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What happens before main() is executed in C and why is it important?

mymicrocontroller.com

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Re: What happens before main() is executed in C and why is it important?

#21

Post by monocasa »

nebulous1 wrote: post #17
jwr wrote: post #13

Earlier quoted context omitted.

On microcontrollers, you often don't have a standard library, or if you do, it is limited, and doesn't do things before main. The article describes pretty well what happens on a microcontroller. I would add that in some (most?) environments, the code that gets executed before main() is easily accessible: it sits in files within the project (written in assembly most of the time).

Cool, he is talking about the loader though, which would imply that this is all language independent? Except for where he mentions things getting executed before main in C++, which I'm assuming would be the doings of the C++ standard library.

It's still pretty language independent. Qemu for instance is a c codebase that depends on static initialization functions that run before main to register the different components.

Re: What happens before main() is executed in C and why is it important?

#22

Post by pdpi »

popmatrix wrote: post #2

It should be noted that relying on static initialization as suggested in the article, while at times useful, can open a new set of problems until initialization order is well-defined (it's out of scope for the article, so I imagine the author is aware and just opting for brevity). Some compilers provide an explicit ordering mechanism (clang and GCC) and the there are proposals in the C++ standards at the moment, but…

> relying on static initialization is something one should think about potentially solving differently

This is such a notorious problem, it even has a name: Static Initialisation Order Fiasco (see: https://isocpp.org/wiki/faq/ctors#static-init-order)

The symmetrical Static Destruction Order Fiasco is also a very fun problem to deal with. The solution proposed for SIOF in the C++ FAQ does nothing to help you deal with this one, though.

Re: What happens before main() is executed in C and why is it important?

#23

Post by monocasa »

RealityVoid wrote: post #9
barbegal wrote: post #7

In embedded applications a lot of things happen before you can even attach a debugger. Most microcontrollers go through a non programmable hardware initialisation sequence which initialises enough hardware for the processor to run. The processor then runs from a section of read only (or at least write protected) memory. This code performs a further stage of initialisation and patching. This can be used by the manufac…

Is this even public for us as developers, can we influence it and it influence us? If not, should we even think about it? I had a lot of curiosity about how JTAG works and how I can build my own debugger for general targets, but is seems documentation for this is thin. I should look over openocd to see how those guys are doing stuff.

It depends heavily on the SoC in question. On an unlocked STM32F4, you can attach JTAG pretty much immediately even if it's way off in the weeds executing garbage, but BCM283x (RPi) has to setup the pinmux for the JTAG first from the ARM core, so you have to already be running some of your own code.

Re: What happens before main() is executed in C and why is it important?

#24

Post by monocasa »

leggomylibro wrote: post #8

One other way to observe these steps a bit more clearly is to look at the startup code provided by a chip's manufacturer or generated by an IDE that targets the chip, such as Keil. Most will put this type of code in '.S' startup assembly files, which often also contain information like the memory addresses for hardware interrupts to use, and linker scripts for telling the compiler about the memory available on the ch…

It's not just "good practice", but instead the startup code is so specific to the platform that there really isn't "generic" startup code.

Re: What happens before main() is executed in C and why is it important?

#27

Post by monocasa »

Osiris wrote: post #26
classics2 wrote: post #25

Strange they looked into the topic and came to the completely incorrect conclusion.

Can you elaborate for those of us unfamiliar with topic?

Doing some of that kind of super low level code in a C++ static initialization constructor is almost certainly a poor choice. Like setting up memory and the stack pointer. Doing hardware init there can make a lot of sense, but you have to be careful from an architectural perspective.

Re: What happens before main() is executed in C and why is it important?

#28

Post by jldugger »

nebulous1 wrote: post #11

This seems to be concentrating on what the system loader is doing without mentioning whatever the statically linked standard library gets up to before calling the main function?

In any other context, yes, you're missing C library predecessors to main(). But it's specifically referencing embedded systems, which is why hardware initialization and memory segments are mentioned.

Re: What happens before main() is executed in C and why is it important?

#29

Post by 2RTZZSro »

RealityVoid wrote: post #4
popmatrix wrote: post #2

It should be noted that relying on static initialization as suggested in the article, while at times useful, can open a new set of problems until initialization order is well-defined (it's out of scope for the article, so I imagine the author is aware and just opting for brevity). Some compilers provide an explicit ordering mechanism (clang and GCC) and the there are proposals in the C++ standards at the moment, but…

In embedded you can usually modify every step from boot to main. They are just pieces of code you can interact with and make them do whatever you want and initialize stuff in whatever order you want, providing you know what you do. Deviating with a war story now, I once modified some startup code so it self-detected its boot location and initialized the ram data according to this location shift. It was fun, but debug…

What was the project for where you wrote code that self-detected its boot location?

Re: What happens before main() is executed in C and why is it important?

#30

Post by pcwalton »

pdpi wrote: post #22
popmatrix wrote: post #2

It should be noted that relying on static initialization as suggested in the article, while at times useful, can open a new set of problems until initialization order is well-defined (it's out of scope for the article, so I imagine the author is aware and just opting for brevity). Some compilers provide an explicit ordering mechanism (clang and GCC) and the there are proposals in the C++ standards at the moment, but…

> relying on static initialization is something one should think about potentially solving differently This is such a notorious problem, it even has a name: Static Initialisation Order Fiasco (see: https://isocpp.org/wiki/faq/ctors#static-init-order ) The symmetrical Static Destruction Order Fiasco is also a very fun problem to deal with. The solution proposed for SIOF in the C++ FAQ does nothing to help you deal wit…

Perhaps worse, lots of new languages, for example Go, copied this mistake from C++. :(
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