Section (2) select_tut
Name
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO — synchronous I/O multiplexing
Synopsis
/* According to POSIX.1-2001, POSIX.1-2008 */ #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 *utimeout) ; |
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) ; |
#include <sys/select.h>
int
pselect( |
int nfds, |
fd_set *readfds, | |
fd_set *writefds, | |
fd_set *exceptfds, | |
const struct timespec *ntimeout, | |
const sigset_t *sigmask) ; |
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Note | ||
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DESCRIPTION
select
() (or pselect
()) is used to efficiently monitor
multiple file descriptors, to see if any of them is, or
becomes, ready; that is, to see whether I/O becomes
possible, or an exceptional condition has occurred on any
of the file descriptors.
Its principal arguments are three sets of file
descriptors: readfds
,
writefds
, and
exceptfds
. Each set
is declared as type fd_set
, and its contents can
be manipulated with the macros FD_CLR
(), FD_ISSET
(), FD_SET
(), and FD_ZERO
(). A newly declared set should
first be cleared using FD_ZERO
(). select
() modifies the contents of the sets
according to the rules described below; after calling
select
() you can test if a file
descriptor is still present in a set with the FD_ISSET
() macro. FD_ISSET
() returns nonzero if a specified
file descriptor is present in a set and zero if it is not.
FD_CLR
() removes a file
descriptor from a set.
Arguments
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 that are immediately available for reading. writefds
-
This set is watched to see if there is space to write data to any of its file descriptors. After
select
() has returned,writefds
will be cleared of all file descriptors except for those that are immediately available for writing. exceptfds
-
This set is watched for exceptional conditions. In practice, only one such exceptional condition is common: the availability of
out-of-band
(OOB) data for reading from a TCP socket. See recv(2), send(2), and tcp(7) for more details about OOB data. (One other less common case where select(2) indicates an exceptional condition occurs with pseudoterminals in packet mode; see ioctl_tty(2).) Afterselect
() has returned,exceptfds
will be cleared of all file descriptors except for those for which an exceptional condition has occurred. nfds
-
This is an integer one more than the maximum of any file descriptor in any of the sets. In other words, while adding file descriptors to each of the sets, you must calculate the maximum integer value of all of them, then increment this value by one, and then pass this as
nfds
. utimeout
-
This is the longest time
select
() may wait before returning, even if nothing interesting happened. If this value is passed as NULL, thenselect
() blocks indefinitely waiting for a file descriptor to become ready.utimeout
can be set to zero seconds, which causesselect
() to return immediately, with information about the readiness of file descriptors at the time of the call. The structure struct timeval is defined as:struct timeval { time_t tv_sec
;/* seconds */long tv_usec
;/* microseconds */}; ntimeout
-
This argument for
pselect
() has the same meaning asutimeout
, 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 that the kernel should unblock (i.e., remove from the signal mask of the calling thread), while the caller is blocked inside the
pselect
() call (see sigaddset(3) and sigprocmask(2)). It may be NULL, in which case the call does not modify the signal mask on entry and exit to the function. In this case,pselect
() will then behave just likeselect
().
Combining signal and data events
pselect
() is useful if you
are waiting for a signal as well as for file descriptor(s)
to become ready for I/O. Programs that receive signals
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
set the signal mask to a set of signals that are to be
received only 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(2). Our
pselect
() call would enable
SIGCHLD
by using an empty
signal mask. Our program would look like:
static volatile sig_atomic_t got_SIGCHLD = 0; static void child_sig_handler(int sig) { got_SIGCHLD = 1; } int main(int argc, char *argv[]) { sigset_t sigmask, empty_mask; struct sigaction sa; fd_set readfds, writefds, exceptfds; int r; sigemptyset(&sigmask); sigaddset(&sigmask, SIGCHLD); if (sigprocmask(SIG_BLOCK, &sigmask, NULL) == −1) { perror(sigprocmask); exit(EXIT_FAILURE); } sa.sa_flags = 0; sa.sa_handler = child_sig_handler; sigemptyset(&sa.sa_mask); if (sigaction(SIGCHLD, &sa, NULL) == −1) { perror(sigaction); exit(EXIT_FAILURE); } sigemptyset(&empty_mask); for (;;) { /* main loop */ /* Initialize readfds, writefds, and exceptfds before the pselect() call. (Code omitted.) */ r = pselect(nfds, &readfds, &writefds, &exceptfds, NULL, &empty_mask); if (r == −1 && errno != EINTR) { /* Handle error */ } if (got_SIGCHLD) { got_SIGCHLD = 0; /* Handle signalled event here; e.g., wait() for all terminated children. (Code omitted.) */ } /* main body of program */ } }
Practical
So what is the point of select
()? Can_zsingle_quotesz_t I just read and write to
my file 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. UNIX programmers
often find themselves in a position where they have to
handle I/O from more than one file descriptor where the
data flow may be intermittent. If you were to merely create
a sequence of read(2) and write(2) 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 ready for I/O. select
() efficiently copes with this
situation.
