select

SELECT(2)                   BSD 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
    
2017-02-09
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