The Linux Programming Interface

Alternative I/O Models 1363

printf("Ready: %d\n", ready);

/* Deal with returned list of events */

u for (j = 0; j < ready; j++) {

printf(" fd=%d; events: %s%s%s\n", evlist[j].data.fd,

(evlist[j].events & EPOLLIN)? "EPOLLIN " : "",

(evlist[j].events & EPOLLHUP)? "EPOLLHUP " : "",

(evlist[j].events & EPOLLERR)? "EPOLLERR " : "");

i if (evlist[j].events & EPOLLIN) {

s = read(evlist[j].data.fd, buf, MAX_BUF);

if (s == -1)

errExit("read");

printf(" read %d bytes: %.*s\n", s, s, buf);

o } else if (evlist[j].events & (EPOLLHUP | EPOLLERR)) {

/* If EPOLLIN and EPOLLHUP were both set, then there might

be more than MAX_BUF bytes to read. Therefore, we close

the file descriptor only if EPOLLIN was not set.

We'll read further bytes after the next epoll_wait(). */

printf(" closing fd %d\n", evlist[j].data.fd);

a if (close(evlist[j].data.fd) == -1)

errExit("close");

numOpenFds--;

}

}

}

printf("All file descriptors closed; bye\n");

exit(EXIT_SUCCESS);

}

––––––––––––––––––––––––––––––––––––––––––––––––––––––– altio/epoll_input.c

63.4.4 A Closer Look at epoll Semantics

We now look at some subtleties of the interaction of open files, file descriptors, and

epoll. For the purposes of this discussion, it is worth reviewing Figure 5-2 (page 95),

which shows the relationship between file descriptors, open file descriptions, and

the system-wide file i-node table.

When we create an epoll instance using epoll_create(), the kernel creates a new

in-memory i-node and open file description, and allocates a new file descriptor in

the calling process that refers to the open file description. The interest list for an

epoll instance is associated with the open file description, not with the epoll file

descriptor. This has the following consequences:

z If we duplicate an epoll file descriptor using dup() (or similar), then the dupli-

cated descriptor refers to the same epoll interest and ready lists as the original

descriptor. We may modify the interest list by specifying either file descriptor