Originální popis anglicky:
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous I/O
multiplexing
Návod, kniha: Linux Programmer's Manual
#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 *utimeout);
int pselect(int nfds, fd_set *readfds,
fd_set *writefds, fd_set *exceptfds,
const struct timespec *ntimeout, sigset_t
*sigmask);
FD_CLR(int fd, fd_set *set);
FD_ISSET(int fd, fd_set *set);
FD_SET(int fd, fd_set *set);
FD_ZERO(fd_set *set);
select (or
pselect) is the pivot function of most C programs that
handle more than one simultaneous file descriptor (or socket handle) in an
efficient manner. Its principal arguments are three arrays of file
descriptors:
readfds,
writefds, and
exceptfds. The way
that
select is usually used is to block while waiting for a
"change of status" on one or more of the file descriptors. A
"change of status" is when more characters become available from the
file descriptor,
or when space becomes available within the kernel's
internal buffers for more to be written to the file descriptor,
or when
a file descriptor goes into error (in the case of a socket or pipe this is
when the other end of the connection is closed).
In summary,
select just watches multiple file descriptors, and is the
standard Unix call to do so.
The arrays of file descriptors are called
file descriptor sets. Each set
is declared as type
fd_set, and its contents can be altered with the
macros
FD_CLR,
FD_ISSET,
FD_SET, and
FD_ZERO.
FD_ZERO is usually the first function to be used on a newly declared
set. Thereafter, the individual file descriptors that you are interested in
can be added one by one with
FD_SET.
select modifies the
contents of the sets according to the rules described below; after calling
select you can test if your file descriptor is still present in the set
with the
FD_ISSET macro.
FD_ISSET returns non-zero if the
descriptor is present and zero if it is not.
FD_CLR removes a file
descriptor from the set although I can't see the use for it in a clean
program.
- readfds
- This set is watched to see if data is available for reading
from any of its file descriptors. After select has returned,
readfds will be cleared of all file descriptors except for those
file descriptors that are immediately available for reading with a
recv() (for sockets) or read() (for pipes, files, and
sockets) call.
- writefds
- This set is watched to see if there is space to write data
to any of its file descriptor. After select has returned,
writefds will be cleared of all file descriptors except for those
file descriptors that are immediately available for writing with a
send() (for sockets) or write() (for pipes, files, and
sockets) call.
- exceptfds
- This set is watched for exceptions or errors on any of the
file descriptors. However, that is actually just a rumor. How you use
exceptfds is to watch for out-of-band (OOB) data. OOB data
is data sent on a socket using the MSG_OOB flag, and hence
exceptfds only really applies to sockets. See recv(2) and
send(2) about this. After select has returned,
exceptfds will be cleared of all file descriptors except for those
descriptors that are available for reading OOB data. You can only ever
read one byte of OOB data though (which is done with recv()), and
writing OOB data (done with send) can be done at any time and will
not block. Hence there is no need for a fourth set to check if a socket is
available for writing OOB data.
- nfds
- This is an integer one more than the maximum of any file
descriptor in any of the sets. In other words, while you are busy adding
file descriptors to your sets, you must calculate the maximum integer
value of all of them, then increment this value by one, and then pass this
as nfds to select.
- utimeout
This is the longest time
select must
wait before returning, even if nothing interesting happened. If this value is
passed as
NULL, then
select blocks indefinitely waiting for an
event.
utimeout can be set to zero seconds, which causes
select
to return immediately. The structure
struct timeval is defined as,
struct timeval {
time_t tv_sec; /* seconds */
long tv_usec; /* microseconds */
};
- ntimeout
This argument has the same meaning as
utimeout but
struct timespec has nanosecond precision as
follows,
struct timespec {
long tv_sec; /* seconds */
long tv_nsec; /* nanoseconds */
};
- sigmask
- This argument holds a set of signals to allow while
performing a pselect call (see sigaddset(3) and
sigprocmask(2)). It can be passed as NULL, in which case it does
not modify the set of allowed signals on entry and exit to the function.
