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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/types.h'.

In the case of a server socket (see section Listening for Connections), we say that "input" is available when there are pending connections that could be accepted (see section 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)
This macro initializes the file descriptor set set to be the empty set.

Macro: void FD_SET (int filedes, fd_set *set)
This macro adds filedes to the file descriptor set set.

Macro: void FD_CLR (int filedes, fd_set *set)
This macro removes filedes from the file descriptor set set.

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

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)
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 it is at end of file. A server socket is considered ready for reading if there is a pending connection which can be accepted with accept; see section Accepting Connections. A client socket is ready for writing when its connection is fully established; see section 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 section 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 section High-Resolution Calendar. 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 section 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:

One of the file descriptor sets specified an invalid file descriptor.
The operation was interrupted by a signal. See section Primitives Interrupted by Signals.
The timeout argument is invalid; one of the components is negative or too large.

Portability Note: The select function is a BSD Unix feature.

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.

#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/time.h>

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. */
                                     &set, NULL, NULL,

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 section Byte Stream Connection Server Example.

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