4.4BSD/usr/share/man/cat2/select.0

SELECT(2)                   BSD Programmer's Manual                  SELECT(2)

NNAAMMEE
     sseelleecctt - synchronous I/O multiplexing

SSYYNNOOPPSSIISS
     ##iinncclluuddee <<uunniissttdd..hh>>
     ##iinncclluuddee <<ssyyss//ttyyppeess..hh>>
     ##iinncclluuddee <<ssyyss//ttiimmee..hh>>

     _i_n_t
     sseelleecctt(_i_n_t _n_f_d_s, _f_d___s_e_t _*_r_e_a_d_f_d_s, _f_d___s_e_t _*_w_r_i_t_e_f_d_s, _f_d___s_e_t _*_e_x_c_e_p_t_f_d_s,
             _s_t_r_u_c_t _t_i_m_e_v_a_l _*_t_i_m_e_o_u_t);

     FFDD__SSEETT(_f_d, _&_f_d_s_e_t);

     FFDD__CCLLRR(_f_d, _&_f_d_s_e_t);

     FFDD__IISSSSEETT(_f_d, _&_f_d_s_e_t);

     FFDD__ZZEERROO(_&_f_d_s_e_t);

DDEESSCCRRIIPPTTIIOONN
     SSeelleecctt() examines the I/O descriptor sets whose addresses are passed in
     _r_e_a_d_f_d_s, _w_r_i_t_e_f_d_s, and _e_x_c_e_p_t_f_d_s 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 _n_f_d_s descriptors are checked in
     each set; i.e., the descriptors from 0 through _n_f_d_s-1 in the descriptor
     sets are examined.  On return, sseelleecctt() replaces the given descriptor
     sets with subsets consisting of those descriptors that are ready for the
     requested operation.  SSeelleecctt() 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:
     FFDD__ZZEERROO(_&_f_d_s_e_t_x) initializes a descriptor set _f_d_s_e_t to the null set.
     FFDD__SSEETT(_f_d, _&_f_d_s_e_t) includes a particular descriptor _f_d in _f_d_s_e_t.
     FFDD__CCLLRR(_f_d, _&_f_d_s_e_t) removes _f_d from _f_d_s_e_t. FFDD__IISSSSEETT(_f_d, _&_f_d_s_e_t) is non-
     zero if _f_d is a member of _f_d_s_e_t, 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 _t_i_m_e_o_u_t is a non-nil pointer, it specifies a maximum interval to wait
     for the selection to complete.  If _t_i_m_e_o_u_t is a nil pointer, the select
     blocks indefinitely.  To affect a poll, the _t_i_m_e_o_u_t argument should be
     non-nil, pointing to a zero-valued timeval structure.

     Any of _r_e_a_d_f_d_s, _w_r_i_t_e_f_d_s, and _e_x_c_e_p_t_f_d_s may be given as nil pointers if
     no descriptors are of interest.

RREETTUURRNN VVAALLUUEESS
     SSeelleecctt() returns the number of ready descriptors that are contained in
     the descriptor sets, or -1 if an error occurred.  If the time limit ex-
     pires, sseelleecctt() returns 0.  If sseelleecctt() returns with an error, including
     one due to an interrupted call, the descriptor sets will be unmodified.

EERRRROORRSS
     An error return from sseelleecctt() 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.

SSEEEE AALLSSOO
     accept(2),  connect(2),  read(2),  write(2),  recv(2),  send(2),  getdta-
     blesize(2)

BBUUGGSS
     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 sufficiently large bit field for select
     remains a problem.  The default size FD_SETSIZE (currently 256) is some-
     what larger 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 be-
     fore the inclusion of <_s_y_s_/_t_y_p_e_s_._h>.

     SSeelleecctt() should probably return the time remaining from the original
     timeout, if any, by modifying the time value in place.  This may be im-
     plemented in future versions of the system.  Thus, it is unwise to assume
     that the timeout value will be unmodified by the sseelleecctt() call.

HHIISSTTOORRYY
     The sseelleecctt function call appeared in 4.2BSD.

4.2 Berkeley Distribution        June 4, 1993                                2