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annotate packages/net/tcpip/current/doc/select.html @ 97:ced4577552cd ecos-sw-2000-06-06

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Merge from eCos master repository on 2000-06-06-08:44:00-BST
author jlarmour
date Tue, 06 Jun 2000 08:39:36 +0000
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1 <html>
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2 <body>
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3 <pre>
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4 NAME
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5 select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
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6 I/O multiplexing
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7
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8 SYNOPSIS
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9 #include &lt;network.h&gt;
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10
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11 int select(int n, fd_set *readfds, fd_set *writefds,
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12 fd_set *exceptfds, struct timeval *timeout);
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13
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14 FD_CLR(int fd, fd_set *set);
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15 FD_ISSET(int fd, fd_set *set);
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16 FD_SET(int fd, fd_set *set);
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17 FD_ZERO(fd_set *set);
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18
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19 DESCRIPTION
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20 select waits for a number of file descriptors to change
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21 status.
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22
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23 Three independent sets of descriptors are watched. Those
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24 listed in readfds will be watched to see if characters
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25 become available for reading, those in writefds will be
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26 watched to see if it is ok to immediately write on them,
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27 and those in exceptfds will be watched for exceptions. On
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28 exit, the sets are modified in place to indicate which
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29 descriptors actually changed status.
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30
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31 Four macros are provided to manipulate the sets. FD_ZERO
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32 will clear a set. FD_SET and FD_CLR add or remove a given
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33 descriptor from a set. FD_ISSET tests to see if a
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34 descriptor is part of the set; this is useful after select
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35 returns.
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36
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37 n is the highest-numbered descriptor in any of the three
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38 sets, plus 1.
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39
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40 timeout is an upper bound on the amount of time elapsed
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41 before select returns. It may be zero, causing select to
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42 return immediately. If timeout is NULL (no timeout),
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43 select can block indefinitely.
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44
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45 RETURN VALUE
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46 On success, select returns the number of descriptors con-
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47 tained in the descriptor sets, which may be zero if the
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48 timeout expires before anything interesting happens. On
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49 error, -1 is returned, and errno is set appropriately; the
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50 sets and timeout become undefined, so do not rely on their
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51 contents after an error.
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52
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53 ERRORS
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54 EBADF An invalid file descriptor was given in one of the
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55 sets.
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56
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57 EINTR A non blocked signal was caught.
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58
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59 EINVAL n is negative.
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60
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61 ENOMEM select was unable to allocate memory for internal
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62 tables.
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63
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64 NOTES
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65 Some code calls select with all three sets empty, n zero,
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66 and a non-null timeout as a fairly portable way to sleep
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67 with subsecond precision.
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68
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69 EXAMPLE
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70 #include <stdio.h>
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71 #include <sys/time.h>
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72 #include <sys/types.h>
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73 #include <unistd.h>
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74
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75 int
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76 main(void)
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77 {
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78 fd_set rfds;
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79 struct timeval tv;
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80 int retval;
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81
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82 /* Watch stdin (fd 0) to see when it has input. */
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83 FD_ZERO(&rfds);
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84 FD_SET(0, &rfds);
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85 /* Wait up to five seconds. */
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86 tv.tv_sec = 5;
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87 tv.tv_usec = 0;
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88
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89 retval = select(1, &rfds, NULL, NULL, &tv);
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90 /* Don't rely on the value of tv now! */
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91
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92 if (retval)
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93 printf("Data is available now.\n");
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94 /* FD_ISSET(0, &rfds) will be true. */
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95 else
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96 printf("No data within five seconds.\n");
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97
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98 exit(0);
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99 }
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100
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101 Generally portable to/from non-BSD systems supporting
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102 clones of the BSD socket layer (including System V vari-
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103 ants). However, note that the System V variant typically
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104 sets the timeout variable before exit, but the BSD variant
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105 does not.
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106 </pre>
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107 </body>
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108 </html>