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select(3c)

Standard C Library Functions                                        select(3C)



NAME
       select,  pselect,  FD_SET,  FD_CLR, FD_ISSET, FD_ZERO - synchronous I/O
       multiplexing

SYNOPSIS
       #include <sys/time.h>

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


       int pselect(int nfds,
            fd_set *restrict readfds, fd_set *restrict writefds,
            fd_set *restrict errorfds,
            const struct timespec *restrict timeout,
            const sigset_t *restrict sigmask);


       void FD_SET(int fd, fd_set *fdset);


       void FD_CLR(int fd, fd_set *fdset);


       int FD_ISSET(int fd, fd_set *fdset);


       void FD_ZERO(fd_set *fdset);

DESCRIPTION
       The  pselect()  function  examines  the  file  descriptor  sets   whose
       addresses  are passed in the readfds, writefds, and errorfds parameters
       to see if some of their descriptors are ready for  reading,  are  ready
       for writing, or have an exceptional condition pending, respectively.


       The  select()  function is equivalent to the pselect() function, except
       as follows:

           o      For the select() function, the timeout period  is  given  in
                  seconds  and  microseconds  in  an  argument  of type struct
                  timeval, whereas for  the  pselect()  function  the  timeout
                  period is given in seconds and nanoseconds in an argument of
                  type struct timespec.


           o      The select() function has no sigmask argument. It behaves as
                  pselect() does when sigmask is a null pointer.


           o      Upon successful completion, the select() function might mod‐
                  ify the object pointed to by the timeout argument.



       The select() and pselect() functions support  regular  files,  terminal
       and  pseudo-terminal  devices,  STREAMS-based  files, FIFOs, pipes, and
       sockets. The behavior of select() and  pselect()  on  file  descriptors
       that refer to other types of file is unspecified.


       The nfds argument specifies the range of file descriptors to be tested.
       The first nfds descriptors are  checked  in  each  set;  that  is,  the
       descriptors  from  zero through nfds-1 in the descriptor sets are exam‐
       ined.


       If the readfs argument is not a null pointer, it points to an object of
       type  fd_set that on input specifies the file descriptors to be checked
       for being ready to read, and on output indicates which file descriptors
       are ready to read.


       If  the  writefs argument is not a null pointer, it points to an object
       of type fd_set that on input  specifies  the  file  descriptors  to  be
       checked  for  being  ready to write, and on output indicates which file
       descriptors are ready to write.


       If the errorfds argument is not a null pointer, it points to an  object
       of  type  fd_set  that  on  input  specifies the file descriptors to be
       checked for error conditions pending, and  on  output  indicates  which
       file descriptors have error conditions pending.


       Upon  successful  completion,  the  objects  pointed  to by the readfs,
       writefs, and errorfds arguments are modified  to  indicate  which  file
       descriptors  are ready for reading, ready for writing, or have an error
       condition pending, respectively, and return the total number  of  ready
       descriptors  in all the output sets. For each file descriptor less than
       nfds, the corresponding bit will be set on successful completion if  it
       was  set  on  input  and the associated condition is true for that file
       descriptor.


       If none of the selected descriptors are ready for the requested  opera‐
       tion,  the  select() or pselect() function blocks until at least one of
       the requested operations becomes ready, until the  timeout  occurs,  or
       until  interrupted by a signal. The timeout parameter controls how long
       the select() or pselect() function takes  before  timing  out.  If  the
       timeout  parameter is not a null pointer, it specifies a maximum inter‐
       val to wait for the selection to complete. If the specified time inter‐
       val  expires  without any requested operation becoming ready, the func‐
       tion returns. If the timeout parameter is a null pointer, then the call
       to  select()  or  pselect()  blocks  indefinitely  until  at  least one
       descriptor meets the specified criteria. To effect a poll, the  timeout
       parameter should not be a null pointer, and should point to a zero-val‐
       ued timespec structure.


