poll_emu.c

Go to the documentation of this file.

/*---------------------------------------------------------------------------*\
  $Id: poll.c,v 1.4 2003/01/01 19:02:29 bmc Exp $

  NAME

        poll - select(2)-based poll() emulation function for BSD systems.

  SYNOPSIS
        #include "poll.h"

        struct pollfd
        {
            int     fd;
            short   events;
            short   revents;
        }

        int poll (struct pollfd *pArray, unsigned long n_fds, int timeout)

  DESCRIPTION

        This file, and the accompanying "poll.h", implement the System V
        poll(2) system call for BSD systems (which typically do not provide
        poll()).  Poll() provides a method for multiplexing input and output
        on multiple open file descriptors; in traditional BSD systems, that
        capability is provided by select().  While the semantics of select()
        differ from those of poll(), poll() can be readily emulated in terms
        of select() -- which is how this function is implemented.

  REFERENCES
        Stevens, W. Richard. Unix Network Programming.  Prentice-Hall, 1990.

  NOTES
        1. This software requires an ANSI C compiler.

  LICENSE

        This software is released under the following license:

                Copyright (c) 1995-2002 Brian M. Clapper
                All rights reserved.

                Redistribution and use in source and binary forms are
                permitted provided that: (1) source distributions retain
                this entire copyright notice and comment; (2) modifications
                made to the software are prominently mentioned, and a copy
                of the original software (or a pointer to its location) are
                included; and (3) distributions including binaries display
                the following acknowledgement: "This product includes
                software developed by Brian M. Clapper <bmc@clapper.org>"
                in the documentation or other materials provided with the
                distribution. The name of the author may not be used to
                endorse or promote products derived from this software
                without specific prior written permission.

                THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS
                OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE
                IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
                PARTICULAR PURPOSE.

        Effectively, this means you can do what you want with the software
        except remove this notice or take advantage of the author's name.
        If you modify the software and redistribute your modified version,
        you must indicate that your version is a modification of the
        original, and you must provide either a pointer to or a copy of the
        original.
\*---------------------------------------------------------------------------*/

/*
HISTORY

** 2008-02-15 Antoine Beyeler <abeyeler at ab-ware dot com>
Trivial modification to change call name (poll() -> poll_emu()).

*/



/*---------------------------------------------------------------------------*\
                                 Includes
\*---------------------------------------------------------------------------*/

#include <unistd.h>                          /* standard Unix definitions */
#include <sys/types.h>                       /* system types */
#include <sys/time.h>                        /* time definitions */
#include <assert.h>                          /* assertion macros */
#include <string.h>                          /* string functions */
#include "poll_emu.h"                        /* this package */


/*---------------------------------------------------------------------------*\
                                  Macros
\*---------------------------------------------------------------------------*/

#ifndef MAX
#define MAX(a,b)        ((a) > (b) ? (a) : (b))
#endif


/*---------------------------------------------------------------------------*\
                             Private Functions
\*---------------------------------------------------------------------------*/

static int map_poll_spec
#if __STDC__ > 0
                        (struct pollfd *pArray,
                          unsigned long  n_fds,
                          fd_set        *pReadSet,
                          fd_set        *pWriteSet,
                          fd_set        *pExceptSet)
#else
                         (pArray, n_fds, pReadSet, pWriteSet, pExceptSet)
                          struct pollfd *pArray;
                          unsigned long  n_fds;
                          fd_set        *pReadSet;
                          fd_set        *pWriteSet;
                          fd_set        *pExceptSet;
#endif
{
    register unsigned long  i;                   /* loop control */
    register struct         pollfd *pCur;        /* current array element */
    register int            max_fd = -1;         /* return value */

    /*
       Map the poll() structures into the file descriptor sets required
       by select().
    */
    for (i = 0, pCur = pArray; i < n_fds; i++, pCur++)
    {
        /* Skip any bad FDs in the array. */

        if (pCur->fd < 0)
            continue;

        if (pCur->events & POLLIN)
        {
            /* "Input Ready" notification desired. */
            FD_SET (pCur->fd, pReadSet);
        }

        if (pCur->events & POLLOUT)
        {
            /* "Output Possible" notification desired. */
            FD_SET (pCur->fd, pWriteSet);
        }

        if (pCur->events & POLLPRI)
        {
            /*
               "Exception Occurred" notification desired.  (Exceptions
               include out of band data.
            */
            FD_SET (pCur->fd, pExceptSet);
        }

        max_fd = MAX (max_fd, pCur->fd);
    }

    return max_fd;
}

static struct timeval *map_timeout
#if __STDC__ > 0
                        (int poll_timeout, struct timeval *pSelTimeout)
#else
                        (poll_timeout, pSelTimeout)
                         int             poll_timeout;
                         struct timeval *pSelTimeout;
#endif
{
    struct timeval *pResult;

