NAME

       select — synchronous I/O multiplexing

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

       #include <sys/select.h>

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

       FD_SET(fd, &fdset);

       FD_CLR(fd, &fdset);

       FD_ISSET(fd, &fdset);

       FD_ZERO(&fdset);

DESCRIPTION

       The  select()  system  call  examines  the  I/O  descriptor  sets  whose addresses are passed in readfds,
       writefds, and exceptfds to see if some of their descriptors are ready for reading, are ready for writing,
       or have an exceptional condition pending, respectively.  The only  exceptional  condition  detectable  is
       out-of-band  data  received  on  a socket.  The first nfds descriptors are checked in each set; i.e., the
       descriptors from 0 through nfds-1 in the descriptor sets are examined.  On return, select() replaces  the
       given  descriptor  sets  with  subsets  consisting  of those descriptors that are ready for the requested
       operation.  The select() system call returns the total number of ready descriptors in all the sets.

       The descriptor sets are stored as bit fields in arrays of integers.  The following  macros  are  provided
       for  manipulating  such  descriptor  sets: FD_ZERO(&fdset) initializes a descriptor set fdset to the null
       set.  FD_SET(fd, &fdset) includes a particular descriptor fd in fdset.   FD_CLR(fd,  &fdset)  removes  fd
       from  fdset.   FD_ISSET(fd, &fdset) is non-zero if fd is a member of fdset, zero otherwise.  The behavior
       of these macros is undefined if a descriptor value is  less  than  zero  or  greater  than  or  equal  to
       FD_SETSIZE,  which  is  normally  at  least  equal  to the maximum number of descriptors supported by the
       system.

       If timeout is not a null pointer, it specifies  the  maximum  interval  to  wait  for  the  selection  to
       complete.  System activity can lengthen the interval by an indeterminate amount.

       If timeout is a null pointer, the select blocks indefinitely.

       To effect a poll, the timeout argument should not be a null pointer, but it should point to a zero-valued
       timeval structure.

       Any of readfds, writefds, and exceptfds may be given as null pointers if no descriptors are of interest.

RETURN VALUES

       The  select()  system  call  returns the number of ready descriptors that are contained in the descriptor
       sets, or -1 if an error occurred.  If the time limit expires, select() returns 0.   If  select()  returns
       with an error, including one due to an interrupted system call, the descriptor sets will be unmodified.

ERRORS

       An error return from select() indicates:

       [EBADF]            One of the descriptor sets specified an invalid descriptor.

       [EFAULT]           One  of  the  arguments  readfds, writefds, exceptfds, or timeout points to an invalid
                          address.

       [EINTR]            A signal was delivered before the time limit expired and before any  of  the  selected
                          events occurred.

       [EINVAL]           The specified time limit is invalid.  One of its components is negative or too large.

       [EINVAL]           The nfds argument was invalid.

SEE ALSO

       accept(2),  connect(2),  getdtablesize(2),  gettimeofday(2),  kqueue(2),  poll(2),  pselect(2),  read(2),
       recv(2), send(2), write(2), clocks(7)

NOTES

       The default size of FD_SETSIZE  is  currently  1024.   In  order  to  accommodate  programs  which  might
       potentially  use  a  larger  number  of open files with select(), it is possible to increase this size by
       having the program define FD_SETSIZE before the inclusion of any header which includes <sys/types.h>.

       If nfds is greater than the number of open files, select() is not guaranteed to examine the  unused  file
       descriptors.  For historical reasons, select() will always examine the first 256 descriptors.

STANDARDS

       The  select()  system call and FD_CLR(), FD_ISSET(), FD_SET(), and FD_ZERO() macros conform with IEEE Std
       1003.1-2001 (“POSIX.1”).

HISTORY

       The select() system call appeared in 4.2BSD.

BUGS

       Version 2 of the Single UNIX Specification (“SUSv2”) allows systems to modify  the  original  timeout  in
       place.   Thus,  it  is  unwise to assume that the timeout value will be unmodified by the select() system
       call.  FreeBSD does not modify the return value, which can cause problems for  applications  ported  from
       other systems.

Debian                                            June 25, 2020                                        SELECT(2)