external/ibm-public/postfix/dist/src/util/writable.c

   1 /*      $NetBSD$        */
   2
   3 /*++
   4 /* NAME
   5 /*      writable 3
   6 /* SUMMARY
   7 /*      test if descriptor is writable
   8 /* SYNOPSIS
   9 /*      #include <iostuff.h>
  10 /*
  11 /*      int     writable(fd)
  12 /*      int     fd;
  13 /* DESCRIPTION
  14 /*      writable() asks the kernel if the specified file descriptor
  15 /*      is writable, i.e. a write operation would not block.
  16 /*
  17 /*      Arguments:
  18 /* .IP fd
  19 /*      File descriptor in the range 0..FD_SETSIZE.
  20 /* DIAGNOSTICS
  21 /*      All system call errors are fatal.
  22 /* LICENSE
  23 /* .ad
  24 /* .fi
  25 /*      The Secure Mailer license must be distributed with this software.
  26 /* AUTHOR(S)
  27 /*      Wietse Venema
  28 /*      IBM T.J. Watson Research
  29 /*      P.O. Box 704
  30 /*      Yorktown Heights, NY 10598, USA
  31 /*--*/
  32
  33 /* System library. */
  34
  35 #include <sys_defs.h>
  36 #include <sys/time.h>
  37 #include <signal.h>
  38 #include <errno.h>
  39 #include <unistd.h>
  40 #include <string.h>
  41
  42 #ifdef USE_SYSV_POLL
  43 #include <poll.h>
  44 #endif
  45
  46 #ifdef USE_SYS_SELECT_H
  47 #include <sys/select.h>
  48 #endif
  49
  50 /* Utility library. */
  51
  52 #include <msg.h>
  53 #include <iostuff.h>
  54
  55 /* writable - see if file descriptor is writable */
  56
  57 int     writable(int fd)
  58 {
  59 #ifndef USE_SYSV_POLL
  60     struct timeval tv;
  61     fd_set  write_fds;
  62     fd_set  except_fds;
  63
  64     /*
  65      * Sanity checks.
  66      */
  67     if (fd >= FD_SETSIZE)
  68         msg_fatal("fd %d does not fit in FD_SETSIZE", fd);
  69
  70     /*
  71      * Initialize.
  72      */
  73     FD_ZERO(&write_fds);
  74     FD_SET(fd, &write_fds);
  75     FD_ZERO(&except_fds);
  76     FD_SET(fd, &except_fds);
  77     tv.tv_sec = 0;
  78     tv.tv_usec = 0;
  79
  80     /*
  81      * Loop until we have an authoritative answer.
  82      */
  83     for (;;) {
  84         switch (select(fd + 1, (fd_set *) 0, &write_fds, &except_fds, &tv)) {
  85         case -1:
  86             if (errno != EINTR)
  87                 msg_fatal("select: %m");
  88             continue;
  89         default:
  90             return (FD_ISSET(fd, &write_fds));
  91         case 0:
  92             return (0);
  93         }
  94     }
  95 #else
  96
  97     /*
  98      * System-V poll() is optimal for polling a few descriptors.
  99      */
 100     struct pollfd pollfd;
 101
 102 #define DONT_WAIT_FOR_EVENT     0
 103
 104     pollfd.fd = fd;
 105     pollfd.events = POLLOUT;
 106     for (;;) {
 107         switch (poll(&pollfd, 1, DONT_WAIT_FOR_EVENT)) {
 108         case -1:
 109             if (errno != EINTR)
 110                 msg_fatal("poll: %m");
 111             continue;
 112         case 0:
 113             return (0);
 114         default:
 115             if (pollfd.revents & POLLNVAL)
 116                 msg_fatal("poll: %m");
 117             return (1);
 118         }
 119     }
 120 #endif
 121 }