select(2)							System Calls Manual							 select(2)

NAME

       select - Synchronous I/O multiplexing

SYNOPSIS

       #include <sys/time.h>

       int select(	int nfds,      fd_set *readfds,      fd_set *writefds,	    fd_set *exceptfds,	    struct timeval *timeout) ;

       void FD_CLR(	 int fd,      fd_set *fdset);

       int FD_ISSET(	  int fd,      fd_set *fdset);

       void FD_SET(	 int fd,      fd_set *fdset);

       void FD_ZERO(	  fd_set *fdset);

STANDARDS

       Interfaces documented on this reference page conform to industry standards as follows:

       select():  XSH5.0, XNS5.0

       Refer to the standards(5) reference page for more information about industry standards and associated tags.

PARAMETERS

       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(, ualarm(, or  settimer(.   Specifies  a  file  descriptor.
	      Points to an I/O descriptor set.

DESCRIPTION

       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, 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 parameters.

       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).  Sec-
       ond, 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 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.

NOTES

       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.

RETURN VALUES

       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  suc-
       cessfully 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
       parameters remain unmodified.

ERRORS

       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 complete  suc-
	      cessfully.

RELATED INFORMATION

       Functions: accept(2), connect(2), getdtablesize(2), poll(2) read(2), recv(2), send(2), setsysinfo(2), write(2)

       Standards: standards(5) delim off

																	 select(2)