SELECT_TUT

NAME

SYNOPSIS

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);  

DESCRIPTION

fBselectfP (or fBpselectfP) 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: fIreadfdsfP, fIwritefdsfP, and fIexceptfdsfP. The way that fBselectfP 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, fIorfP when space becomes available within the kernel's internal buffers for more to be written to the file descriptor, fIorfP 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, fBselectfP just watches multiple file descriptors, and is the standard Unix call to do so.

The arrays of file descriptors are called fIfile descriptor setsfP. Each set is declared as type fBfd_setfP, and its contents can be altered with the macros fBFD_CLRfP, fBFD_ISSETfP, fBFD_SETfP, and fBFD_ZEROfP. fBFD_ZEROfP 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_SETfP. fBselectfP modifies the contents of the sets according to the rules described below; after calling fBselectfP you can test if your file descriptor is still present in the set with the fBFD_ISSETfP macro. fBFD_ISSETfP returns non-zero if the descriptor is present and zero if it is not. fBFD_CLRfP removes a file descriptor from the set although I can't see the use for it in a clean program.

ARGUMENTS

fIreadfdsfP

This set is watched to see if data is available for reading from any of its file descriptors. After fBselectfP has returned, fIreadfdsfP 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.

fIwritefdsfP

This set is watched to see if there is space to write data to any of its file descriptor. After fBselectfP has returned, fIwritefdsfP 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.

fIexceptfdsfP

This set is watched for exceptions or errors on any of the file descriptors. However, that is actually just a rumor. How you use fIexceptfdsfP is to watch for fIout-of-bandfP (OOB) data. OOB data is data sent on a socket using the fBMSG_OOBfP flag, and hence fIexceptfdsfP only really applies to sockets. See fBrecvfP(2) and fBsendfP(2) about this. After fBselectfP has returned, fIexceptfdsfP 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 fBsendfP) 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.

fInfdsfP

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 fInfdsfP to fBselectfP.

fIutimeoutfP

This is the longest time fBselectfP must wait before returning, even if nothing interesting happened. If this value is passed as fBNULLfP, then fBselectfP blocks indefinitely waiting for an event. fIutimeoutfP can be set to zero seconds, which causes fBselectfP to return immediately. The structure fBstruct timevalfP is defined as,

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

fIntimeoutfP

This argument has the same meaning as fIutimeoutfP but fBstruct timespecfP has nanosecond precision as follows,

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

fIsigmaskfP

This argument holds a set of signals to allow while performing a fBpselectfP call (see fBsigaddsetfP(3) and fBsigprocmaskfP(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 fBselectfP.

COMBINING SIGNAL AND DATA EVENTS

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 fBpselectfP 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 fBsigprocmaskfP and before the fBselectfP. 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 fBpselectfP system call as yet so this is all probably much of a mute point.

PRACTICAL

So what is the point of fBselectfP? 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 fBreadfP and fBwritefP 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. fBselectfP efficiently copes with this situation.

A classic example of fBselectfP comes from the fBselectfP 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.);
        /* FD_ISSET(0, &rfds) will be true. */
    else
        printf("No data within five seconds.);

    exit(0);
}

PORT FORWARDING EXAMPLE

Here is an example that better demonstrates the true utility of fBselectfP. 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,
            (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,
                 "Usagetfwd <listen-port> <forward-to-port> <forward-to-ip-address>);
        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,
                            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 fBtelnetfP 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.

SELECT LAW

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

fB1.fP

You should always try use fBselectfP 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.

fB2.fP

The value fInfdsfP must be properly calculated for efficiency as explained above.

fB3.fP

No file descriptor must be added to any set if you do not intend to check its result after the fBselectfP call, and respond appropriately. See next rule.

fB4.fP

After fBselectfP returns, all file descriptors in all sets fImustfP be checked. Any file descriptor that is available for writing fImustfP be written to, and any file descriptor available for reading fImustfP be read, etc.

fB5.fP

The functions fBread()fP, fBrecv()fP, fBwrite()fP, and fBsend()fP do fInotfP 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.

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.

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 fBEINTRfP or fBEAGAINfP (fBEWOULDBLOCKfP) 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 fBEINTRfP. If your program does not set non-blocking IO, you will not get fBEAGAINfP. Nonetheless you should still cope with these errors for completeness.

fB8.fP

Never call fBread()fP, fBrecv()fP, fBwrite()fP, or fBsend()fP with a buffer length of zero.

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), fIbutfP only returns zero once (a followup read or write will return -1). Should any of these functions return 0 or -1, you should fInotfP 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.

fB10.fP

The timeout value must be initialized with each new call to fBselectfP, since some operating systems modify the structure. fBpselectfP however does not modify its timeout structure.

fB11.fP

I have heard that the Windows socket layer does not cope with OOB data properly. It also does not cope with fBselectfP 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.)

USLEEP EMULATION

On systems that do not have a fBusleepfP function, you can call fBselectfP 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.

RETURN VALUE

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

If fBselectfP 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 fBerrnofP being set appropriately. In the case of an error, the returned sets and the timeout struct contents are undefined and should not be used. fBpselectfP however never modifies fIntimeoutfP.

ERRORS

fBEBADFfP

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 fBclosefP 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.

fBEINTRfP

An interrupting signal was caught like fBSIGINTfP or fBSIGCHLDfP etc. In this case you should rebuild your file descriptor sets and retry.

fBEINVALfP

Occurs if fInfdsfP is negative or an invalid value is specified in fIutimeoutfP or fIntimeoutfP.

fBENOMEMfP

Internal memory allocation failure.

NOTES

The poll(2) system call has the same functionality as fBselectfP, but with less subtle behavior. It is less portable than fBselectfP.

CONFORMING TO

The fBpselectfP 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 fIsigmaskfP parameter.

SEE ALSO

AUTHORS

Index

NAME

SYNOPSIS

DESCRIPTION

ARGUMENTS

COMBINING SIGNAL AND DATA EVENTS

PRACTICAL

PORT FORWARDING EXAMPLE

SELECT LAW

USLEEP EMULATION

RETURN VALUE

ERRORS

NOTES

CONFORMING TO

SEE ALSO

AUTHORS