swapcontext and free memory

swapcontext and free memory

[xcomp]

Hi all,
I have written a small test case which simply creates a new context, swaps to it and returns to the main function. I am allocating memory for the ucontext_t structures and the stack on which the context is executed using malloc. Because of this I also want to deallocate the memory again using free. The problem is I can't deallocate the memory after swapping back. It runs only if all three free() calls are commented out. Otherwise a segmentation fault occurs. Does swapcontext handle this on its own? Can anyone explain this behavior? I didn't find anything concerning this problem in the web.
I am running this program on Ubuntu 8.04 with the following system information: Linux ubuntu8041 2.6.24-23-generic #1 SMP i686 GNU/Linux

The test program is the following one:
---
#include <stdlib.h>
#include <stdio.h>

#include <ucontext.h>

#define STACKSIZE 4096

void thread_function() {
	printf("\nthread_function was called...");
}

int main(int argc, char** argv) {
	printf("\nStart");

	ucontext_t *main_context, *thread_context;

	main_context = (ucontext*) malloc(sizeof(ucontext));
	thread_context = (ucontext*) malloc(sizeof(ucontext));

	void* thread_stack = malloc(STACKSIZE);

	printf("\n\nAllocated memory:\n\tmain_context:\t%p\n\tthread_context:\t%p\n\tthread_stack:\t%p", main_context, thread_context, thread_stack);

	getcontext(thread_context);
	thread_context->uc_stack.ss_sp = thread_stack;
	thread_context->uc_stack.ss_size = sizeof(thread_stack);
	thread_context->uc_stack.ss_flags = 0;
	thread_context->uc_flags = 0;
	thread_context->uc_link = main_context;
	sigemptyset(&(thread_context->uc_sigmask));
	makecontext(thread_context, thread_function, 0);

	swapcontext(main_context, thread_context);

	printf("\n\nswapcontext() returned. Freeing memory starts now...");

	free(thread_stack);
	printf("\nthread_stack was deallocated...");

	free(thread_context);
	printf("\nthread_context was deallocated...");

	free(main_context);
	printf("\nmain_context was deallocated...");

	printf("\n\nEnd");
}
---

Best,
Matthias :-)

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Re: swapcontext and free memory

[Tiago Maluta]

xcomp@arcor.de wrote:
> Hi all,
> I have written a small test case which simply creates a new context, swaps to it and returns to the main function. I am allocating memory for the ucontext_t structures and the stack on which the context is executed using malloc.
Because of this I also want to deallocate the memory again using free.
The problem is I can't deallocate the memory after swapping back. It
runs only if all three free() calls are commented out. Otherwise a
segmentation fault occurs. Does swapcontext handle this on its own? Can
anyone explain this behavior? I didn't find anything concerning this
problem in the web.
> I am running this program on Ubuntu 8.04 with the following system information: Linux
ubuntu8041 2.6.24-23-generic #1 SMP i686 GNU/Linux

(...)

> 	main_context = (ucontext*) malloc(sizeof(ucontext));
> 	thread_context = (ucontext*) malloc(sizeof(ucontext));

changing:

main_context = (ucontext_t*) malloc(sizeof(ucontext_t));
thread_context = (ucontext_t*) malloc(sizeof(ucontext_t));

and compiling with free() I got:

#./a.out
start

Allocated memory:
        main_context:   0x804b008
        thread_context: 0x804b168
        thread_stack:   0x804b2c8
thread_function was called...

swapcontext() returned. Freeing memory starts now...
thread_stack was deallocated...
thread_context was deallocated...
main_context was deallocated...

I'm using glibc version 2.8.

--tm

Re: swapcontext and free memory

[Jon Mayo]

On Wed, Mar 18, 2009 at 12:22 PM, Tiago Maluta
<maluta_tiago@yahoo.com.br> wrote:
> xcomp@arcor.de wrote:
>> Hi all,
>> I have written a small test case which simply creates a new context, swaps to it and returns to the main function. I am allocating memory for the ucontext_t structures and the stack on which the context is executed using malloc.
> Because of this I also want to deallocate the memory again using free.
> The problem is I can't deallocate the memory after swapping back. It
> runs only if all three free() calls are commented out. Otherwise a
> segmentation fault occurs. Does swapcontext handle this on its own? Can
> anyone explain this behavior? I didn't find anything concerning this
> problem in the web.
>> I am running this program on Ubuntu 8.04 with the following system information: Linux
> ubuntu8041 2.6.24-23-generic #1 SMP i686 GNU/Linux
>
> (...)
>
>>       main_context = (ucontext*) malloc(sizeof(ucontext));
>>       thread_context = (ucontext*) malloc(sizeof(ucontext));
>
> changing:
>
> main_context = (ucontext_t*) malloc(sizeof(ucontext_t));
> thread_context = (ucontext_t*) malloc(sizeof(ucontext_t));
>
> and compiling with free() I got:
>
> #./a.out
> start
>
> Allocated memory:
>        main_context:   0x804b008
>        thread_context: 0x804b168
>        thread_stack:   0x804b2c8
> thread_function was called...
>
> swapcontext() returned. Freeing memory starts now...
> thread_stack was deallocated...
> thread_context was deallocated...
> main_context was deallocated...
>
> I'm using glibc version 2.8.
>
> --tm
>
>
>

I think you should be able to free() them fine. Could you post a short
bit of source that exhibits the problem? Also I tend to mmap() some
anonymous memory instead of using malloc() for things like makecontext
and sigaltstack. Because I want to put guard pages on my stacks due to
them generally being very small.

-- 
Jon Mayo
<jon.mayo@gmail.com>
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