User level thread - c

Question

I am trying to create user level thread. Here is a sample of my code. Can any body help me what is the problem in this program.
#include<stdio.h>
#include<ucontext.h>
int thread_counter = 0;
int thread1, thread2;
int who_am_i;
struct TCB {
ucontext_t context;
void (* fun_ptr)();
};
struct TCB tcb[3];
char stack[2][8192];
//----------------------
int thread_create(void (*fun)()) {
static volatile int s;
thread_counter++;
s = 0;
getcontext(&tcb[thread_counter].context);
if(!s) {
tcb[thread_counter].context.uc_stack.ss_sp = stack[thread_counter];
tcb[thread_counter].context.uc_stack.ss_size = sizeof(stack[thread_counter]);
tcb[thread_counter].context.uc_link = &tcb[0].context;
tcb[thread_counter].fun_ptr = fun;
s = 1;
}
else {
tcb[who_am_i].fun_ptr();
}
return thread_counter;
}
void thread_yield(int next_thread) {
static volatile int switched;
switched = 0;
getcontext(&tcb[who_am_i].context);
if(!switched) {
switched = 1;
who_am_i = next_thread;
setcontext(&tcb[next_thread].context);
}
}
//----------------------
void f1() {
printf("start f1\n");
thread_yield(thread2);
printf("finish f1:\n");
}
void f2() {
printf("start f2\n");
thread_yield(thread1);
printf("finish f2\n");
}
//----------------------
int main() {
thread1 = thread_create(f1);
thread2 = thread_create(f2);
who_am_i = 0;
thread_yield(thread1);
return 0;
}
Thread is not switching properly. When I run it, it gives following output:
start f1
start f2
finish f2
Thank you

Answer 1

You have an undefined behavior situation.
In thread_create you increase thread_counter the first thing you do. So when you create the second thread thread_counter will be 2. Then you access stack[2] which will give you undefined behavior.
You also hardcode the uc_link member of the context to &tcb[0].context, which is never initialized due to your "premature" increment of thread_counter.
But the main problem is that you don't actually create a new context, you just get the context for the current thread. You should use makecontext for each thread instead.