描述符。每个结构体的events域是监视该文件描述符的事件掩码，由用户来设置这个域。revents域是文件描述符的操作结果事件掩码。内核在调用返回时设置这个域。events域中请求的任何事件都可能在revents域中返回。

目前，poll与select都可以用最新的epoll模型取代，并且，epoll模型有着更好的效率。epoll的讲解，可以参考博文http://blog.csdn.net/pngynghay/article/details/8169455

0 0