It will then behave just like select.
pselect must be used if you are waiting for a signal as well as data from
a file descriptor. Programs that receive signals as events normally use the
signal handler only to raise a global flag. The global flag will indicate that
the event must be processed in the main loop of the program. A signal will
cause the
select (or
pselect) call to return with
errno
set to
EINTR. This behavior is essential so that signals can be
processed in the main loop of the program, otherwise
select would block
indefinitely. Now, somewhere in the main loop will be a conditional to check
the global flag. So we must ask: what if a signal arrives after the
conditional, but before the
select call? The answer is that
select would block indefinitely, even though an event is actually
pending. This race condition is solved by the
pselect call. This call
can be used to mask out signals that are not to be received except within the
pselect call. For instance, let us say that the event in question was
the exit of a child process. Before the start of the main loop, we would block
SIGCHLD using
sigprocmask. Our
pselect call would enable
SIGCHLD by using the virgin signal mask. Our program would look like:
int child_events = 0;
void child_sig_handler (int x) {
child_events++;
signal (SIGCHLD, child_sig_handler);
}
int main (int argc, char **argv) {
sigset_t sigmask, orig_sigmask;
sigemptyset (&sigmask);
sigaddset (&sigmask, SIGCHLD);
sigprocmask (SIG_BLOCK, &sigmask,
&orig_sigmask);
signal (SIGCHLD, child_sig_handler);
for (;;) { /* main loop */
for (; child_events > 0; child_events--) {
/* do event work here */
}
r = pselect (nfds, &rd, &wr, &er, 0, &orig_sigmask);
/* main body of program */
}
}
Note that the above
pselect call can be replaced with:
sigprocmask (SIG_BLOCK, &orig_sigmask, 0);
r = select (nfds, &rd, &wr, &er, 0);
sigprocmask (SIG_BLOCK, &sigmask, 0);
but then there is still the possibility that a signal could arrive after the
first
sigprocmask and before the
select. If you do do this, it
is prudent to at least put a finite timeout so that the process does not
block. At present glibc probably works this way. The Linux kernel does not
have a native
pselect system call as yet so this is all probably much
of a moot point.
So what is the point of
select? Can't I just read and write to my
descriptors whenever I want? The point of select is that it watches multiple
descriptors at the same time and properly puts the process to sleep if there
is no activity. It does this while enabling you to handle multiple
simultaneous pipes and sockets. Unix programmers often find themselves in a
position where they have to handle IO from more than one file descriptor where
the data flow may be intermittent. If you were to merely create a sequence of
read and
write calls, you would find that one of your calls may
block waiting for data from/to a file descriptor, while another file
descriptor is unused though available for data.
select efficiently
copes with this situation.
A classic example of
select comes from the
select man page:
#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");
exit(0);
}
Here is an example that better demonstrates the true utility of
select.
The listing below a TCP forwarding program that forwards from one TCP port to
another.
