关于select的exceptfds参数

[liangchencf]

哪位了解select的第四个参数，看了很多代码都忽略了这个参数，只知道是检测异常的文件描述符的，具体怎么用不得而知，网上也搜不到。谢了

[wuwusansan]

内核检测在timeout时间内完成的

[xixuer_20070803]

int select( int width,//最大句柄数加1

fd_set *read_fds,//监视的可读文件句柄集合。

fd_set *write_fds,//监视的可写文件句柄集合。

fd_set *excepr_fds,//监视的异常文件句柄集合。

struct timeval *timeout);本次select()的超时结束时间。

只是出现异常的文件或socket句柄集合，（因为这个函数要判断socket或文件可读写然后操作，中间异常的话用这个吧）

[yylklshmyt20090217]

struct timeval *timeout
select等待描述符活动时间 超过这个时间 则select这个等待描述符过程结束

[勤奋的沉沦]

SELECT(2) Linux Programmer’s Manual SELECT(2)

NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O multiplexing

SYNOPSIS
/* According to POSIX.1-2001 */
#include <sys/select.h>

/* According to earlier standards */
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);

void FD_CLR(int fd, fd_set *set);
int FD_ISSET(int fd, fd_set *set);
void FD_SET(int fd, fd_set *set);
void FD_ZERO(fd_set *set);

#define _XOPEN_SOURCE 600
#include <sys/select.h>

int pselect(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, const struct timespec *timeout,
const sigset_t *sigmask);

DESCRIPTION
select() and pselect() allow a program to monitor multiple file descriptors, waiting until one or more of the file
descriptors become "ready" for some class of I/O operation (e.g., input possible). A file descriptor is considered
ready if it is possible to perform the corresponding I/O operation (e.g., read(2)) without blocking.

The operation of select() and pselect() is identical, with three differences:

(i) select() uses a timeout that is a struct timeval (with seconds and microseconds), while pselect() uses a struct
timespec (with seconds and nanoseconds).

(ii) select() may update the timeout argument to indicate how much time was left. pselect() does not change this
argument.

(iii) select() has no sigmask argument, and behaves as pselect() called with NULL sigmask.

Three independent sets of file descriptors are watched. Those listed in readfds will be watched to see if characters
become available for reading (more precisely, to see if a read will not block; in particular, a file descriptor is also
ready on end-of-file), those in writefds will be watched to see if a write will not block, and those in exceptfds will
be watched for exceptions. On exit, the sets are modified in place to indicate which file descriptors actually changed
status. Each of the three file descriptor sets may be specified as NULL if no file descriptors are to be watched for
the corresponding class of events.

Four macros are provided to manipulate the sets. FD_ZERO() clears a set. FD_SET() and FD_CLR() respectively add and
remove a given file descriptor from a set. FD_ISSET() tests to see if a file descriptor is part of the set; this is
useful after select() returns.

nfds is the highest-numbered file descriptor in any of the three sets, plus 1.

timeout is an upper bound on the amount of time elapsed before select() returns. It may be zero, causing select() to
return immediately. (This is useful for polling.) If timeout is NULL (no timeout), select() can block indefinitely.

sigmask is a pointer to a signal mask (see sigprocmask(2)); if it is not NULL, then pselect() first replaces the current
signal mask by the one pointed to by sigmask, then does the ‘select’ function, and then restores the original signal
mask.

Other than the difference in the precision of the timeout argument, the following pselect() call:

ready = pselect(nfds, &readfds, &writefds, &exceptfds,
timeout, &sigmask);

is equivalent to atomically executing the following calls:

sigset_t origmask;

sigprocmask(SIG_SETMASK, &sigmask, &origmask);
ready = select(nfds, &readfds, &writefds, &exceptfds, timeout);
sigprocmask(SIG_SETMASK, &origmask, NULL);

The reason that pselect() is needed is that if one wants to wait for either a signal or for a file descriptor to become
ready, then an atomic test is needed to prevent race conditions. (Suppose the signal handler sets a global flag and
returns. Then a test of this global flag followed by a call of select() could hang indefinitely if the signal arrived
just after the test but just before the call. By contrast, pselect() allows one to first block signals, handle the sig-
nals that have come in, then call pselect() with the desired sigmask, avoiding the race.)

The timeout
The time structures involved are defined in <sys/time.h> and look like

struct timeval {
long tv_sec; /* seconds */
long tv_usec; /* microseconds */
};

and

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

(However, see below on the POSIX.1-2001 versions.)

Some code calls select() with all three sets empty, n zero, and a non-NULL timeout as a fairly portable way to sleep
with subsecond precision.

On Linux, select() modifies timeout to reflect the amount of time not slept; most other implementations do not do this.
(POSIX.1-2001 permits either behaviour.) This causes problems both when Linux code which reads timeout is ported to
other operating systems, and when code is ported to Linux that reuses a struct timeval for multiple select()s in a loop
without reinitializing it. Consider timeout to be undefined after select() returns.

RETURN VALUE
On success, select() and pselect() return the number of file descriptors contained in the three returned descriptor sets
(that is, the total number of bits that are set in readfds, writefds, exceptfds) which may be zero if the timeout
expires before anything interesting happens. On error, -1 is returned, and errno is set appropriately; the sets and
timeout become undefined, so do not rely on their contents after an error.
ERRORS
EBADF An invalid file descriptor was given in one of the sets. (Perhaps a file descriptor that was already closed, or
one on which an error has occurred.)

EINTR A signal was caught.

EINVAL nfds is negative or the value contained within timeout is invalid.

ENOMEM unable to allocate memory for internal tables.

EXAMPLE
#include <stdio.h>
#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int
main(void) {
fd_set rfds;
struct timeval tv;
int retval;

/* Watch stdin (fd 0) to see when it has input. */
FD_ZERO(&rfds);
FD_SET(0, &rfds);

/* Wait up to five seconds. */
tv.tv_sec = 5;
tv.tv_usec = 0;

retval = select(1, &rfds, NULL, NULL, &tv);
/* Don’t rely on the value of tv now! */
:
if (retval == -1)
perror("select()");
else if (retval)
printf("Data is available now.\n");
/* FD_ISSET(0, &rfds) will be true. */
else
printf("No data within five seconds.\n");

return 0;
}

[challenge99]

msdn

In summary, a socket will be identified in a particular set when select returns if:

readfds:
If listen has been called and a connection is pending, accept will succeed.
Data is available for reading (includes OOB data if SO_OOBINLINE is enabled).
Connection has been closed/reset/terminated.

writefds:
If processing a connect call (nonblocking), connection has succeeded.
Data can be sent.

exceptfds:
If processing a connect call (nonblocking), connection attempt failed.
OOB data is available for reading (only if SO_OOBINLINE is disabled).