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# select

    SELECT(2)                     System Calls Manual                    SELECT(2)

    NAME
         select - synchronous I/O multiplexing

    SYNOPSIS
         #include <sys/types.h>
         #include <sys/time.h>
         #include <unistd.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
         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.  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.  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(&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 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.

    RETURN VALUES
         select() returns the number of ready descriptors that are contained in
         the descriptor sets, or -1 is an error occurred.  If the time limit
         expires, select() returns 0.  If select() returns with an error, includ-
         ing one due to an interrupted call, the descriptor sets will be unmodi-
         fied.

    ERRORS
         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 com-
                            ponents is negative or too large.

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

    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 256), but that is somewhat smaller than the current
         kernel limit to the number of open files.  However, in order to accommo-
         date 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 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 = (fd_set *)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() works 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 (i.e., httpd(8)).

    HISTORY
         The select() function call appeared in 4.2BSD.

    BSD                             March 25, 1994                             BSD

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