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Annotation of 43BSDReno/share/doc/ps1/06.sysman/2.1.t, 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: .\"    @(#)2.1.t       6.2 (Berkeley) 5/12/86
        !             6: .\"
        !             7: .sh "Generic operations
        !             8: .PP
        !             9: .PP
        !            10: Many system abstractions support the
        !            11: operations \fIread\fP, \fIwrite\fP and \fIioctl\fP.  We describe
        !            12: the basics of these common primitives here.
        !            13: Similarly, the mechanisms whereby normally synchronous operations
        !            14: may occur in a non-blocking or asynchronous fashion are
        !            15: common to all system-defined abstractions and are described here.
        !            16: .NH 3
        !            17: Read and write
        !            18: .PP
        !            19: The \fIread\fP and \fIwrite\fP system calls can be applied
        !            20: to communications channels, files, terminals and devices.
        !            21: They have the form:
        !            22: .DS
        !            23: cc = read(fd, buf, nbytes);
        !            24: result int cc; int fd; result caddr_t buf; int nbytes;
        !            25: 
        !            26: cc = write(fd, buf, nbytes);
        !            27: result int cc; int fd; caddr_t buf; int nbytes;
        !            28: .DE
        !            29: The \fIread\fP call transfers as much data as possible from the
        !            30: object defined by \fIfd\fP to the buffer at address \fIbuf\fP of
        !            31: size \fInbytes\fP.  The number of bytes transferred is
        !            32: returned in \fIcc\fP, which is \-1 if a return occurred before
        !            33: any data was transferred because of an error or use of non-blocking
        !            34: operations.
        !            35: .PP
        !            36: The \fIwrite\fP call transfers data from the buffer to the
        !            37: object defined by \fIfd\fP.  Depending on the type of \fIfd\fP,
        !            38: it is possible that the \fIwrite\fP call will accept some portion
        !            39: of the provided bytes; the user should resubmit the other bytes
        !            40: in a later request in this case.
        !            41: Error returns because of interrupted or otherwise incomplete operations
        !            42: are possible.
        !            43: .PP
        !            44: Scattering of data on input or gathering of data for output
        !            45: is also possible using an array of input/output vector descriptors.
        !            46: The type for the descriptors is defined in \fI<sys/uio.h>\fP as:
        !            47: .DS
        !            48: ._f
        !            49: struct iovec {
        !            50:        caddr_t iov_msg;        /* base of a component */
        !            51:        int     iov_len;        /* length of a component */
        !            52: };
        !            53: .DE
        !            54: The calls using an array of descriptors are:
        !            55: .DS
        !            56: cc = readv(fd, iov, iovlen);
        !            57: result int cc; int fd; struct iovec *iov; int iovlen;
        !            58: 
        !            59: cc = writev(fd, iov, iovlen);
        !            60: result int cc; int fd; struct iovec *iov; int iovlen;
        !            61: .DE
        !            62: Here \fIiovlen\fP is the count of elements in the \fIiov\fP array.
        !            63: .NH 3
        !            64: Input/output control
        !            65: .PP
        !            66: Control operations on an object are performed by the \fIioctl\fP
        !            67: operation:
        !            68: .DS
        !            69: ioctl(fd, request, buffer);
        !            70: int fd, request; caddr_t buffer;
        !            71: .DE
        !            72: This operation causes the specified \fIrequest\fP to be performed
        !            73: on the object \fIfd\fP.  The \fIrequest\fP parameter specifies
        !            74: whether the argument buffer is to be read, written, read and written,
        !            75: or is not needed, and also the size of the buffer, as well as the
        !            76: request.
        !            77: Different descriptor types and subtypes within descriptor types
        !            78: may use distinct \fIioctl\fP requests.  For example,
        !            79: operations on terminals control flushing of input and output
        !            80: queues and setting of terminal parameters; operations on
        !            81: disks cause formatting operations to occur; operations on tapes
        !            82: control tape positioning.
        !            83: .PP
        !            84: The names for basic control operations are defined in \fI<sys/ioctl.h>\fP.
        !            85: .NH 3
        !            86: Non-blocking and asynchronous operations
        !            87: .PP
        !            88: A process that wishes to do non-blocking operations on one of
        !            89: its descriptors sets the descriptor in non-blocking mode as
        !            90: described in section 1.5.4.  Thereafter the \fIread\fP call will
        !            91: return a specific EWOULDBLOCK error indication if there is no data to be
        !            92: \fIread\fP.  The process may
        !            93: \fIselect\fP the associated descriptor to determine when a read is
        !            94: possible.
        !            95: .PP
        !            96: Output attempted when a descriptor can accept less than is requested
        !            97: will either accept some of the provided data, returning a shorter than normal
        !            98: length, or return an error indicating that the operation would block.
        !            99: More output can be performed as soon as a \fIselect\fP call indicates
        !           100: the object is writeable.
        !           101: .PP
        !           102: Operations other than data input or output
        !           103: may be performed on a descriptor in a non-blocking fashion.
        !           104: These operations will return with a characteristic error indicating
        !           105: that they are in progress
        !           106: if they cannot complete immediately.  The descriptor
        !           107: may then be \fIselect\fPed for \fIwrite\fP to find out
        !           108: when the operation has been completed.  When \fIselect\fP indicates
        !           109: the descriptor is writeable, the operation has completed.
        !           110: Depending on the nature of the descriptor and the operation,
        !           111: additional activity may be started or the new state may be tested.

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