Symbian OS is an operating system with associated libraries, user interface frameworks and reference implementations of common tools, produced by Symbian Ltd.. It is a descendant of Psion's EPOC.
Symbian is currently owned by BenQ, Ericsson, Panasonic, Nokia, Siemens AG and Sony Ericsson.
There are a number of smartphone user interface platforms based on Symbian OS, including open platforms UIQ, Nokia's Series 60, Series 80 and Series 90 and closed platforms such as that developed for NTT DoCoMo's FOMA handsets. This adaptability allows Symbian OS to be used on smartphones with a variety of form factors (e.g. clam-shell or "monoblock"/"candybar", keypad- or pen-driven).
Symbian OS, with its roots in Psion Software's EPOC (which itself had similarities to the internals of VMS, a grown-up POSIX compatible operating system for mini-computers in the 1980s) is structured like many desktop operating systems, with pre-emptive multitasking, multithreading, and memory protection.
Symbian OS's major advantage is the fact that it was built for handheld devices, with limited resources, that may be running for months or years. There is a strong emphasis on conserving memory, using Symbian-specific programming idioms such as descriptors and a cleanup stack. Together with other techniques, these keep memory usage low and memory leaks rare. There are similar techniques for conserving disk space (though the disks on Symbian devices are usually flash memory). Furthermore, all Symbian OS programming is event-based, and the CPU is switched off when applications are not directly dealing with an event. This is achieved through a programming idiom called active objects. Correct use of these techniques helps ensure longer battery life.
All of this makes Symbian OS's flavour of C++ very specialised, with a steep learning curve. However, many Symbian OS devices can also be programmed in OPL, Python, Visual Basic, Simkin, and Perl - together with the J2ME and Personal Java flavours of Java.
Symbian OS competes with other mobile operating systems, such as Windows Mobile, Palm OS, and Linux. It also competes with the embedded operating systems used on lower-end phones, such as NOS and OSE, which tend to be maintained by the phone companies themselves. Symbian OS' major advantage over these embedded operating systems is its modularity - there is runtime linking between dynamically linked shared libraries (DLLs, see dynamic linking) on the device, and an emphasis on plug-in architectures. This makes complex phones quicker to develop, though this is sometimes offset by the complexity of Symbian OS C++ and the awkwardness of going to another company for an OS (instead of doing it in-house).
The advantages over other 'open' OS competitors (such as Linux and Windows Mobile, the last one is not Open Source) are more debatable. Phone vendors and network operators like the customisability of Symbian OS relative to Windows. This customisability, though, makes integrating a Symbian OS phone more difficult. It's possible that Linux goes too far in the other direction, and is simply too hard to make a phone from at the moment. Symbian OS's ground-up design for mobile devices should make it more power- and memory-efficient, as well as being flexible.
Security and Malware
Symbian OS has been subject to a variety of viruses, the best known of which is Cabir. Usually these send themselves from phone to phone by Bluetooth. So far, none have taken advantage of any flaws in Symbian OS - instead, they have all asked the user whether they would like to install the software, with somewhat prominent warnings that it can't be trusted.
However, of course, the average mobile phone user shouldn't have to worry about such things, so Symbian OS 9 is adopting a capability model. Installed software will theoretically be unable to do damaging things (such as costing the user money by sending network data) without being digitally signed - thus making it traceable. Developers can apply to have their software signed via the Symbian Signed program.
A common question is whether Symbian OS is "open". It is not open in the sense of Open Source software - the source code is not publicly available. However, nearly all the source code is provided to Symbian OS phone manufacturers and many other partners. Moreover, the APIs are publicly documented and anyone can develop software for Symbian OS. This contrasts with traditional embedded phone operating systems, which typically cannot accept any aftermarket software except Java applications.
Symbian is also open in terms of the Open Standards it supports.