swapcontext and free memory

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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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