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NAME    [Toc]    [Back]

     select - synchronous I/O multiplexing

SYNOPSIS    [Toc]    [Back]

     #include <sys/types.h>
     #include <sys/time.h>
     #include <string.h>
     #include <unistd.h>

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

     FD_SET(fd, &fdset);

     FD_CLR(fd, &fdset);

     FD_ISSET(fd, &fdset);


DESCRIPTION    [Toc]    [Back]

     select()  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 first nfds descriptors  are
checked in
     each set; i.e., the descriptors from 0 through nfds-1 in the
     sets are examined.  On return, select() replaces  the  given
     sets  with  subsets consisting of those descriptors that are
ready for the
     requested operation.  select() 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
     FD_CLR(fd,  &fdset)  removes  fd  from  fdset.  FD_ISSET(fd,
&fdset) is nonzero
 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 a non-null pointer, it specifies a maximum interval to wait
     for the selection to complete.  If timeout is a null  pointer, the select
     blocks indefinitely.  To effect a poll, the timeout argument
should be
     non-null,  pointing  to  a  zero-valued  timeval  structure.
timeout is not
     changed  by select(), and may be reused on subsequent calls;
however, it
     is good style to re-initialize it before each invocation  of

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

RETURN VALUES    [Toc]    [Back]

     select() 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 call, the descriptor sets will be

ERRORS    [Toc]    [Back]

     An error return from select() indicates:

     [EFAULT]      One or more of readfds, writefds, or exceptfds
points outside
 the process's allocated address space.

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

     [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.

SEE ALSO    [Toc]    [Back]

     accept(2), connect(2),  gettimeofday(2),  poll(2),  read(2),
     send(2), write(2), getdtablesize(3)

HISTORY    [Toc]    [Back]

     The select() function call appeared in 4.2BSD.

BUGS    [Toc]    [Back]

     Although  the  provision of getdtablesize(3) was intended to
allow user
     programs to be written independent of the  kernel  limit  on
the number of
     open  files, the dimension of a sufficiently large bit field
for select
     remains a problem.  The default bit size of fd_set is  based
on the symbol
     FD_SETSIZE  (currently  1024),  but that is somewhat smaller
than the current
 kernel limit to the number of open files.  However,  in
order to accommodate
 programs which might potentially use a larger number of open
     files with select, it is  possible  to  increase  this  size
within a program
     by  providing  a  larger definition of FD_SETSIZE before the
inclusion of
     <sys/types.h>.  The kernel will cope, and the  userland  libraries provided
     with the system are also ready for large numbers of file descriptors.

     Alternatively, to be really safe, it is possible to allocate
fd_set bitarrays
 dynamically.  The idea is to permit a program to work
     even if it is execve(2)'d with 4000 file descriptors pre-allocated.  The
     following  illustrates  the technique which is used by userland libraries:

           fd_set *fdsr;
           int max = fd;

           fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
           if (fdsr == NULL) {
                   return (-1);
           FD_SET(fd, fdsr);
           n = select(max+1, fdsr, NULL, NULL, &tv);

     Alternatively, it is possible to use the poll(2)  interface.
poll(2) is
     more efficient when the size of select()'s fd_set bit-arrays
are very
     large, and for fixed numbers of file  descriptors  one  need
not size and
     dynamically allocate a memory object.

     select()  should  probably  have been designed to return the
time remaining
     from the original timeout, if any,  by  modifying  the  time
value in place.
     Even though some systems stupidly act in this different way,
it is unlikely
 this semantic will ever be commonly  implemented,  as
the change
     causes massive source code compatibility problems.  Furthermore, recent
     new standards have dictated the current behaviour.  In  general, due to
     the existence of those brain-damaged non-conforming systems,
it is unwise
     to assume that the timeout value will be unmodified  by  the
select() call,
     and  the  caller  should reinitialize it on each invocation.
     the delta is easily done by calling  gettimeofday(2)  before
and after the
     call  to  select(),  and  using  timersub() (as described in

     Internally to the kernel, select() works poorly if  multiple
     wait  on the same file descriptor.  Given that, it is rather
surprising to
     see that many daemons are written that way (i.e., httpd(8)).

OpenBSD      3.6                          March      25,     1994
[ Back ]
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