    /*
       Map the poll() timeout value into a select() timeout.  The possible
       values of the poll() timeout value, and their meanings, are:

       VALUE    MEANING

       -1       wait indefinitely (until signal occurs)
        0       return immediately, don't block
       >0       wait specified number of milliseconds

       select() uses a "struct timeval", which specifies the timeout in
       seconds and microseconds, so the milliseconds value has to be mapped
       accordingly.
    */

    assert (pSelTimeout != (struct timeval *) NULL);

    switch (poll_timeout)
    {
        case -1:
            /*
               A NULL timeout structure tells select() to wait indefinitely.
            */
            pResult = (struct timeval *) NULL;
            break;

        case 0:
            /*
               "Return immediately" (test) is specified by all zeros in
               a timeval structure.
            */
            pSelTimeout->tv_sec  = 0;
            pSelTimeout->tv_usec = 0;
            pResult = pSelTimeout;
            break;

        default:
            /* Wait the specified number of milliseconds. */
            pSelTimeout->tv_sec  = poll_timeout / 1000; /* get seconds */
            poll_timeout        %= 1000;                /* remove seconds */
            pSelTimeout->tv_usec = poll_timeout * 1000; /* get microseconds */
            pResult = pSelTimeout;
            break;
    }


    return pResult;
}

static void map_select_results
#if __STDC__ > 0
                         (struct pollfd *pArray,
                          unsigned long  n_fds,
                          fd_set        *pReadSet,
                          fd_set        *pWriteSet,
                          fd_set        *pExceptSet)
#else
                         (pArray, n_fds, pReadSet, pWriteSet, pExceptSet)
                          struct pollfd *pArray;
                          unsigned long  n_fds;
                          fd_set        *pReadSet;
                          fd_set        *pWriteSet;
                          fd_set        *pExceptSet;
#endif
{
    register unsigned long  i;                   /* loop control */
    register struct         pollfd *pCur;        /* current array element */

    for (i = 0, pCur = pArray; i < n_fds; i++, pCur++)
    {
        /* Skip any bad FDs in the array. */

        if (pCur->fd < 0)
            continue;

        /* Exception events take priority over input events. */

        pCur->revents = 0;
        if (FD_ISSET (pCur->fd, pExceptSet))
            pCur->revents |= POLLPRI;

        else if (FD_ISSET (pCur->fd, pReadSet))
            pCur->revents |= POLLIN;

        if (FD_ISSET (pCur->fd, pWriteSet))
            pCur->revents |= POLLOUT;
    }

    return;
}

/*---------------------------------------------------------------------------*\
                             Public Functions
\*---------------------------------------------------------------------------*/

int poll_emu

#if __STDC__ > 0
        (struct pollfd *pArray, unsigned long n_fds, int timeout)
#else
        (pArray, n_fds, timeout)
         struct        pollfd *pArray;
         unsigned long n_fds;
         int           timeout;
#endif

{
    fd_set  read_descs;                          /* input file descs */
    fd_set  write_descs;                         /* output file descs */
    fd_set  except_descs;                        /* exception descs */
    struct  timeval stime;                       /* select() timeout value */
    int     ready_descriptors;                   /* function result */
    int     max_fd;                              /* maximum fd value */
    struct  timeval *pTimeout;                   /* actually passed */

    FD_ZERO (&read_descs);
    FD_ZERO (&write_descs);
    FD_ZERO (&except_descs);

    assert (pArray != (struct pollfd *) NULL);

    /* Map the poll() file descriptor list in the select() data structures. */

    max_fd = map_poll_spec (pArray, n_fds,
                            &read_descs, &write_descs, &except_descs);

    /* Map the poll() timeout value in the select() timeout structure. */

    pTimeout = map_timeout (timeout, &stime);

    /* Make the select() call. */

    ready_descriptors = select (max_fd + 1, &read_descs, &write_descs,
                                &except_descs, pTimeout);

    if (ready_descriptors >= 0)
    {
        map_select_results (pArray, n_fds,
                            &read_descs, &write_descs, &except_descs);
    }

    return ready_descriptors;
}