       The use of a timeout does not affect  any  pending  timers  set  up  by
       alarm(2), ualarm(3C), or setitimer(2).


       If  sigmask is not a null pointer, then the pselect() function replaces
       the signal mask of the process by the set of signals pointed to by sig‐
       mask  before examining the descriptors, and restores the signal mask of
       the process before returning.


       A descriptor is considered ready for reading when a call  to  an  input
       function  with  O_NONBLOCK  clear  would  not block, whether or not the
       function would transfer data successfully. (The function  might  return
       data,  an end-of-file indication, or an error other than one indicating
       that it is blocked, and in each of these cases the descriptor  will  be
       considered ready for reading.)


       A  descriptor  is considered ready for writing when a call to an output
       function with O_NONBLOCK clear would not  block,  whether  or  not  the
       function would transfer data successfully.


       If  a  socket  has  a pending error, it is considered to have an excep‐
       tional condition pending. Otherwise, what  constitutes  an  exceptional
       condition  is  file type-specific. For a file descriptor for use with a
       socket, it is protocol-specific except as noted below. For  other  file
       types,  if  the  operation  is  meaningless for a particular file type,
       select() or pselect() indicates that the descriptor is ready  for  read
       or  write  operations  and  indicates that the descriptor has no excep‐
       tional condition pending.


       If a descriptor refers to a socket, the implied input function  is  the
       recvmsg(3C)  function  with  parameters requesting normal and ancillary
       data, such that the presence of either type causes  the  socket  to  be
       marked  as readable. The presence of out-of-band data is checked if the
       socket option SO_OOBINLINE has been enabled,  as  out-of-band  data  is
       enqueued  with  normal data. If the socket is currently listening, then
       it is marked as readable if an incoming  connection  request  has  been
       received, and a call to the accept function completes without blocking.


       If  a descriptor refers to a socket, the implied output function is the
       sendmsg(3C) function supplying an amount of normal data  equal  to  the
       current value of the SO_SNDLOWAT option for the socket. If a non-block‐
       ing call to the connect function has been made for a  socket,  and  the
       connection  attempt  has  either  succeeded or failed leaving a pending
       error, the socket is marked as writable.


       A socket is considered to have an exceptional condition  pending  if  a
       receive  operation  with O_NONBLOCK clear for the open file description
       and with the MSG_OOB flag set would  return  out-of-band  data  without
       blocking.  (It  is  protocol-specific whether the MSG_OOB flag would be
       used to read out-of-band data.) A socket will  also  be  considered  to
       have  an  exceptional  condition pending if an out-of-band data mark is
       present in the receive queue.


       A file descriptor for a socket that is listening for  connections  will
       indicate  that it is ready for reading, when connections are available.
       A file descriptor for a socket that is connecting  asynchronously  will
       indicate  that  it  is  ready  for  writing, when a connection has been
       established.


       Selecting true for reading on a socket descriptor  upon  which  a  lis‐
       ten(3C)  call has been performed indicates that a subsequent accept(3C)
       call on that descriptor will not block.


       If the timeout argument is not a null pointer, it points to  an  object
       of  type  struct  timeval that specifies a maximum interval to wait for
       the selection to complete. If the timeout argument points to an  object
       of type struct timeval whose members are 0, select() does not block. If
       the timeout argument is a null pointer, select() blocks until an  event
       causes  one  of the masks to be returned with a valid (non-zero) value.
       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 success‐
       fully and returns 0.


       If the readfs, writefs, and errorfds arguments are  all  null  pointers
       and  the  timeout argument is not a null pointer, select() or pselect()
       blocks for the time specified, or until interrupted by a signal. If the
       readfs,  writefs,  and errorfds arguments are all null pointers and the
       timeout argument is a null pointer, select() blocks  until  interrupted
       by a signal.


       File  descriptors  associated with regular files always select true for
       ready to read, ready to write, and error conditions.