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/time.h>
#include <sys/types.h>
#include <string.h>
#include <signal.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <errno.h>
static int forward_port;
#undef max
#define max(x,y) ((x) > (y) ? (x) : (y))
static int listen_socket (int listen_port) {
struct sockaddr_in a;
int s;
int yes;
if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) {
perror ("socket");
return -1;
}
yes = 1;
if (setsockopt
(s, SOL_SOCKET, SO_REUSEADDR,
(char *) &yes, sizeof (yes)) < 0) {
perror ("setsockopt");
close (s);
return -1;
}
memset (&a, 0, sizeof (a));
a.sin_port = htons (listen_port);
a.sin_family = AF_INET;
if (bind
(s, (struct sockaddr *) &a, sizeof (a)) < 0) {
perror ("bind");
close (s);
return -1;
}
printf ("accepting connections on port %d\n",
(int) listen_port);
listen (s, 10);
return s;
}
static int connect_socket (int connect_port,
char *address) {
struct sockaddr_in a;
int s;
if ((s = socket (AF_INET, SOCK_STREAM, 0)) < 0) {
perror ("socket");
close (s);
return -1;
}
memset (&a, 0, sizeof (a));
a.sin_port = htons (connect_port);
a.sin_family = AF_INET;
if (!inet_aton
(address,
(struct in_addr *) &a.sin_addr.s_addr)) {
perror ("bad IP address format");
close (s);
return -1;
}
if (connect
(s, (struct sockaddr *) &a,
sizeof (a)) < 0) {
perror ("connect()");
shutdown (s, SHUT_RDWR);
close (s);
return -1;
}
return s;
}
#define SHUT_FD1 { \
if (fd1 >= 0) { \
shutdown (fd1, SHUT_RDWR); \
close (fd1); \
fd1 = -1; \
} \
}
#define SHUT_FD2 { \
if (fd2 >= 0) { \
shutdown (fd2, SHUT_RDWR); \
close (fd2); \
fd2 = -1; \
} \
}
#define BUF_SIZE 1024
int main (int argc, char **argv) {
int h;
int fd1 = -1, fd2 = -1;
char buf1[BUF_SIZE], buf2[BUF_SIZE];
int buf1_avail, buf1_written;
int buf2_avail, buf2_written;
if (argc != 4) {
fprintf (stderr,
"Usage\n\tfwd <listen-port> \
<forward-to-port> <forward-to-ip-address>\n");
exit (1);
}
signal (SIGPIPE, SIG_IGN);
forward_port = atoi (argv[2]);
h = listen_socket (atoi (argv[1]));
if (h < 0)
exit (1);
for (;;) {
int r, nfds = 0;
fd_set rd, wr, er;
FD_ZERO (&rd);
FD_ZERO (&wr);
FD_ZERO (&er);
FD_SET (h, &rd);
nfds = max (nfds, h);
if (fd1 > 0 && buf1_avail < BUF_SIZE) {
FD_SET (fd1, &rd);
nfds = max (nfds, fd1);
}
if (fd2 > 0 && buf2_avail < BUF_SIZE) {
FD_SET (fd2, &rd);
nfds = max (nfds, fd2);
}
if (fd1 > 0
&& buf2_avail - buf2_written > 0) {
FD_SET (fd1, &wr);
nfds = max (nfds, fd1);
}
if (fd2 > 0
&& buf1_avail - buf1_written > 0) {
FD_SET (fd2, &wr);
nfds = max (nfds, fd2);
}
if (fd1 > 0) {
FD_SET (fd1, &er);
nfds = max (nfds, fd1);
}
if (fd2 > 0) {
FD_SET (fd2, &er);
nfds = max (nfds, fd2);
}
r = select (nfds + 1, &rd, &wr, &er, NULL);
if (r == -1 && errno == EINTR)
continue;
if (r < 0) {
perror ("select()");
exit (1);
}
if (FD_ISSET (h, &rd)) {
unsigned int l;
struct sockaddr_in client_address;
memset (&client_address, 0, l =
sizeof (client_address));
r = accept (h, (struct sockaddr *)
&client_address, &l);
if (r < 0) {
perror ("accept()");
} else {
SHUT_FD1;
SHUT_FD2;
buf1_avail = buf1_written = 0;
buf2_avail = buf2_written = 0;
fd1 = r;
fd2 =
connect_socket (forward_port,
argv[3]);
if (fd2 < 0) {
SHUT_FD1;
} else
printf ("connect from %s\n",
inet_ntoa
(client_address.sin_addr));
}
}
/* NB: read oob data before normal reads */
if (fd1 > 0)
if (FD_ISSET (fd1, &er)) {
char c;
errno = 0;
r = recv (fd1, &c, 1, MSG_OOB);
if (r < 1) {
SHUT_FD1;
} else
send (fd2, &c, 1, MSG_OOB);
}
if (fd2 > 0)
if (FD_ISSET (fd2, &er)) {
char c;
errno = 0;
r = recv (fd2, &c, 1, MSG_OOB);
if (r < 1) {
SHUT_FD1;
} else
send (fd1, &c, 1, MSG_OOB);
}
if (fd1 > 0)
if (FD_ISSET (fd1, &rd)) {
