Getting Started with QNX Neutrino - QNX Software Systems

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Advanced topics © 2009, QNX Software Systems GmbH & Co. KG. structure, if only for this one purpose.) Finally, you’ll need to define your own OCB typedef, so that the prototypes for the code are all correct. Let’s look at the OCB typedef first, and then we’ll see how to override the functions: #define IOFUNC_OCB_T struct my_ocb #include This tells the included file, , that the manifest constant IOFUNC_OCB_T now points to your new and improved OCB structure. It’s very important to keep in mind that the “normal” OCB must appear as the first entry in your extended OCB! This is because the POSIX helper library passes around a pointer to what it expects is a normal OCB — it doesn’t know about your extended OCB, so therefore the first data element at the pointer location must be the normal OCB. Here’s our extended OCB: typedef struct my_ocb { iofunc_ocb_t normal_ocb; int my_extra_flags; ... } my_ocb_t; Finally, here’s the code that illustrates how to override the allocation and deallocation functions in the mount structure: // declare iofunc_mount_t iofunc_funcs_t mount; mount_funcs; // set up the mount functions structure // with our allocate/deallocate functions // _IOFUNC_NFUNCS is from the .h file mount_funcs.nfuncs = _IOFUNC_NFUNCS; // your new OCB allocator mount_funcs.ocb_calloc = my_ocb_calloc; // your new OCB deallocator mount_funcs.ocb_free = my_ocb_free; // set up the mount structure memset (&mount, 0, sizeof (mount)); Then all you have to do is bind the mount functions to the mount structure, and the mount structure to the attributes structure: ... mount.funcs = &mount_funcs; attr.mount = &mount; The my_ocb_calloc() and my_ocb_free() functions are responsible for allocating and initializing an extended OCB and for freeing the OCB, respectively. They are prototyped as: 270 Chapter 5 • Resource Managers April 30, 2009
© 2009, QNX Software Systems GmbH & Co. KG. Advanced topics IOFUNC_OCB_T * my_ocb_calloc (resmgr_context_t *ctp, IOFUNC_ATTR_T *attr); void my_ocb_free (IOFUNC_OCB_T *ocb); This means that the my_ocb_calloc() function gets passed both the internal resource manager context and the attributes structure. The function is responsible for returning an initialized OCB. The my_ocb_free() function gets passed the OCB and is responsible for releasing the storage for it. OCB monitor More efficient allocation It’s important to realize that the OCB may be allocated by functions other than the normal io_open() handler — for example, the memory manager may allocate an OCB. The impact of this is that your OCB allocating function must be able to initialize the OCB with the attr argument. There are two interesting uses for these two functions (that have nothing to do with extending the OCB): • OCB allocation/deallocation monitor • more efficient allocation/deallocation In this case, you can simply “tie in” to the allocator/deallocator and monitor the usage of the OCBs (for example, you may wish to limit the total number of OCBs outstanding at any given time). This may prove to be a good idea if you’re not taking over the io_open() outcall, and yet still need to intercept the creation of (and possibly deletion of) OCBs. Another use for overriding the library’s built-in OCB allocator/deallocator is that you may wish to keep the OCBs on a free list, instead of the library’s calloc() and free() functions. If you’re allocating and deallocating OCBs at a high rate, this may prove to be more efficient. Extending the attributes structure You may wish to extend the attributes structure in cases where you need to store additional device information. Since the attributes structure is associated on a “per-device” basis, this means that any extra information you store there will be accessible to all OCBs that reference that device (since the OCB contains a pointer to the attributes structure). Often things like serial baud rate, etc. are stored in extended attributes structures. Extending the attributes structure is much simpler than dealing with extended OCBs, simply because attributes structures are allocated and deallocated by your code anyway. April 30, 2009 Chapter 5 • Resource Managers 271
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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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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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Chapter 4 Interrupts In this chapte
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Writing interrupt handlers © 2009,
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