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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?

#51

Post by pjc50 »

monocasa wrote: post #40
tedunangst wrote: post #38

Earlier quoted context omitted.

The article is not advocating initializing the stack pointer with a C++ constructor.

> * Memory segments are initialized. Memory segments such as .bss (for uninitialized data), .data (for initialized data such as static variables, global variables, local static variables, addresses of functions, and function pointers), and .text (where the actual code resides) are initialized and a valid stack is set up. > * Command line arguments are received. This may not be relevant in embedded systems as in embed…

You can't really initialize the stack pointer in a constructor, or indeed in C++ at all; there's no syntax for it and the compiler may use the stack to allocate local variables in the function prologue.

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

#52

Post by monocasa »

pjc50 wrote: post #51
monocasa wrote: post #40

Earlier quoted context omitted.

> * Memory segments are initialized. Memory segments such as .bss (for uninitialized data), .data (for initialized data such as static variables, global variables, local static variables, addresses of functions, and function pointers), and .text (where the actual code resides) are initialized and a valid stack is set up. > * Command line arguments are received. This may not be relevant in embedded systems as in embed…

You can't really initialize the stack pointer in a constructor, or indeed in C++ at all; there's no syntax for it and the compiler may use the stack to allocate local variables in the function prologue.

You can if you're super duper careful and throw in a little inline asm, depending on the architecture. ARM for instance is very likely not to need to spill to the stack on tiny little leaf functions.

It's an absolutely terrible idea, but I've seen engineers be so afraid of asm files that they'd try something like this.

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

#53

Post by pjmlp »

pcwalton wrote: post #30
pdpi wrote: post #22

Earlier quoted context omitted.

> 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++. :(

Actually, if you mean package initialization blocks, they were already present in Mesa derived languages.

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

#54

Post by Myrmornis »

> Command line arguments are received.

Nit: that’s not the best wording. if the executable is being executed by something like `exec` then these are just “arguments”; the command line (i.e. a shell) isn’t involved. Do embedded systems support something like `exec`?

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

#55

Post by Bromskloss »

monocasa wrote: post #34
makomk wrote: post #31

Earlier quoted context omitted.

The generic compiler-provided startup code is fine for AVRs under most circumstances, since they're only produced by a single manufacturer and don't have fancy features like PLLs which need to be initialized. It's the more complicated chips where it gets a little hairy, especially if you have to deal with things like external RAM.

I guess I rarely see the case for an AVR when you're not prototyping on an Arduino. You can get chips that are an order of magnitude more powerful per $/watt/and any other metric you'd want to use.

Have you read "The Amazing $1 Microcontroller" [0], which might have been posted here before? It's a survey of available microcontrollers, with the intent of helping you choose one. If you have read it, are you in agreement with the article, for example on how AVRs compare to others?

[0] https://jaycarlson.net/microcontrollers/

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

#56

Post by monocasa »

Bromskloss wrote: post #55
monocasa wrote: post #34

Earlier quoted context omitted.

I guess I rarely see the case for an AVR when you're not prototyping on an Arduino. You can get chips that are an order of magnitude more powerful per $/watt/and any other metric you'd want to use.

Have you read "The Amazing $1 Microcontroller" [0], which might have been posted here before? It's a survey of available microcontrollers, with the intent of helping you choose one. If you have read it, are you in agreement with the article, for example on how AVRs compare to others? [0] https://jaycarlson.net/microcontrollers/

Yeah, he put a lot of work into that, and I love that he published all of the information.

My biggest complaint though is that he expresses GPIO cycle time and interrupt latency in cycles, rather than normalized on real time.

Particularly when comparing a STM32F0 to an AVR, the perf looks almost even, until you realize that the STM32F0 is clocked 2.5x faster.

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

#57

Post by doomjunky »

To me one curious thing about the main() function without arguments is, these arguments are still pushed onto the stack. You can find them by dereferencing a pointer of another stack variable and playing around with the offset.

    // Just tested with mingw-gcc on Win8.1
    // Maybe you need to play around with the offset, +28
    int main() {
        int i; // Another stack variable

        printf("%d\n", (long long)&i + 28); // &argv
        printf("%d\n", *((void**)((long long)&i + 28))); // argv
        printf("%s\n", ((char**)*((void**)((long long)&i + 28)))[0]); // argv[0] 

        return 0;
    }
Exclaimer: I don't know how this behaves in an embedded environment.

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

#58

Post by kevin_thibedeau »

monocasa wrote: post #34
makomk wrote: post #31

Earlier quoted context omitted.

The generic compiler-provided startup code is fine for AVRs under most circumstances, since they're only produced by a single manufacturer and don't have fancy features like PLLs which need to be initialized. It's the more complicated chips where it gets a little hairy, especially if you have to deal with things like external RAM.

I guess I rarely see the case for an AVR when you're not prototyping on an Arduino. You can get chips that are an order of magnitude more powerful per $/watt/and any other metric you'd want to use.

Low latency IO and computational performance isn't always needed.

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

#60

Post by ronsor »

doomjunky wrote: post #57

To me one curious thing about the main() function without arguments is, these arguments are still pushed onto the stack. You can find them by dereferencing a pointer of another stack variable and playing around with the offset. // Just tested with mingw-gcc on Win8.1 // Maybe you need to play around with the offset, +28 int main() { int i; // Another stack variable printf("%d\n", (long long)&i + 28); // &argv printf(…

totally unportable and undefined behavior
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