C vs C++ in embedded Linux

Question

I am developing an application for embedded Linux (ARM). It will execute 500 times/sec, therefore speed is important. I would prefer to use C++ but I am afraid it will be slower than C even if I avoid fancy features like virtual functions. Is there a reason to use C or it's just as fine to write in C++?

Answer 1

In C++ you have things like template metaprogramming that resolve in compile time several situations where C or any other procedural programming language would have to do in runtime.

I should say more. Template metaprogramming and some class inheritance tricks are really amazing. It can save you a lot of processing time that you'd spend otherwise by "ifing" and "switching".

This means that C++ can be actually faster than C if well conducted.

Obviously you can program "in C" using C++ and you'd have no penalty at all. If you're not too fond of C++ I'd advise you to do a "C on C++" or "C with C++ extensions" just to take vantage of C++ improvements, but the real advantage you'll have is by programming the C++ way. There you will see that C++ is, good part of times, or faster or cleaner than C or, at least as fast as.

Have no fear. Face C++. After the stdc++ (against libc) there will be almost no overhead in code size. If your application is from median to high in size, it will be diluted.

I use C++ from simple 8-bit ATmega to Marvell's ARM9, passing through AVR32 UC3 and Cortex-M3 and always find it profitable.

If you need specific advise in a given situation, feel free to ask.

Answer 2

I am using ARM9 board for hardware control and I am using both C and C++ Application in 500 Mhz board. It is up to you to use the language and how you implement your logic to implement the functionality. Because I have not found any problem running my Application over the day controlling the hardware.

While writting your program, carefully select your variable, check it against extra instruction/loop, initialization. also use Gcc optimization flag while compilation.

I have not any problem running my Qt Application and C program on 500 Mhz ARM 9 board.

Answer 3

The real key to size and speed efficient code, embedded or otherwise, is for the programmer to fully understand the implications of his orher decisions.

To an extent, C++ provides more opportunities where something expensive can look deceptively innocent. Equivalent functionality to C++ features often requires more ink on the page, which may lead to a little more contemplation of its potential expense. But it's by no means an absolute - C (and its libraries) has the risk of deceptively innocent expense too.

Ultimately there is no substitute for understanding what you have asked for in each line of code.

Answer 4

C++ in general suffers no run time penalty over C - (except for a few things like RTTI).

Except in a few odd circumstances the compiler should be able to determine which virtual function to call at compile time and so add no overhead.

Edit: Ok with such a variety of compilers, CPUs, runtime libs, OSes there are some features of C++ that might create slower code, there are some features that might create faster code.

But can we all agree that C++ isn't automatically excluded from embedded use anymore ?

Answer 5

You can use C++ but be extra careful.

For size, keep a close eye on your linker map file. You can find it including tons of stuff you don't need, just from an innocent-looking declaration.

For speed, profile or random-pause often. It's super easy to do more news and deletes than you really need, especially with container classes, and be really careful with things like iterators. Often they're doing you un-asked-for favors.

You can step through the code at the assembly-language level to make sure it's only doing what you actually need, which should be about the same as the C code.

Answer 6

The main reason for choosing C over C++ is size of the compiled binary, which can be a real restriction for embedded systems.

On performance, there's no measurable difference, if you use the language right. You can write slow C code just as easily as slow C++, as long as you're aware of the under-the-hood mechanisms of what you're writing you should be fine with either.

Answer 7

C++ is fine as long as you have enough RAM and flash in your embedded system. The C++ runtime library (libstdc++) is big, and comes in addition to the C standard library (libc) even if you use C++ only.

Answer 8

As long as you limit the features you use, you won't have much, if any, performance hit in C++ over C. The features you'll want to avoid include: exceptions, RTTI, and keep your class hierarchy as flat as possible (and use virtual functions sparingly).