       On failure, the objects pointed to by the readfs, writefs, and errorfds
       arguments are not modified. If the timeout interval expires without the
       specified condition being true for any of the specified  file  descrip‐
       tors, the objects pointed to by the readfs, writefs, and errorfds argu‐
       ments have all bits set to 0.


       File descriptor masks of type fd_set can be initialized and tested with
       the macros FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO().

       FD_CLR(fd, &fdset)      Clears  the  bit  for the file descriptor fd in
                               the file descriptor set fdset.


       FD_ISSET(fd, &fdset)    Returns a non-zero value if  the  bit  for  the
                               file  descriptor fd is set in the file descrip‐
                               tor set pointed to by fdset, and 0 otherwise.


       FD_SET(fd, &fdset)      Sets the bit for the file descriptor fd in  the
                               file descriptor set fdset.


       FD_ZERO(&fdset)         Initializes  the  file  descriptor set fdset to
                               have zero bits for all file descriptors.



       The behavior of these macros is undefined if the fd  argument  is  less
       than  0 or greater than or equal to FD_SETSIZE, or if fd is not a valid
       file descriptor, or if any of the arguments are expressions  with  side
       effects.

RETURN VALUES
       On  successful completion, select() and pselect() return the total num‐
       ber of bits set in the bit masks. Otherwise, −1 is returned  and  errno
       is set to indicate the error.


       The  FD_CLR(),  FD_SET(),  and  FD_ZERO()  macros  return no value. The
       FD_ISSET() macro returns a non-zero value  if  the  bit  for  the  file
       descriptor  fd  is  set in the file descriptor set pointed to by fdset,
       and 0 otherwise.

ERRORS
       The select() and pselect() functions will fail if:

       EBADF     One or more of the file  descriptor  sets  specified  a  file
                 descriptor that is not a valid open file descriptor.


       EINTR     The  function  was  interrupted  before  any  of the selected
                 events occurred and before the timeout interval expired.

                 If SA_RESTART has been set for the interrupting signal, it is
                 implementation-dependent whether select() restarts or returns
                 with EINTR.


       EINVAL    An invalid timeout interval was specified.


       EINVAL    The nfds argument is less than 0 or greater than FD_SETSIZE.


       EINVAL    One of the specified file descriptors refers to a  STREAM  or
                 multiplexer  that  is  linked  (directly or indirectly) down‐
                 stream from a multiplexer.


       EINVAL    A component of the  pointed-to  time  limit  is  outside  the
                 acceptable  range:  t_sec  must be between 0 and 10^8, inclu‐
                 sive. t_usec must be greater than or equal  to  0,  and  less
                 than 10^6.


USAGE
       The  poll(2)  function is preferred over this function. It must be used
       when the number of file descriptors exceeds FD_SETSIZE.


       The use of a timeout does not affect  any  pending  timers  set  up  by
       alarm(2), ualarm(3C) or setitimer(2).


       On successful completion, the object pointed to by the timeout argument
       may be modified.

ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i)


       ATTRIBUTE TYPEATTRIBUTE VALUE _ Interface StabilityCommitted _ MT-Leve‐
       lAsync-Signal-Safe _ StandardSee standards(7).


SEE ALSO
       alarm(2),   fcntl(2),   poll(2),   read(2),   setitimer(2),   write(2),
       accept(3C), listen(3C), ualarm(3C), attributes(7), standards(7)

NOTES
       The default value for FD_SETSIZE (currently 1024) is  larger  than  the
       default limit on the number of open files. To accommodate 32-bit appli‐
       cations that wish to use a larger number of open files  with  select(),
       it  is  possible  to  increase this size at compile time by providing a
       larger definition of FD_SETSIZE before the inclusion of any system-sup‐
       plied  header.  The maximum supported size for FD_SETSIZE is 65536. The
       default value is already 65536 for 64-bit applications.



Oracle Solaris 11.4               16 Aug 2017                       select(3C)
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