.\" This manpage is copyright (C) 2001 Paul Sheer.
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.TH SELECT_TUT 2 "October 21, 2001" "Linux 2.4" "Linux Programmer's Manual"
.SH NAME
select, pselect, FD_CLR, FD_ISSET, FD_SET, FD_ZERO \- synchronous I/O multiplexing
.SH SYNOPSIS
.B #include <sys/time.h>
.br
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.BI "int select(int " nfds ", fd_set *" readfds ,
.BI "fd_set *" writefds ", fd_set *" exceptfds ,
.BI "struct timeval *" utimeout );
.sp
.BI "int pselect(int " nfds ", fd_set *" readfds ,
.BI "fd_set *" writefds ", fd_set *" exceptfds ,
.BI "const struct timespec *" ntimeout ", sigset_t *" sigmask );
.sp
.BI "FD_CLR(int " fd ", fd_set *" set );
.br
.BI "FD_ISSET(int " fd ", fd_set *" set );
.br
.BI "FD_SET(int " fd ", fd_set *" set );
.br
.BI "FD_ZERO(fd_set *" set );
.fi
.SH DESCRIPTION

\fBselect\fP (or \fBpselect\fP) 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:
\fIreadfds\fP, \fIwritefds\fP, and \fIexceptfds\fP. The way that
\fBselect\fP 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, \fIor\fP
when space becomes available within the kernel's internal buffers for
more to be written to the file descriptor, \fIor\fP 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, \fBselect\fP just watches multiple file descriptors,
and is the standard Unix call to do so.

The arrays of file descriptors are called \fIfile descriptor sets\fP.
Each set is declared as type \fBfd_set\fP, and its contents can be
altered with the macros \fBFD_CLR\fP, \fBFD_ISSET\fP, \fBFD_SET\fP,  and
\fBFD_ZERO\fP. \fBFD_ZERO\fP 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 \fBFD_SET\fP.
\fBselect\fP modifies the contents of the sets according to the rules
described below; after calling \fBselect\fP you can test if your file
descriptor is still present in the set with the \fBFD_ISSET\fP macro.
\fBFD_ISSET\fP returns non-zero if the descriptor is present and zero if
it is not. \fBFD_CLR\fP removes a file descriptor from the set although
I can't see the use for it in a clean program.

.SH ARGUMENTS
.TP
\fIreadfds\fP
This set is watched to see if data is available for reading from any of
its file descriptors. After \fBselect\fP has returned, \fIreadfds\fP will be
cleared of all file descriptors except for those file descriptors that
are immediately available for reading with a \fBrecv()\fP (for sockets) or
\fBread()\fP (for pipes, files, and sockets) call.
.TP
\fIwritefds\fP
This set is watched to see if there is space to write data to any of
its file descriptor. After \fBselect\fP has returned, \fIwritefds\fP will be
cleared of all file descriptors except for those file descriptors that
are immediately available for writing with a \fBsend()\fP (for sockets) or
\fBwrite()\fP (for pipes, files, and sockets) call.
.TP
\fIexceptfds\fP
This set is watched for exceptions or errors on any of the file
descriptors. However, that is actually just a rumor. How you use
\fIexceptfds\fP is to watch for \fIout\-of\-band\fP (OOB) data. OOB data
is data sent on a socket using the \fBMSG_OOB\fP flag, and hence
\fIexceptfds\fP only really applies to sockets. See \fBrecv\fP(2) and
\fBsend\fP(2) about this. After \fBselect\fP has returned,
\fIexceptfds\fP 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 \fBrecv()\fP), and
writing OOB data (done with \fBsend\fP) 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.
.TP
\fInfds\fP
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 \fInfds\fP to
\fBselect\fP.
.TP
\fIutimeout\fP
.RS
This is the longest time \fBselect\fP must wait before returning, even
if nothing interesting happened. If this value is passed as \fBNULL\fP,
then \fBselect\fP blocks indefinitely waiting for an event.
\fIutimeout\fP can be set to zero seconds, which causes \fBselect\fP to
return immediately. The structure \fBstruct timeval\fP is defined as,
.PP
.nf
struct timeval {
    time_t tv_sec;    /* seconds */
    long tv_usec;     /* microseconds */
};
.fi
.RE
.TP
\fIntimeout\fP
.RS
This argument has the same meaning as \fIutimeout\fP but \fBstruct timespec\fP
has nanosecond precision as follows,
.PP
.nf
struct timespec {
    long tv_sec;    /* seconds */
    long tv_nsec;   /* nanoseconds */
};
.fi
.RE
.TP
\fIsigmask\fP
This argument holds a set of signals to allow while performing a \fBpselect\fP
call (see \fBsigaddset\fP(3) and \fBsigprocmask\fP(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 \fBselect\fP.

