poll同select，用于监控file descriptor事件，推荐用poll的升级版epool来实现功能，但在简单应用中使用poll更方便。
#include <poll.h> int poll(struct pollfd *fds, nfds_t nfds, int timeout); struct pollfd { int fd; short events; short revents; };
fds是文件描述符集的数组，nfds指定数组元素个数。
pollfd中fd是打开文件的文件描述符；events是输入参数，指定监控事件；revents是输出参数，被内核填充，只是实际发生的事件。
timeout指定超时毫秒数（milliseconds）。
成功返回正数，表示非0revents的fd个数，0表示超时，错误返回-1并设置errno。
On success, a positive number is returned; this is the number of structures which have nonzero 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.
应用举例
监控一个网络socket是否发生错误（刚链接时）。
int usock_wait_ready(int fd, int msecs) { struct pollfd fds[1]; int res; fds[0].fd = fd; fds[0].events = POLLOUT; res = poll(fds, 1, msecs); if (res < 0) { return errno; } else if (res == 0) { return -ETIMEDOUT; } else { int err = 0; socklen_t optlen = sizeof(err); res = getsockopt(fd, SOL_SOCKET, SO_ERROR, &err, &optlen); if (res) return errno; if (err) return err; } return 0; }