r =
read (fd1, buf1 + buf1_avail,
BUF_SIZE - buf1_avail);
if (r < 1) {
SHUT_FD1;
} else
buf1_avail += r;
}
if (fd2 > 0)
if (FD_ISSET (fd2, &rd)) {
r =
read (fd2, buf2 + buf2_avail,
BUF_SIZE - buf2_avail);
if (r < 1) {
SHUT_FD2;
} else
buf2_avail += r;
}
if (fd1 > 0)
if (FD_ISSET (fd1, &wr)) {
r =
write (fd1,
buf2 + buf2_written,
buf2_avail -
buf2_written);
if (r < 1) {
SHUT_FD1;
} else
buf2_written += r;
}
if (fd2 > 0)
if (FD_ISSET (fd2, &wr)) {
r =
write (fd2,
buf1 + buf1_written,
buf1_avail -
buf1_written);
if (r < 1) {
SHUT_FD2;
} else
buf1_written += r;
}
/* check if write data has caught read data */
if (buf1_written == buf1_avail)
buf1_written = buf1_avail = 0;
if (buf2_written == buf2_avail)
buf2_written = buf2_avail = 0;
/* one side has closed the connection, keep
writing to the other side until empty */
if (fd1 < 0
&& buf1_avail - buf1_written == 0) {
SHUT_FD2;
}
if (fd2 < 0
&& buf2_avail - buf2_written == 0) {
SHUT_FD1;
}
}
return 0;
}
The above program properly forwards most kinds of TCP connections including OOB
signal data transmitted by
telnet servers. It handles the tricky
problem of having data flow in both directions simultaneously. You might think
it more efficient to use a
fork() call and devote a thread to each
stream. This becomes more tricky than you might suspect. Another idea is to
set non-blocking IO using an
ioctl() call. This also has its problems
because you end up having to have inefficient timeouts.
The program does not handle more than one simultaneous connection at a time,
although it could easily be extended to do this with a linked list of buffers
- one for each connection. At the moment, new connections cause the current
connection to be dropped.
Many people who try to use
select come across behavior that is difficult
to understand and produces non-portable or borderline results. For instance,
the above program is carefully written not to block at any point, even though
it does not set its file descriptors to non-blocking mode at all (see
ioctl(2)). It is easy to introduce subtle errors that will remove the
advantage of using
select, hence I will present a list of essentials to
watch for when using the
select call.
- 1.
- You should always try use select without a timeout.
Your program should have nothing to do if there is no data available. Code
that depends on timeouts is not usually portable and difficult to
debug.
- 2.
- The value nfds must be properly calculated for
efficiency as explained above.
- 3.
- No file descriptor must be added to any set if you do not
intend to check its result after the select call, and respond
appropriately. See next rule.
- 4.
- After select returns, all file descriptors in all
sets must be checked. Any file descriptor that is available for
writing must be written to, and any file descriptor available for
reading must be read, etc.
- 5.
- The functions read(), recv(), write(),
and send() do not necessarily read/write the full amount of
data that you have requested. If they do read/write the full amount, its
because you have a low traffic load and a fast stream. This is not always
going to be the case. You should cope with the case of your functions only
managing to send or receive a single byte.
- 6.
- Never read/write only in single bytes at a time unless your
are really sure that you have a small amount of data to process. It is
extremely inefficient not to read/write as much data as you can buffer
each time. The buffers in the example above are 1024 bytes although they
could easily be made as large as the maximum possible packet size on your
local network.
