Getting Started with QNX Neutrino - QNX Software Systems

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Examples © 2009, QNX Software Systems GmbH & Co. KG. Maintenance of context information Updating POSIX information The code Both the “Data area size considerations” and the “Handling of EOF case” scenarios will require that context be maintained in the OCB passed to your io_read() function, specifically the offset member. One final consideration: when data is read from a resource manager, the POSIX access time (atime) variable needs to be updated. This is so that a client stat() function will show that someone has indeed accessed the device. Here’s the code that addresses all the above points. We’ll go through it step-by-step in the discussion that follows: /* * io_read1.c */ #include #include #include #include // our data string char *data_string = "Hello, world!\n"; int io_read (resmgr_context_t *ctp, io_read_t *msg, iofunc_ocb_t *ocb) { int sts; int nbytes; int nleft; int off; int xtype; struct _xtype_offset *xoffset; // 1) verify that the device is opened for read if ((sts = iofunc_read_verify (ctp, msg, ocb, NULL)) != EOK) { return (sts); } // 2) check for and handle an XTYPE override xtype = msg -> i.xtype & _IO_XTYPE_MASK; if (xtype == _IO_XTYPE_OFFSET) { xoffset = (struct _xtype_offset *) (&msg -> i + 1); off = xoffset -> offset; } else if (xtype == _IO_XTYPE_NONE) { off = ocb -> offset; } else { // unknown, fail it return (ENOSYS); } // 3) how many bytes are left? nleft = ocb -> attr -> nbytes - off; // 4) how many bytes can we return to the client? nbytes = min (nleft, msg -> i.nbytes); 256 Chapter 5 • Resource Managers April 30, 2009
© 2009, QNX Software Systems GmbH & Co. KG. Examples // 5) if returning data, write it to client if (nbytes) { MsgReply (ctp -> rcvid, nbytes, data_string + off, nbytes); // 6) set up POSIX stat() "atime" data ocb -> attr -> flags |= IOFUNC_ATTR_ATIME | IOFUNC_ATTR_DIRTY_TIME; // 7) advance the lseek() index by the number of bytes // read if not _IO_XTYPE_OFFSET if (xtype == _IO_XTYPE_NONE) { ocb -> offset += nbytes; } } else { // 8) not returning data, just unblock client MsgReply (ctp -> rcvid, EOK, NULL, 0); } } // 9) indicate we already did the MsgReply to the library return (_RESMGR_NOREPLY); Step 1 Here we ensured that the client’s open() call had in fact specified that the device was to be opened for reading. If the client opened the device for writing only, and then attempted to perform a read from it, it would be considered an error. In that case, the helper function iofunc_read_verify() would return EBADF, and not EOK, so we’d return that value to the library, which would then pass it along to the client. Step 2 Here we checked to see if the client had specified an xtype-override — a per-message override (e.g., because while the device had been opened in non-blocking mode, this specifies for this one request that we’d like blocking behavior). Note that the blocking aspect of the “xtype” override can be noted by the iofunc_read_verify() function’s last parameter — since we’re illustrating a very simple example, we just passed in a NULL indicating that we don’t care about this aspect. More important, however, is to see how particular “xtype” modifiers are handled. An interesting one is the _IO_XTYPE_OFFSET modifier, which, if present, indicates that the message passed from the client contains an offset and that the read operation should not modify the “current file position” of the file descriptor (this is used by the function pread(), for example). If the _IO_XTYPE_OFFSET modifier is not present, then the read operation can go ahead and modify the “current file position.” We use the variable xtype to store the “xtype” that we received in the message, and the variable off to represent the current offset that we should be using during processing. You’ll see some additional handling of the _IO_XTYPE_OFFSET modifier below, in step 7. If there is a different “xtype override” than _IO_XTYPE_OFFSET (and not the no-op one of _IO_XTYPE_NONE), we fail the request with ENOSYS. This simply means that we don’t know how to handle it, and we therefore return the error up to the client. April 30, 2009 Chapter 5 • Resource Managers 257
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QNX Neutrino RTOS Getting Started w
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Contents About This Guide xi What y
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© 2009, QNX Software Systems GmbH
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List of Figures A process as a cont
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About This Guide April 30, 2009 Abo
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Typographical conventions © 2009,
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Foreword to the First Edition by Pe
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Pulses © 2009, QNX Software System
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Message passing over a network © 2
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Chapter 3 Clocks, Timers, and Getti
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Using timers © 2009, QNX Software
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Advanced topics © 2009, QNX Softwa
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Chapter 4 Interrupts In this chapte
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Neutrino and interrupts © 2009, QN
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Writing interrupt handlers © 2009,
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TIME TIME Writing interrupt handler
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Summary © 2009, QNX Software Syste
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Seeking professional help © 2009,
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Seeking professional help © 2009,
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Appendix C Sample Programs In this
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atoz.c © 2009, QNX Software System
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atoz.c © 2009, QNX Software System
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time1.c © 2009, QNX Software Syste
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tp1.c © 2009, QNX Software Systems
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tt1.c © 2009, QNX Software Systems
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Getting Started with QNX Neutrino - QNX Software Systems

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