Context Switching using swap context() and a custom yield function - c

Question

I am writing my own user level thread library and am running into some issues. Below is the code of the library I have written thus far after the edits provided by you guys and also some introspection from my end:
struct tcb {
int thread_id;
int thread_pri;
ucontext_t thread_context;
struct tcb *next;
} *ready_head;
typedef struct tcb tcb;
tcb *running_head;
tcb *tmp,*tmp1,*tmp2,*temp;
// ucontext_t *ready;
// head = NULL;
// running_head = head;
void t_shutdown()
{
free(temp);
free(tmp);
free(tmp1);
free(ready_head);
free(running_head);
}
void t_yield()
{
// tmp2 = ready_head;
tmp1 = running_head;
/* tmp = running_head;
running_head = ready_head;
while(tmp1->next != NULL)
tmp1 = tmp1->next;
tmp1->next = tmp;
*/
insert(tmp1);
running_head = ready_head;
ready_head = ready_head->next;
printf("yield1\n");
swapcontext(&running_head->thread_context, &tmp1->thread_context);
printf("yield2\n");
// setcontext(&ready_head->thread_context);
}
void insert(tcb *a)
{
tcb *b;
if (ready_head == NULL)
{
ready_head = a;
}
else
{
b = ready_head;
while(b->next)
b = b->next;
b->next = a;
}
printf("insert\n");
}
void t_init()
{
tmp = (tcb *)malloc(sizeof(tcb));
getcontext(&tmp->thread_context); /* let tmp be the context of main() */
tmp->next = NULL;
running_head = tmp;
ready_head = NULL;
}
int t_create(void (*fct)(int), int id, int pri)
{
size_t sz = 0x10000;
temp = (tcb *)malloc(sizeof(tcb));
getcontext(&temp->thread_context);
temp->thread_id = id;
temp->thread_pri = pri;
temp->thread_context.uc_stack.ss_sp = malloc(sz); /* new statement */
temp->thread_context.uc_stack.ss_size = sz;
temp->thread_context.uc_stack.ss_flags = 0;
temp->thread_context.uc_link = &tmp->thread_context;
makecontext(&temp->thread_context, fct, 1, id);
insert(temp);
printf("1\n");
}
I am testing this library on the following test program:
void assign(void)
{
int i;
for (i = 0; i < 3; i++)
printf("in assign(1): %d\n", i);
t_yield();
for (i = 10; i < 13; i++)
printf("in assign(2): %d\n", i);
t_yield();
for (i = 20; i < 23; i++)
printf("in assign(3): %d\n", i);
}
int main(int argc, char **argv)
{
t_init();
t_create(assign, 1, 1);
printf("in main(): 0\n");
t_yield();
printf("in main(): 1\n");
t_yield();
printf("in main(): 2\n");
t_yield();
printf("done...\n");
return (0);
}
The expected output is:
in main(): 0
in assign(1): 0
in assign(1): 1
in assign(1): 2
in main(): 1
in assign(2): 10
in assign(2): 11
in assign(2): 12
in main(): 2
in assign(3): 20
in assign(3): 21
in assign(3): 22
done...
However, I am getting:
in main(): 0
in main(): 1
in main(): 2
done...
I now know that there is an issue with the swapcontext() as that does not seem to be working at all. I have tried to use printf statements to debug my program but am still unable to get this to work! Please help ....

Answer 1

It appears to me that in your t_yield() function you are actually swapping the ready_head context with the ready_head context. You might want to try
swapcontext(&tmp->thread_context, &running_head->thread_context);
instead.