- 7.
- The functions read(), recv(), write(),
and send() as well as the select() call can return -1 with
an errno of EINTR or EAGAIN (EWOULDBLOCK) which are
not errors. These results must be properly managed (not done properly
above). If your program is not going to receive any signals then it is
unlikely you will get EINTR. If your program does not set
non-blocking IO, you will not get EAGAIN. Nonetheless you should
still cope with these errors for completeness.
- 8.
- Never call read(), recv(), write(), or
send() with a buffer length of zero.
- 9.
- Except as indicated in 7., the functions
read(), recv(), write(), and send() never have
a return value less than 1 except if an error has occurred. For instance,
a read() on a pipe where the other end has died returns zero (so
does an end-of-file error), but only returns zero once (a followup
read or write will return -1). Should any of these functions return 0 or
-1, you should not pass that descriptor to select ever again. In
the above example, I close the descriptor immediately, and then set it to
-1 to prevent it being included in a set.
- 10.
- The timeout value must be initialized with each new call to
select, since some operating systems modify the structure.
pselect however does not modify its timeout structure.
- 11.
- I have heard that the Windows socket layer does not cope
with OOB data properly. It also does not cope with select calls
when no file descriptors are set at all. Having no file descriptors set is
a useful way to sleep the process with sub-second precision by using the
timeout. (See further on.)
On systems that do not have a
usleep function, you can call
select
with a finite timeout and no file descriptors as follows:
struct timeval tv;
tv.tv_sec = 0;
tv.tv_usec = 200000; /* 0.2 seconds */
select (0, NULL, NULL, NULL, &tv);
This is only guarenteed to work on Unix systems, however.
On success,
select returns the total number of file descriptors still
present in the file descriptor sets.
If
select timed out, then the file descriptors sets should be all empty
(but may not be on some systems). However the return value will definitely be
zero.
A return value of -1 indicates an error, with
errno being set
appropriately. In the case of an error, the returned sets and the timeout
struct contents are undefined and should not be used.
pselect however
never modifies
ntimeout.
- EBADF
- A set contained an invalid file descriptor. This error
often occurs when you add a file descriptor to a set that you have already
issued a close on, or when that file descriptor has experienced
some kind of error. Hence you should cease adding to sets any file
descriptor that returns an error on reading or writing.
- EINTR
- An interrupting signal was caught like SIGINT or
SIGCHLD etc. In this case you should rebuild your file descriptor
sets and retry.
- EINVAL
- Occurs if nfds is negative or an invalid value is
specified in utimeout or ntimeout.
- ENOMEM
- Internal memory allocation failure.
Generally speaking, all operating systems that support sockets, also support
select. Some people consider
select to be an esoteric and rarely
used function. Indeed, many types of programs become extremely complicated
without it.
select can be used to solve many problems in a portable and
efficient way that naive programmers try to solve with threads, forking, IPCs,
signals, memory sharing and other dirty methods.
pselect is a newer
function that is less commonly used.
The
poll(2) system call has the same functionality as
select, but
with less subtle behavior. It is less portable than
select.
4.4BSD (the
select function first appeared in 4.2BSD). Generally portable
to/from non-BSD systems supporting clones of the BSD socket layer (including
System V variants). However, note that the System V variant typically sets the
timeout variable before exit, but the BSD variant does not.
The
pselect function is defined in IEEE Std 1003.1g-2000 (POSIX.1g). It
is found in glibc2.1 and later. Glibc2.0 has a function with this name, that
however does not take a
sigmask parameter.
accept(2),
connect(2),
ioctl(2),
poll(2),
read(2),
recv(2),
select(2),
send(2),
sigprocmask(2),
write(2),
sigaddset(3),
sigdelset(3),
sigemptyset(3),
sigfillset(3),
sigismember(3)
This man page was written by Paul Sheer.
