SELECT(2)           RISC/os Reference Manual            SELECT(2)



NAME
     select - synchronous I/O multiplexing

SYNOPSIS
   Headers
     For -systype svr3:

     #include <bsd/sys/types.h>
     #include <bsd/sys/time.h>

     For -systype bsd43:

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

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

     FD_SET(fd, &fdset)
     FD_CLR(fd, &fdset)
     FD_ISSET(fd, &fdset)
     FD_ZERO(&fdset)
     int fd;
     fd_set fdset;

DESCRIPTION
     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 writ-
     ing, 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 descriptor sets are
     examined (nfds being the number of file descriptors for the
     entire file system).  On return, select replaces the given
     descriptor sets with subsets consisting of those descriptors
     that are ready for the requested operation.  The total
     number of ready descriptors in all the sets is returned in
     nfound.

     The descriptor sets are stored as bit fields in arrays of
     integers.  The following macros are provided for manipulat-
     ing 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
     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



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     descriptors supported by the system.

     If timeout is a non-zero pointer, it specifies a maximum
     interval to wait for the selection to complete.  If timeout
     is a zero pointer, the select blocks indefinitely.  To
     affect a poll, the timeout argument should be non-zero,
     pointing to a zero-valued timeval structure.

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

RETURN VALUE
     select returns the number of ready descriptors that are con-
     tained in the descriptor sets, or -1 if an error occurred.
     If the time limit expires then select returns 0.  If select
     returns with an error, including one due to an interrupted
     call, the descriptor sets will be unmodified.

ERRORS
     An error return from select indicates:

     [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
     accept(2), connect(2), getdtablesize(2), read(2), recv(2),
     send(2), write(2).

BUGS
     Although the provision of getdtablesize(2) was intended to
     allow user programs to be written independent of the kernel
     limit on the number of open files, the dimension of a suffi-
     ciently large bit field for select remains a problem.  The
     default size FD_SETSIZE (currently 256) is somewhat larger
     than the current kernel limit to the number of open files.
     However, in order to accommodate programs which might poten-
     tially 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>.

     select should probably return the time remaining from the
     original timeout, if any, by modifying the time value in
     place.  This may be implemented in future versions of the
     system.  Thus, it is unwise to assume that the timeout value



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     will be unmodified by the select call.

NOTE
     When these routines are used in a program which is compiled
     in -systype svr3, they are not resolved by libc.a.  See
     intro(3) for more information.

















































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