Messages corresponding to 10 and 11 in the figure will be sent later when the

Messages corresponding to 10 and 11 in the figure

This preview shows page 36 - 38 out of 140 pages.

Messages corresponding to (10) and (11) in the figure will be sent later when the operation can be completed. If the request can be fully satisfied, or an error occurs, either the count of bytes transferred or the error code is returned in the status field of the return message to the FS. In this case a message will be sent immediately from the FS back to the process that made the original call, to wake it up. Reading from a terminal is fundamentally different from reading from a disk device. The disk driver issues a command to the disk hardware and eventually data will be returned, barring a mechanical or electrical failure. The computer can display a prompt upon the screen, but there is no way for it to force a person sitting at the keyboard to start typing. For that matter, there is no guarantee that anybody will be sitting there at all. In order to make the speedy return that is required, do_read (line 13953) starts by storing information that will enable the request to be completed later, when and if input arrives.
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There are a few error checks to be made first. It is an error if the device is still expecting input to fulfill a previous request, or if the parameters in the message are invalid (lines 13964 to 13972). If these tests are passed, information about the request is copied into the proper fields in the device's tp->tty_table entry on lines 13975 to 13979. The last step, setting tp->tty_inleft to the number of characters requested, is important. This variable is used to determine when the read request is satisfied. In canonical mode tp->tty_inleft is decremented by one for each character returned, until an end of line is received, at which point it is suddenly reduced to zero. In noncanonical mode it is handled differently, but in any case it is reset to zero whenever the call is satisfied, whether by a timeout or by receiving at least the minimum number of bytes requested. When tp->tty_inleft reaches zero, a reply message is sent. As we will see, reply messages can be generated in several places. It is sometimes necessary to check whether a reading process still expects a reply; a nonzero value of tp->tty_inleft serves as a flag for that purpose. [Page 337] In canonical mode a terminal device waits for input until either the number of characters asked for in the call has been received, or the end of a line or the end of the file is reached. The ICANON bit in the termios structure is tested on line 13981 to see if canonical mode is in effect for the terminal. If it is not set, the termios MIN and TIME values are checked to determine what action to take. In Fig. 3-31 we saw how MIN and TIME interact to provide different ways a read call can behave. TIME is tested on line 13983. A value of zero corresponds to the left-hand column in Fig. 3-31, and in this case no further tests are needed at this point. If TIME is nonzero, then MIN is tested. If it is zero, settimer is called to start the timer that will terminate the DEV_READ request after a delay, even if no bytes have been received. Tp-
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