.SH COMBINING SIGNAL AND DATA EVENTS
\fBpselect\fP 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 \fBselect\fP (or \fBpselect\fP)
call to return with \fBerrno\fP set to \fBEINTR\fP. This behavior is
essential so that signals can be processed in the main loop of the
program, otherwise \fBselect\fP 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
\fBselect\fP call? The answer is that \fBselect\fP would block
indefinitely, even though an event is actually pending. This race
condition is solved by the \fBpselect\fP call. This call can be used to
mask out signals that are not to be received except within the
\fBpselect\fP 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 \fBsigprocmask\fP. Our \fBpselect\fP
call would enable \fBSIGCHLD\fP by using the virgin signal mask. Our
program would look like:
.PP
.nf
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 */
    }
}
.fi
.PP
Note that the above \fBpselect\fP call can be replaced with:
.PP
.nf
        sigprocmask (SIG_BLOCK, &orig_sigmask, 0);
        r = select (nfds, &rd, &wr, &er, 0);
        sigprocmask (SIG_BLOCK, &sigmask, 0);
.fi
.PP
but then there is still the possibility that a signal
could arrive after the first \fBsigprocmask\fP and before
the \fBselect\fP. 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 \fBpselect\fP
system call as yet so this is all probably much of a
moot point.
.PP


.SH PRACTICAL

So what is the point of \fBselect\fP? 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 \fBread\fP and \fBwrite\fP 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. \fBselect\fP efficiently copes with this situation.

A classic example of \fBselect\fP comes from the \fBselect\fP
man page:

.nf
#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);
}
.fi


.SH PORT FORWARDING EXAMPLE

Here is an example that better demonstrates the true utility of
\fBselect\fP.
The listing below is a TCP forwarding program that forwards
from one TCP port to another.
.PP
.nf
#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;
}
.fi
.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 \fBfork()\fP
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
\fBioctl()\fP 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.

.SH SELECT LAW

Many people who try to use \fBselect\fP 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 \fBioctl\fP(2)). It is easy to introduce
subtle errors that will remove the advantage of using \fBselect\fP,
hence I will present a list of essentials to watch for when using the
\fBselect\fP call.

