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
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 Listening), 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)
This macro initializes the file descriptor set set to be the
— 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 file descriptor set set, and zero (false) otherwise.
Next, here is the description of the select function itself.
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 not 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 Accepting Connections. A client socket is ready for writing when
its connection is fully established; see Connecting.
“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 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 Interrupted Primitives.
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 timeout argument is invalid; one of the components is negative
or too large.
Portability Note: The select function is a BSD Unix
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.
input_timeout (int filedes, unsigned int seconds)
struct timeval timeout;
/* Initialize the file descriptor 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,
fprintf (stderr, "select returned %d.\n",
input_timeout (STDIN_FILENO, 5));
There is another example showing the use of select to multiplex
input from multiple sockets in Server Example.
Published under the terms of the GNU General Public License