select - Synchronous I/O multiplexing
struct timeval *timeout ); void FD_CLR(
fd_set *fdset ); int FD_ISSET(
fd_set *fdset ); void FD_SET(
fd_set *fdset ); void FD_ZERO(
fd_set *fdset );
Interfaces documented on this reference page conform to
industry standards as follows:
select(): XSH4.2, XSH5.0, XNS5.0
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies the number of open objects that may be ready for
reading or writing or that have exceptions pending. The
nfds parameter cannot be greater than FD_SETSIZE. 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() function. 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. 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. 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() function blocks indefinitely. To poll, the
timeout parameter should be specified as a nonzero
value and point to a zero-valued timeval structure.
The use of a timeout does not affect any pending
timers set up by alarm(), unalarm(), or settimer().
Specifies a file descriptor. Points to an I/O
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, respectively.
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 characteristics:
for any selected I/O descriptor set pointed to by the
readfds, writefds, or 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 parameters.
When it returns successfully, select() first 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). Then, the function 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 following macros: 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 value.
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 normally at least equal to the
maximum number of file descriptors supported by the system.
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 unspecified.
For sockets, a file descriptor for a socket that is listening
for connections indicates that it is ready for
reading when connections are available. A file descriptor
for a socket that is connecting asynchronously indicates
that it is ready for writing after a connection is established.
By default, this function supports up to 4K open file
descriptors per process. However, up to 64K open file
descriptors per process is supported if an application has
enabled this capability by using the SSI_FD_NEWMAX operation
in a setsysinfo() call and the RLIMIT_NOFILE argument
in a setrlimit() call. For summary information on
retrieving and setting limits on the number of open file
descriptors per process, see the description of the
open_max_hard and open_max_soft system attributes in
In addition to the requirement that limits greater than 4K
must be enabled by the setsysinfo() and setrlimit() calls,
programmers 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. If not set to
be a higher value in the program before compilation,
FD_SETSIZE is set in the <sys/select.h> header file to be
For a more in-depth discussion of this subject, refer to
the section about tuning open file limits in the System
Configuration and Tuning manual.
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
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
When select() returns an error, including a process interrupt,
the I/O descriptor sets pointed to by the readfds,
writefds, and exceptfds parameters remain unmodified.
The select() function sets errno to the specified values
for the following conditions:
One or more of the I/O descriptor sets specified an
invalid file descriptor. A signal was delivered before
the time limit specified by the timeout parameter expired
and before any of the selected events occurred. The time
limit specified by the timeout parameter is invalid.
The nfds parameter is less than 0, or greater than
or equal to FD_SETSIZE.
One of the specified file descriptors refers to a
STREAM or multiplexer that is linked (directly or
indirectly) downstream from a multiplexer. [Tru64
UNIX] Allocation of internal data structures
failed. A later call to the select() function may
Functions: accept(2), connect(2), getdtablesize(2),
poll(2) read(2), recv(2), send(2), setsysinfo(2), write(2)
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