### 13.9 Waiting for Input or Output ¶

Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets.

You cannot normally use `read` for this purpose, because this blocks the program until input is available on one particular file descriptor; input on other channels won’t wake it up. You could set nonblocking mode and poll each file descriptor in turn, but this is very inefficient.

A better solution is to use the `select` function. This blocks the program until input or output is ready on a specified set of file descriptors, or until a timer expires, whichever comes first. This facility is declared in the header file `sys/select.h`.

In the case of a server socket (see Listening for Connections), we say that “input” is available when there are pending connections that could be accepted (see Accepting Connections). `accept` for server sockets blocks and interacts with `select` just as `read` does for normal input.

The file descriptor sets for the `select` function are specified as `fd_set` objects. Here is the description of the data type and some macros for manipulating these objects.

Data Type: fd_set ¶

  The `fd_set` data type represents file descriptor sets for the `select` function. It is actually a bit array.

Macro: `int` FD_SETSIZE ¶

  The value of this macro is the maximum number of file descriptors that a `fd_set` object can hold information about. On systems with a fixed maximum number, `FD_SETSIZE` is at least that number. On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an `fd_set`; if you get a file descriptor with a value as high as `FD_SETSIZE`, you cannot put that descriptor into an `fd_set`.

Macro: `void` FD_ZERO `(fd_set *set)` ¶

  Preliminary: | MT-Safe race:set | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This macro initializes the file descriptor set set to be the empty set.

Macro: `void` FD_SET `(int filedes, fd_set *set)` ¶

  Preliminary: | MT-Safe race:set | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This macro adds filedes to the file descriptor set set.

  The filedes parameter must not have side effects since it is evaluated more than once.

Macro: `void` FD_CLR `(int filedes, fd_set *set)` ¶

  Preliminary: | MT-Safe race:set | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This macro removes filedes from the file descriptor set set.

  The filedes parameter must not have side effects since it is evaluated more than once.

Macro: `int` FD_ISSET `(int filedes, const fd_set *set)` ¶

  Preliminary: | MT-Safe race:set | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  This macro returns a nonzero value (true) if filedes is a member of the file descriptor set set, and zero (false) otherwise.

  The filedes parameter must not have side effects since it is evaluated more than once.

Next, here is the description of the `select` function itself.

Function: `int` select `(int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout)` ¶

  Preliminary: | MT-Safe race:read-fds race:write-fds race:except-fds | AS-Safe | AC-Safe | See POSIX Safety Concepts.

  The `select` function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired.

  The file descriptors specified by the read-fds argument are checked to see if they are ready for reading; the write-fds file descriptors are checked to see if they are ready for writing; and the except-fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.

  A file descriptor is considered ready for reading if a `read` call will not block. This usually includes the read offset being at the end of the file or there is an error to report. A server socket is considered ready for reading if there is a pending connection which can be accepted with `accept`; see Accepting Connections. A client socket is ready for writing when its connection is fully established; see Making a Connection.

  “Exceptional conditions” does not mean errors—errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (See Sockets, for information on urgent messages.)

  The `select` function checks only the first nfds file descriptors. The usual thing is to pass `FD_SETSIZE` as the value of this argument.

  The timeout specifies the maximum time to wait. If you pass a null pointer for this argument, it means to block indefinitely until one of the file descriptors is ready. Otherwise, you should provide the time in `struct timeval` format; see Time Types. Specify zero as the time (a `struct timeval` containing all zeros) if you want to find out which descriptors are ready without waiting if none are ready.

  The normal return value from `select` is the total number of ready file descriptors in all of the sets. Each of the argument sets is overwritten with information about the descriptors that are ready for the corresponding operation. Thus, to see if a particular descriptor desc has input, use `FD_ISSET (desc, read-fds)` after `select` returns.

  If `select` returns because the timeout period expires, it returns a value of zero.

  Any signal will cause `select` to return immediately. So if your program uses signals, you can’t rely on `select` to keep waiting for the full time specified. If you want to be sure of waiting for a particular amount of time, you must check for `EINTR` and repeat the `select` with a newly calculated timeout based on the current time. See the example below. See also Primitives Interrupted by Signals.

  If an error occurs, `select` returns `-1` and does not modify the argument file descriptor sets. The following `errno` error conditions are defined for this function:

  `EBADF`

    One of the file descriptor sets specified an invalid file descriptor.

  `EINTR`

    The operation was interrupted by a signal. See Primitives Interrupted by Signals.

  `EINVAL`

    The timeout argument is invalid; one of the components is negative or too large.

Here is an example showing how you can use `select` to establish a timeout period for reading from a file descriptor. The `input_timeout` function blocks the calling process until input is available on the file descriptor, or until the timeout period expires.

    #define _GNU_SOURCE 1
    #include <errno.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/select.h>

    int
    input_timeout (int filedes, unsigned int seconds)
    {
      fd_set set;
      struct timeval timeout;

      /* Initialize the file descriptor set. */
      FD_ZERO (&set);
      FD_SET (filedes, &set);

      /* Initialize the timeout data structure. */
      timeout.tv_sec = seconds;
      timeout.tv_usec = 0;

      /* select returns 0 if timeout, 1 if input available, -1 if error. */
      return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                         &set, NULL, NULL,
                                         &timeout));
    }

    int
    main (void)
    {
      fprintf (stderr, "select returned %d.\n",
               input_timeout (STDIN_FILENO, 5));
      return 0;
    }

There is another example showing the use of `select` to multiplex input from multiple sockets in Byte Stream Connection Server Example.

For an alternate interface to this functionality, see `poll` (see Other low-level-I/O-related functions).