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authorDaniel Silverstone <dsilvers@digital-scurf.org>2017-02-04 09:41:13 +0000
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+[[!meta title="Caveat RISC OS"]]
+[[!meta author="James Bursa"]]
+[[!meta date="2010-03-01T02:57:12Z"]]
+
+
+[[!toc]] When working on software
+that must run under RISC OS, there are many issues that you must keep in
+mind that might not be obvious or apparent to people who have never
+written software for RISC OS before. This page describes some of them.
+
+Some of the NetSurf Project's sponsors have donated us the equipment and
+bandwidth required to give accepted students access to an open-access
+RISC OS computer over the internet, so they can test and experiment with
+it. This is done via an IP KVM using a rather ghastly, but usable, Java
+applet in a web page. The machine is an A9 Home donated by Advantage Six
+Ltd.
+
+RISC OS is co-operatively multi-tasked
+--------------------------------------
+
+RISC OS GUI applications must 'yield' to the OS before another process
+can be scheduled. This is done via the Wimp\_Poll system call. In
+NetSurf, this is wrapped inside gui\_multitask(), which handles yielding
+to the OS, getting GUI events from it, and running scheduled functions.
+
+RISC OS has a limited number of shared-namespace file descriptors
+-----------------------------------------------------------------
+
+RISC OS has 255 file handles - and that's it. You can't possibly have
+more files than that open at anyone time, and there is a single pool of
+file handles shared by all processes. Don't leave files open that you
+don't need to keep open, and don't blindly close other handles.
+
+RISC OS has a limited number of shared-namespace socket descriptors
+-------------------------------------------------------------------
+
+Unlike on other operating systems, RISC OS's sockets are allocated from
+a different pool of numbers. They may, for example, overlap with file
+descriptors. Also note that as with file descriptors, there is a single
+pool of them shared by all processes. There's also a limit on the number
+you can have open at once. This can be as low as 64.
+
+RISC OS's environment variables are global to the system
+--------------------------------------------------------
+
+RISC OS uses environment variables extensively: much of the
+configuration of the system is done via them. They are shared by all
+processes. Because of this, RISC OS has a naming convention for them so
+they don't clash. If you really feel the need to use an environment
+variable, you should prefix its name with NetSurf\$.
+
+Memory is valuable
+------------------
+
+RISC OS machines may have as little as 64MB of RAM, and the OS provides
+no support for swap or paging. Additionally, much of the memory
+allocated via malloc() will end up being allocated in what is called a
+Dynamic Area. The upshot of this is that if you fragment the heap too
+much, you've basically wasted memory, and no process will be able to use
+the spaces you have freed.
+
+RISC OS hardware is slow
+------------------------
+
+The fastest RISC OS computer you can buy is a 600MHz ARM clone with no
+floating point and a tiny cache. Make sure your code is tight, but
+remember readability and maintenability is paramount!
+
+RISC OS is delicate and lacks wide-spread memory protection
+-----------------------------------------------------------
+
+It's trivial to take down the entire OS if you're not careful, where on
+other OSes you'd just get a SIGSEGV. Be careful!
+
+RISC OS lacks shared libraries
+------------------------------
+
+While the GTK version of NetSurf's binary is less than 600kB, RISC OS
+has no shared libraries, meaning it's over 3MB. Given the speed of RISC
+OS machines, this is already a problem. If you want to make use of a new
+library, make sure it's not too huge, and that it doesn't depend on a
+dozen other libraries.
+
+[[!inline raw=yes pages="Documentation"]]
+