Are there global constructors in the Linux kernel?
Good day I have been doing a little kernel following some repositories of GitHub, videos and wikis, and I have found a topic that leaves me with doubts, the theme is the global constructors. Suppose I write a program in c, for the user space, for my code to run I must have the main function int main() { /*Code*/} The gcc compiler searches for this function to run my program, but before that happens, I can use the global constructors and write a code that runs before the main function, but in the case of writing a kernel, why do I need to run lines of code before the kernel is executed ?, I understand that when writing a kernel there is no main function and that it is disabled to indicate it to the compiler. In the Linux kernel are there global constructors? If so, what things are executed before starting the kernel? att: cristian vargas.
Hi, [sending again after having replied just to the OP by mistake, sorry!] Il 09/10/19 05:44, CRISTIAN ANDRES VARGAS GONZALEZ ha scritto:
The gcc compiler searches for this function to run my program, but before that happens, I can use the global constructors and write a code that runs before the main function, but in the case of writing a kernel, why do I need to run lines of code before the kernel is executed ?, I understand that when writing a kernel there is no main function and that it is disabled to indicate it to the compiler. In the Linux kernel are there global constructors? If so, what things are executed before starting the kernel?
I am not a Linux expert, but I think I know enough on this specific topic to give at least a partial answer: the global constructors feature depends on the initialization code in the C runtime environment, which depends on the operating system. In the case of the kernel, there is not another operating system around, so everything that must happen at initialization time must be done by the kernel itself, including setting up the stack, zeroing uninitialized data and calling whatever you want to be called. There cannot be any automatic way to do these things, because there is nobody except the kernel itself who can do that. This book has a nice and rather detailed write-up on what happens when Linux boots (at least on x86_64): https://0xax.gitbooks.io/linux-insides/ You will see that nothing is called "automatically" like for global constructors: everything is called explicitly and at the right moment. This is also necessary because early kernel initialization routines cannot assume that the system is in a consistent state, since their job is to bring it to be. Therefore you must control precisely what must happen when. Hope this helps! Giovanni. -- Giovanni Mascellani <g.mascellani@gmail.com> Postdoc researcher - Université Libre de Bruxelles -- Giovanni Mascellani <g.mascellani@gmail.com> Postdoc researcher - Université Libre de Bruxelles
Hi, Cristian. There are, a kinda of ... kernel/taskstats.c: | late_initcall(taskstats_init); kernel/rcu/update.c: | early_initcall(check_cpu_stall_init); There is a whole set of macros STAGE_initcall() in include/linux/init.h Arguments to these macros invokations are accounted and called by kernel initialization code at strictly defined stages of initialization. See https://0xax.gitbooks.io/linux-insides/content/Concepts/linux-cpu-3.html The only thing that is run before kernel is <a series of> platform-specific boot loaders, maybe under hypervisor control. Compiler has no power to insert something in between. Regards, Konstantin. CRISTIAN ANDRES VARGAS GONZALEZ, 09 Oct 2019 06:44 MSK:
Good day
I have been doing a little kernel following some repositories of GitHub, videos and wikis, and I have found a topic that leaves me with doubts, the theme is the global constructors.
Suppose I write a program in c, for the user space, for my code to run I must have the main function int main() { /*Code*/}
The gcc compiler searches for this function to run my program, but before that happens, I can use the global constructors and write a code that runs before the main function, but in the case of writing a kernel, why do I need to run lines of code before the kernel is executed ?, I understand that when writing a kernel there is no main function and that it is disabled to indicate it to the compiler. In the Linux kernel are there global constructors? If so, what things are executed before starting the kernel? att: cristian vargas.
participants (3)
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CRISTIAN ANDRES VARGAS GONZALEZ -
Giovanni Mascellani -
Konstantin Andreev