.TP
\fB1.\fP
You should always try use \fBselect\fP 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.
.TP
\fB2.\fP
The value \fInfds\fP must be properly calculated for efficiency as
explained above.
.TP
\fB3.\fP
No file descriptor must be added to any set if you do not intend
to check its result after the \fBselect\fP call, and respond
appropriately. See next rule.
.TP
\fB4.\fP
After \fBselect\fP returns, all file descriptors in all sets
\fImust\fP be checked. Any file descriptor that is available
for writing \fImust\fP be written to, and any file descriptor
available for reading \fImust\fP be read, etc.
.TP
\fB5.\fP
The functions \fBread()\fP, \fBrecv()\fP, \fBwrite()\fP, and
\fBsend()\fP do \fInot\fP 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.
.TP
\fB6.\fP
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.
.TP
\fB7.\fP
The functions \fBread()\fP, \fBrecv()\fP, \fBwrite()\fP, and
\fBsend()\fP as well as the \fBselect()\fP call can return \-1 with an
errno of \fBEINTR\fP or \fBEAGAIN\fP (\fBEWOULDBLOCK\fP) 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 \fBEINTR\fP. If your program does not
set non-blocking IO, you will not get \fBEAGAIN\fP. Nonetheless
you should still cope with these errors for completeness.
.TP
\fB8.\fP
Never call \fBread()\fP, \fBrecv()\fP, \fBwrite()\fP, or \fBsend()\fP
with a buffer length of zero.
.TP
\fB9.\fP
Except as indicated in \fB7.\fP, the functions \fBread()\fP,
\fBrecv()\fP, \fBwrite()\fP, and \fBsend()\fP never have a return value
less than 1 except if an error has occurred. For instance, a
\fBread()\fP on a pipe where the other end has died returns zero (so
does an end-of-file error), \fIbut\fP only returns zero
once (a followup read or write will return \-1). Should
any of these functions return 0 or \-1, you should \fInot\fP
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.
.TP
\fB10.\fP
The timeout value must be initialized with each new call to \fBselect\fP,
since some operating systems modify the structure. \fBpselect\fP
however does not modify its timeout structure.
.TP
\fB11.\fP
I have heard that the Windows socket layer does not cope with OOB data
properly. It also does not cope with \fBselect\fP 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.)

.SH USLEEP EMULATION

On systems that do not have a \fBusleep\fP function, you can call
\fBselect\fP with a finite timeout and no file descriptors as
follows:
.PP
.nf
    struct timeval tv;
    tv.tv_sec = 0;
    tv.tv_usec = 200000;  /* 0.2 seconds */
    select (0, NULL, NULL, NULL, &tv);
.fi
.PP
This is only guaranteed to work on Unix systems, however.

.SH RETURN VALUE

On success, \fBselect\fP returns the total number of file descriptors
still present in the file descriptor sets.

If \fBselect\fP 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 \fBerrno\fP being
set appropriately. In the case of an error, the returned sets and
the timeout struct contents are undefined and should not be used.
\fBpselect\fP however never modifies \fIntimeout\fP.

.SH ERRORS
.TP
\fBEBADF\fP
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
\fBclose\fP 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.
.TP
\fBEINTR\fP
An interrupting signal was caught like \fBSIGINT\fP or \fBSIGCHLD\fP etc.
In this case you should rebuild your file descriptor sets and retry.
.TP
\fBEINVAL\fP
Occurs if \fInfds\fP is negative or an invalid value is specified
in \fIutimeout\fP or \fIntimeout\fP.
.TP
\fBENOMEM\fP
Internal memory allocation failure.

.SH NOTES
Generally speaking, all operating systems that support sockets, also
support \fBselect\fP. Some people consider \fBselect\fP to be an
esoteric and rarely used function. Indeed, many types of programs become
extremely complicated without it. \fBselect\fP 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. \fBpselect\fP is a newer function that is less commonly
used.
.PP
The
.BR poll (2)
system call has the same functionality as \fBselect\fP,
but with less subtle behavior. It is less portable than \fBselect\fP.

.SH CONFORMING TO
4.4BSD (the \fBselect\fP 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.
.PP
The \fBpselect\fP 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 \fIsigmask\fP parameter.

.SH SEE ALSO
.BR accept (2),
.BR connect (2),
.BR ioctl (2),
.BR poll (2),
.BR read (2),
.BR recv (2),
.BR select (2),
.BR send (2),
.BR sigprocmask (2),
.BR write (2),
.BR sigaddset (3),
.BR sigdelset (3),
.BR sigemptyset (3),
.BR sigfillset (3),
.BR sigismember (3)

.SH AUTHORS
This man page was written by Paul Sheer.