select - Synchronous I/O multiplexing
struct timeval *timeout) ;
Interfaces documented on this reference page conform to industry standards
select(): XSH5.0, XNS5.0
Refer to the standards(5) reference page for more information about indus-
try standards and associated tags.
nfds Specifies the number of open objects that may be ready for read-
ing or writing or that have exceptions pending. The nfds parame-
ter cannot be greater than FD_SETSIZE.
readfds Points to an I/O descriptor set consisting of file descriptors of
objects opened for reading. When the readfds parameter is a null
pointer, the read I/O descriptor set is ignored by the select()
writefds Points to an I/O descriptor set consisting of file descriptors
for objects opened for writing. When the writefds parameter is a
null pointer, the write I/O descriptor set is ignored.
exceptfds Points to an I/O descriptor set consisting of file descriptors
for objects opened for reading or writing that have an exception
pending. When the exceptfds parameter is a null pointer, the
exception I/O descriptor set is ignored.
timeout Points to a type timeval structure that specifies the maximum
time to wait for a response to a select() function. When the
timeout parameter has a nonzero value, the maximum time interval
to wait for the select() function to complete is specified by
values stored in space reserved by the type timeval structure
pointed to by the timeout parameter.
When the timeout parameter is a null pointer, the select() func-
tion blocks indefinitely. To poll, the timeout parameter should
be specified as a nonzero value and point to a zero-valued
The use of a timeout does not affect any pending timers set up
by alarm(), ualarm(), or settimer().
fd Specifies a file descriptor.
fdset Points to an I/O descriptor set.
The select() function checks the status of objects identified by bit masks
called I/O descriptor sets. Each I/O descriptor set consists of an array
of bits whose relative position and state represent a file descriptor and
the status of its corresponding object. There is an I/O descriptor set for
reading, writing, and for pending exceptions. These I/O descriptor sets
are pointed to by the readfds, writefds, and exceptfds parameters, respec-
tively. The I/O descriptor sets provide a means of monitoring the read,
write, and exception status of objects represented by file descriptors.
The status of nfds-1 file descriptors in each referenced I/O descriptor set
is checked when the select() function is called. The select() function
returns a modified I/O descriptor set, which has the following characteris-
tics: for any selected I/O descriptor set pointed to by the readfds, wri-
tefds, and exceptfds parameters, if the state of any bit corresponding with
an active file descriptor is set on entry, when the object represented by
the set bit is ready for reading, writing, or its exception condition has
been satisfied, a corresponding bit position is also set in the returned
I/O descriptor set pointed to by the readfds, writefds, or exceptfds param-
When it returns successfully, select() does the following: First, it
replaces the original I/O descriptor sets with the corresponding I/O
descriptor sets (that have a set bit for each file descriptor representing
those objects that are ready for the requested operation). Second, it
returns the total number of ready objects represented by set bits in all
the I/O descriptor sets.
Note: When objects are ready for the requested operation, the operation
will not block. This does not necessarily mean, however, that data is
available for the operation. See read(2) and write(2) for information on
when these calls would block.
After an I/O descriptor set is created, it may be modified with the follow-
Clears the I/O descriptor bit specified by file descriptor fd in
the I/O descriptor set addressed by fdset.
Returns a nonzero value when the I/O descriptor bit for fd is
included in the I/O descriptor set addressed by fdset. Otherwise
0 (zero) is returned.
Includes the particular I/O descriptor bit specified by fd in the
I/O descriptor set addressed by fdset.
Initializes the I/O descriptor set addressed by fdset to a null
The behavior of these macros is undefined when parameter fd has a value
less than 0 (zero) or greater than or equal to FD_SETSIZE, which is nor-
mally at least equal to the maximum number of file descriptors supported by
The select() function supports regular files, terminal and pseudo-terminal
devices, STREAMS-based files, FIFOs, and pipes. The behavior of the
select() function on file descriptors that refer to other types of files is
For sockets, a file descriptor for a socket that is listening for connec-
tions indicates that it is ready for reading when connections are avail-
able. A file descriptor for a socket that is connecting asynchronously
indicates that it is ready for writing after a connection is established.
This function supports up to 64K open file descriptors per process if that
capability is enabled. If not already defined, FD_SETSIZE is set in the
<<sys/select.h>> header file to be 4K.
New applications can use any of these features for processes using more
than 4K open file descriptors. When they do so, however, they must specify
an alternate value for FD_SETSIZE--one that does not exceed 64K--before
they include <<sys/select.h>> in a program to be compiled.
Upon successful completion, the select() function returns the number of
ready objects represented by corresponding file descriptor bits in the I/O
descriptor sets. When an error occurs, -1 is returned and errno is set to
indicate the error.
If the time limit expires before any event occurs that would cause one of
the masks to be set to a non-zero value, select() completes successfully
and returns 0 (zero).
When select() returns an error, including a process interrupt, the I/O
descriptor sets pointed to by the readfds, writefds, and exceptfds parame-
ters remain unmodified.
The select() function sets errno to the specified values for the following
[EBADF] One or more of the I/O descriptor sets specified an invalid file
[EINTR] A signal was delivered before the time limit specified by the
timeout parameter expired and before any of the selected events
[EINVAL] The time limit specified by the timeout parameter is invalid.
The nfds parameter is less than 0, or greater than or equal to
One of the specified file descriptors refers to a STREAM or mul-
tiplexer that is linked (directly or indirectly) downstream from
[EAGAIN] [Tru64 UNIX] Allocation of internal data structures failed. A
later call to the select() function may complete successfully.
Functions: accept(2), connect(2), getdtablesize(2), poll(2) read(2),
recv(2), send(2), setsysinfo(2), write(2)