Select law
Many people who try to use select
() come across behavior that is
difficult to understand and produces nonportable 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 nonblocking mode. It
is easy to introduce subtle errors that will remove the
advantage of using select
(),
so here is a list of essentials to watch for when using
select
().
-
You should always try to 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 is difficult to debug. -
The value
nfds
must be properly calculated for efficiency as explained above. -
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. -
After
select
() returns, all file descriptors in all sets should be checked to see if they are ready. -
The functions read(2), recv(2), write(2), and send(2) do
not
necessarily read/write the full amount of data that you have requested. If they do read/write the full amount, it_zsingle_quotesz_s 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 managing to send or receive only a single byte. -
Never read/write only in single bytes at a time unless you 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 below are 1024 bytes although they could easily be made larger.
-
Calls to read(2), recv(2), write(2), send(2), and
select
() can fail with the error EINTR, and calls to read(2), recv(2) write(2), and send(2) can fail witherrno
set to EAGAIN (EWOULDBLOCK). 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 nonblocking I/O, you will not get EAGAIN. -
Never call read(2), recv(2), write(2), or send(2) with a buffer length of zero.
-
If the functions read(2), recv(2), write(2), and send(2) fail with errors other than those listed in
7.
, or one of the input functions returns 0, indicating end of file, then you shouldnot
pass that file descriptor toselect
() again. In the example below, I close the file descriptor immediately, and then set it to −1 to prevent it being included in a set. -
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. -
Since
select
() modifies its file descriptor sets, if the call is being used in a loop, then the sets must be reinitialized before each call.
Usleep emulation
On systems that do not have a usleep(3) 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 guaranteed to work only on UNIX systems, however.
RETURN VALUE
On success, select
() returns
the total number of file descriptors still present in the
file descriptor sets.
If select
() timed out, then
the return value will be zero. The file descriptors set
should be all empty (but may not be on some systems).
A return value of −1 indicates an error, with
errno
being set appropriately.
In the case of an error, the contents of the returned sets
and the struct timeout
contents are undefined and should not be used. pselect
() however never modifies ntimeout
.
NOTES
Generally speaking, all operating systems that support
sockets also support select
().
select
() can be used to solve
many problems in a portable and efficient way that naive
programmers try to solve in a more complicated manner using
threads, forking, IPCs, signals, memory sharing, and so
on.
The poll(2) system call has the
same functionality as select
(),
and is somewhat more efficient when monitoring sparse file
descriptor sets. It is nowadays widely available, but
historically was less portable than select
().
The Linux-specific epoll(7) API provides an interface that is more efficient than select(2) and poll(2) when monitoring large numbers of file descriptors.
EXAMPLE
Here is an example that better demonstrates the true
utility of select
(). The
listing below is 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 addr; int lfd; int yes; lfd = socket(AF_INET, SOCK_STREAM, 0); if (lfd == −1) { perror(socket); return −1; } yes = 1; if (setsockopt(lfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes)) == −1) { perror(setsockopt); close(lfd); return −1; } memset(&addr, 0, sizeof(addr)); addr.sin_port = htons(listen_port); addr.sin_family = AF_INET; if (bind(lfd, (struct sockaddr *) &addr, sizeof(addr)) == −1) { perror(bind); close(lfd); return −1; } printf(accepting connections on port %d , listen_port); listen(lfd, 10); return lfd; } static int connect_socket(int connect_port, char *address) { struct sockaddr_in addr; int cfd; cfd = socket(AF_INET, SOCK_STREAM, 0); if (cfd == −1) { perror(socket); return −1; } memset(&addr, 0, sizeof(addr)); addr.sin_port = htons(connect_port); addr.sin_family = AF_INET; if (!inet_aton(address, (struct in_addr *) &addr.sin_addr.s_addr)) { fprintf(stderr, inet_aton(): bad IP address format ); close(cfd); return −1; } if (connect(cfd, (struct sockaddr *) &addr, sizeof(addr)) == −1) { perror(connect()); shutdown(cfd, SHUT_RDWR); close(cfd); return −1; } return cfd; } #define SHUT_FD1 do { if (fd1 >= 0) { shutdown(fd1, SHUT_RDWR); close(fd1); fd1 = −1; } } while (0) #define SHUT_FD2 do { if (fd2 >= 0) { shutdown(fd2, SHUT_RDWR); close(fd2); fd2 = −1; } } while (0) #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 = 0, buf1_written = 0; int buf2_avail = 0, buf2_written = 0; if (argc != 4) { fprintf(stderr, Usage fwd <listen−port> <forward−to−port> <forward−to−ip−address> ); exit(EXIT_FAILURE); } signal(SIGPIPE, SIG_IGN); forward_port = atoi(argv[2]); h = listen_socket(atoi(argv[1])); if (h == −1) exit(EXIT_FAILURE); for (;;) { int ready, nfds = 0; ssize_t nbytes; fd_set readfds, writefds, exceptfds; FD_ZERO(&readfds); FD_ZERO(&writefds); FD_ZERO(&exceptfds); FD_SET(h, &readfds); nfds = max(nfds, h); if (fd1 > 0 && buf1_avail < BUF_SIZE) FD_SET(fd1, &readfds); /* Note: nfds is updated below, when fd1 is added to exceptfds. */ if (fd2 > 0 && buf2_avail < BUF_SIZE) FD_SET(fd2, &readfds); if (fd1 > 0 && buf2_avail − buf2_written > 0) FD_SET(fd1, &writefds); if (fd2 > 0 && buf1_avail − buf1_written > 0) FD_SET(fd2, &writefds); if (fd1 > 0) { FD_SET(fd1, &exceptfds); nfds = max(nfds, fd1); } if (fd2 > 0) { FD_SET(fd2, &exceptfds); nfds = max(nfds, fd2); } ready = select(nfds + 1, &readfds, &writefds, &exceptfds, NULL); if (ready == −1 && errno == EINTR) continue; if (ready == −1) { perror(select()); exit(EXIT_FAILURE); } if (FD_ISSET(h, &readfds)) { socklen_t addrlen; struct sockaddr_in client_addr; int fd; addrlen = sizeof(client_addr); memset(&client_addr, 0, addrlen); fd = accept(h, (struct sockaddr *) &client_addr, &addrlen); if (fd == −1) { perror(accept()); } else { SHUT_FD1; SHUT_FD2; buf1_avail = buf1_written = 0; buf2_avail = buf2_written = 0; fd1 = fd; fd2 = connect_socket(forward_port, argv[3]); if (fd2 == −1) SHUT_FD1; else printf(connect from %s , inet_ntoa(client_addr.sin_addr)); /* Skip any events on the old, closed file descriptors. */ continue; } } /* NB: read OOB data before normal reads */ if (fd1 > 0 && FD_ISSET(fd1, &exceptfds)) { char c; nbytes = recv(fd1, &c, 1, MSG_OOB); if (nbytes < 1) SHUT_FD1; else send(fd2, &c, 1, MSG_OOB); } if (fd2 > 0 && FD_ISSET(fd2, &exceptfds)) { char c; nbytes = recv(fd2, &c, 1, MSG_OOB); if (nbytes < 1) SHUT_FD2; else send(fd1, &c, 1, MSG_OOB); } if (fd1 > 0 && FD_ISSET(fd1, &readfds)) { nbytes = read(fd1, buf1 + buf1_avail, BUF_SIZE − buf1_avail); if (nbytes < 1) SHUT_FD1; else buf1_avail += nbytes; } if (fd2 > 0 && FD_ISSET(fd2, &readfds)) { nbytes = read(fd2, buf2 + buf2_avail, BUF_SIZE − buf2_avail); if (nbytes < 1) SHUT_FD2; else buf2_avail += nbytes; } if (fd1 > 0 && FD_ISSET(fd1, &writefds) && buf2_avail > 0) { nbytes = write(fd1, buf2 + buf2_written, buf2_avail − buf2_written); if (nbytes < 1) SHUT_FD1; else buf2_written += nbytes; } if (fd2 > 0 && FD_ISSET(fd2, &writefds) && buf1_avail > 0) { nbytes = write(fd2, buf1 + buf1_written, buf1_avail − buf1_written); if (nbytes < 1) SHUT_FD2; else buf1_written += nbytes; } /* 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; } exit(EXIT_SUCCESS); }
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(2) call and devote a
thread to each stream. This becomes more tricky than you
might suspect. Another idea is to set nonblocking I/O using
fcntl(2). This also has its
problems because you end up using 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.
SEE ALSO
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), epoll(7)
COLOPHON
This page is part of release 5.04 of the Linux man-pages
project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
This manpage is copyright (C) 2001 Paul Sheer. %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END very minor changes, aeb Modified 5 June 2002, Michael Kerrisk <mtk.manpagesgmail.com> 2006-05-13, mtk, removed much material that is redundant with select.2 various other changes 2008-01-26, mtk, substantial changes and rewrites |