.\" This manpage is copyright (C) 2001 Paul Sheer.
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" very minor changes, aeb
.\"
.\" Modified 5 June 2002, Michael Kerrisk <mtk.manpages@gmail.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
.\"
.TH SELECT_TUT 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
select, pselect \- synchronous I/O multiplexing
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
See
.BR select (2)
.SH DESCRIPTION
The
.BR select ()
and
.BR pselect ()
system calls are 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.
.PP
This page provides background and tutorial information
on the use of these system calls.
For details of the arguments and semantics of
.BR select ()
and
.BR pselect (),
see
.BR select (2).
.\"
.SS Combining signal and data events
.BR 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
.BR select ()
(or
.BR pselect ())
call to return with \fIerrno\fP set to \fBEINTR\fP.
This behavior is essential so that signals can be processed
in the main loop of the program, otherwise
.BR select ()
would block indefinitely.
.PP
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
.BR select ()
call?
The answer is that
.BR select ()
would block indefinitely, even though an event is actually pending.
This race condition is solved by the
.BR 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
.BR 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 \fBSIGCHLD\fP using
.BR sigprocmask (2).
Our
.BR pselect ()
call would enable
.B SIGCHLD
by using an empty signal mask.
Our program would look like:
.PP
.EX
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 */
    }
}
.EE
.SS Practical
So what is the point of
.BR select ()?
Can't I just read and write to my file descriptors whenever I want?
The point of
.BR 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
.BR read (2)
and
.BR 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.
.BR select ()
efficiently copes with this situation.
.SS Select law
Many people who try to use
.BR 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
.BR select (),
so here is a list of essentials to watch for when using
.BR select ().
.TP 4
1.
You should always try to use
.BR 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.
.TP
2.
The value \fInfds\fP must be properly calculated for efficiency as
explained above.
.TP
3.
No file descriptor must be added to any set if you do not intend
to check its result after the
.BR select ()
call, and respond appropriately.
See next rule.
.TP
4.
After
.BR select ()
returns, all file descriptors in all sets
should be checked to see if they are ready.
.TP
5.
The functions
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
do \fInot\fP necessarily read/write the full amount of data
that you have requested.
If they do read/write the full amount, it'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.
.TP
6.
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.
.TP
7.
Calls to
.BR read (2),
.BR recv (2),
.BR write (2),
.BR send (2),
and
.BR select ()
can fail with the error
\fBEINTR\fP,
and calls to
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
can fail with
.I errno
set to \fBEAGAIN\fP (\fBEWOULDBLOCK\fP).
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 \fBEINTR\fP.
If your program does not set nonblocking I/O,
you will not get \fBEAGAIN\fP.
.\" Nonetheless, you should still cope with these errors for completeness.
.TP
8.
Never call
.BR read (2),
.BR recv (2),
.BR write (2),
or
.BR send (2)
with a buffer length of zero.
.TP
9.
If the functions
.BR read (2),
.BR recv (2),
.BR write (2),
and
.BR send (2)
fail with errors other than those listed in \fB7.\fP,
or one of the input functions returns 0, indicating end of file,
then you should \fInot\fP pass that file descriptor to
.BR select ()
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.
.TP
10.
The timeout value must be initialized with each new call to
.BR select (),
since some operating systems modify the structure.
.BR pselect ()
however does not modify its timeout structure.
.TP
11.
Since
.BR select ()
modifies its file descriptor sets,
if the call is being used in a loop,
then the sets must be reinitialized before each call.
.\" "I have heard" does not fill me with confidence, and doesn't
.\" belong in a man page, so I've commented this point out.
.\" .TP
.\" 11.
.\" I have heard that the Windows socket layer does not cope with OOB data
.\" properly.
.\" It also does not cope with
.\" .BR select ()
.\" calls when no file descriptors are set at all.
.\" Having no file descriptors set is a useful
.\" way to sleep the process with subsecond precision by using the timeout.
.\" (See further on.)
.SH RETURN VALUE
See
.BR select (2).
.SH NOTES
Generally speaking,
all operating systems that support sockets also support
.BR select ().
.BR 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.
.PP
The
.BR poll (2)
system call has the same functionality as
.BR select (),
and is somewhat more efficient when monitoring sparse
file descriptor sets.
It is nowadays widely available, but historically was less portable than
.BR select ().
.PP
The Linux-specific
.BR epoll (7)
API provides an interface that is more efficient than
.BR select (2)
and
.BR poll (2)
when monitoring large numbers of file descriptors.
.SH EXAMPLES
Here is an example that better demonstrates the true utility of
.BR select ().
The listing below is a TCP forwarding program that forwards
from one TCP port to another.
.PP
.\" SRC BEGIN (select.c)
.EX
#include <arpa/inet.h>
#include <errno.h>
#include <netinet/in.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <unistd.h>
\&
static int forward_port;
\&
#undef max
#define max(x, y) ((x) > (y) ? (x) : (y))
\&
static int
listen_socket(int listen_port)
{
    int                 lfd;
    int                 yes;
    struct sockaddr_in  addr;
\&
    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\en", listen_port);
    listen(lfd, 10);
    return lfd;
}
\&
static int
connect_socket(int connect_port, char *address)
{
    int                 cfd;
    struct sockaddr_in  addr;
\&
    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\en");
        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 {                                \e
                     if (fd1 >= 0) {                 \e
                         shutdown(fd1, SHUT_RDWR);   \e
                         close(fd1);                 \e
                         fd1 = \-1;                   \e
                     }                               \e
                 } while (0)
\&
#define SHUT_FD2 do {                                \e
                     if (fd2 >= 0) {                 \e
                         shutdown(fd2, SHUT_RDWR);   \e
                         close(fd2);                 \e
                         fd2 = \-1;                   \e
                     }                               \e
                 } while (0)
\&
#define BUF_SIZE 1024
\&
int
main(int argc, char *argv[])
{
    int      h;
    int      ready, nfds;
    int      fd1 = \-1, fd2 = \-1;
    int      buf1_avail = 0, buf1_written = 0;
    int      buf2_avail = 0, buf2_written = 0;
    char     buf1[BUF_SIZE], buf2[BUF_SIZE];
    fd_set   readfds, writefds, exceptfds;
    ssize_t  nbytes;
\&
    if (argc != 4) {
        fprintf(stderr, "Usage\en\etfwd <listen\-port> "
                "<forward\-to\-port> <forward\-to\-ip\-address>\en");
        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 (;;) {
        nfds = 0;
\&
        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\en",
                           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);
}
.EE
.\" SRC END
.PP
The above program properly forwards most kinds of TCP connections
including OOB signal data transmitted by \fBtelnet\fP servers.
It handles the tricky problem of having data flow in both directions
simultaneously.
You might think it more efficient to use a
.BR 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
.BR fcntl (2).
This also has its problems because you end up using
inefficient timeouts.
.PP
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\[em]one for each connection.
At the moment, new
connections cause the current connection to be dropped.
.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
.BR select (2),
.BR send (2),
.BR sigprocmask (2),
.BR write (2),
.BR epoll (7)
.\" .SH AUTHORS
.\" This man page was written by Paul Sheer.