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Annotation of 43BSDTahoe/man/man2/select.2, revision 1.1

1.1     ! root        1: .\" Copyright (c) 1983 Regents of the University of California.
        !             2: .\" All rights reserved.  The Berkeley software License Agreement
        !             3: .\" specifies the terms and conditions for redistribution.
        !             4: .\"
        !             5: .\"    @(#)select.2    6.5 (Berkeley) 5/15/86
        !             6: .\"
        !             7: .TH SELECT 2 "May 15, 1986"
        !             8: .UC 5
        !             9: .SH NAME
        !            10: select \- synchronous I/O multiplexing
        !            11: .SH SYNOPSIS
        !            12: .nf
        !            13: .ft B
        !            14: #include <sys/types.h>
        !            15: #include <sys/time.h>
        !            16: .PP
        !            17: .ft B
        !            18: nfound = select(nfds, readfds, writefds, exceptfds, timeout)
        !            19: int nfound, nfds;
        !            20: fd_set *readfds, *writefds, *exceptfds;
        !            21: struct timeval *timeout;
        !            22: .PP
        !            23: .ft B
        !            24: FD_SET(fd, &fdset)     
        !            25: FD_CLR(fd, &fdset)     
        !            26: FD_ISSET(fd, &fdset)   
        !            27: FD_ZERO(&fdset)        
        !            28: int fd;
        !            29: fd_set fdset;
        !            30: .fi
        !            31: .SH DESCRIPTION
        !            32: .I Select
        !            33: examines the I/O descriptor sets whose addresses are passed in
        !            34: .IR readfds ,
        !            35: .IR writefds ,
        !            36: and
        !            37: .I exceptfds
        !            38: to see if some of their descriptors
        !            39: are ready for reading, are ready for writing, or have an exceptional
        !            40: condition pending, respectively.
        !            41: The first
        !            42: .I nfds
        !            43: descriptors are checked in each set;
        !            44: i.e. the descriptors from 0 through
        !            45: .IR nfds -1
        !            46: in the descriptor sets are examined.
        !            47: On return,
        !            48: .I select
        !            49: replaces the given descriptor sets
        !            50: with subsets consisting of those descriptors that are ready
        !            51: for the requested operation.
        !            52: The total number of ready descriptors in all the sets is returned in
        !            53: .IR nfound .
        !            54: .PP
        !            55: The descriptor sets are stored as bit fields in arrays of integers.
        !            56: The following macros are provided for manipulating such descriptor sets:
        !            57: .I "FD_ZERO(&fdset)"
        !            58: initializes a descriptor set
        !            59: .I fdset
        !            60: to the null set.
        !            61: .I "FD_SET(fd, &fdset)"
        !            62: includes a particular descriptor
        !            63: .I fd
        !            64: in
        !            65: .IR fdset .
        !            66: .I "FD_CLR(fd, &fdset)"
        !            67: removes
        !            68: .I fd
        !            69: from
        !            70: .IR fdset .
        !            71: .I "FD_ISSET(fd, &fdset)"
        !            72: is nonzero if
        !            73: .I fd
        !            74: is a member of
        !            75: .IR fdset ,
        !            76: zero otherwise.
        !            77: The behavior of these macros is undefined if
        !            78: a descriptor value is less than zero or greater than or equal to
        !            79: .IR FD_SETSIZE ,
        !            80: which is normally at least equal
        !            81: to the maximum number of descriptors supported by the system.
        !            82: .PP
        !            83: If
        !            84: .I timeout
        !            85: is a non-zero pointer, it specifies a maximum interval to wait for the
        !            86: selection to complete.  If 
        !            87: .I timeout
        !            88: is a zero pointer, the select blocks indefinitely.  To affect a poll, the
        !            89: .I timeout
        !            90: argument should be non-zero, pointing to a zero-valued timeval structure.
        !            91: .PP
        !            92: Any of
        !            93: .IR readfds ,
        !            94: .IR writefds ,
        !            95: and
        !            96: .I exceptfds
        !            97: may be given as zero pointers if no descriptors are of interest.
        !            98: .SH "RETURN VALUE
        !            99: .I Select
        !           100: returns the number of ready descriptors that are contained in
        !           101: the descriptor sets,
        !           102: or \-1 if an error occurred.
        !           103: If the time limit expires then
        !           104: .I select
        !           105: returns 0.
        !           106: If
        !           107: .I select
        !           108: returns with an error,
        !           109: including one due to an interrupted call,
        !           110: the descriptor sets will be unmodified.
        !           111: .SH "ERRORS
        !           112: An error return from \fIselect\fP indicates:
        !           113: .TP 15
        !           114: [EBADF]
        !           115: One of the descriptor sets specified an invalid descriptor.
        !           116: .TP 15
        !           117: [EINTR]
        !           118: A signal was delivered before the time limit expired and
        !           119: before any of the selected events occurred.
        !           120: .TP 15
        !           121: [EINVAL]
        !           122: The specified time limit is invalid.  One of its components is
        !           123: negative or too large.
        !           124: .SH SEE ALSO
        !           125: accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2)
        !           126: .SH BUGS
        !           127: Although the provision of
        !           128: .IR getdtablesize (2)
        !           129: was intended to allow user programs to be written independent
        !           130: of the kernel limit on the number of open files, the dimension
        !           131: of a sufficiently large bit field for select remains a problem.
        !           132: The default size FD_SETSIZE (currently 256) is somewhat larger than
        !           133: the current kernel limit to the number of open files.
        !           134: However, in order to accommodate programs which might potentially
        !           135: use a larger number of open files with select, it is possible
        !           136: to increase this size within a program by providing
        !           137: a larger definition of FD_SETSIZE before the inclusion of <sys/types.h>.
        !           138: .PP
        !           139: .I Select
        !           140: should probably return the time remaining from the original timeout,
        !           141: if any, by modifying the time value in place.
        !           142: This may be implemented in future versions of the system.
        !           143: Thus, it is unwise to assume that the timeout value will be unmodified
        !           144: by the
        !           145: .I select
        !           146: call.

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