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SELECT(2)                   BSD System Calls Manual                  SELECT(2)

NAME
     select, pselect -- synchronous I/O multiplexing

SYNOPSIS
     #include <&lt;sys/select.h>&gt;

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

     int
     pselect(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds,
         const struct timespec *timeout, const sigset_t *mask);

     FD_SET(fd, &amp;fdset);

     FD_CLR(fd, &amp;fdset);

     FD_ISSET(fd, &amp;fdset);

     FD_ZERO(&amp;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 writing, or have an exceptional condi-
     tion pending, respectively.  Exceptional conditions include the presence
     of out-of-band data on a socket.  The first nfds descriptors are checked
     in each set; i.e., the descriptors from 0 through nfds-1 in the descrip-
     tor sets are examined.  On return, select() replaces the given descriptor
     sets with subsets consisting of those descriptors that are ready for the
     requested operation.  select() returns the total number of ready descrip-
     tors 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(&amp;fdset) initializes a descriptor set fdset to the null set.
     FD_SET(fd, &amp;fdset) includes a particular descriptor fd in fdset.
     FD_CLR(fd, &amp;fdset) removes fd from fdset.  FD_ISSET(fd, &amp;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 max-
     imum 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 select().

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

     The pselect() function is similar to select() except that it specifies
     the timeout using a timespec structure.  Also, if mask is a non-null
     pointer, pselect() atomically sets the calling thread's signal mask to
     the signal set pointed to by mask for the duration of the function call.
     In this case, the original signal mask will be restored before pselect()
     returns.

RETURN VALUES
     If successful, select() and pselect() return the number of ready descrip-
     tors that are contained in the descriptor sets.  If a descriptor is
     included in multiple descriptor sets, each inclusion is counted sepa-
     rately.  If the time limit expires before any descriptors become ready,
     they return 0.

     Otherwise, if select() or pselect() return with an error, including one
     due to an interrupted call, they return -1, and the descriptor sets will
     be unmodified.

ERRORS
     An error return from select() or pselect() 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 descriptors became
                        ready.

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

SEE ALSO
     accept(2), clock_gettime(2), connect(2), gettimeofday(2), poll(2),
     read(2), recv(2), send(2), write(2), getdtablesize(3)

HISTORY
     The select() system call first appeared in 4.1cBSD.  The pselect() system
     call has been available since OpenBSD 5.4.

BUGS
     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 cur-
     rent 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
     any headers.  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 bit-
     arrays dynamically.  The idea is to permit a program to work properly
     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 = calloc(howmany(max+1, NFDBITS), sizeof(fd_mask));
           if (fdsr == NULL) {
                   ...
                   return (-1);
           }
           FD_SET(fd, fdsr);
           n = select(max+1, fdsr, NULL, NULL, &tv);
           ...
           free(fdsr);

     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.  Calculating
     the delta is easily done by calling gettimeofday(2) before and after the
     call to select(), and using timersub() (as described in getitimer(2)).

     Internally to the kernel, select() and pselect() work poorly if multiple
     processes wait on the same file descriptor.  Given that, it is rather
     surprising to see that many daemons are written that way.

BSD                             April 29, 2017                             BSD