Question

Hello I am writing some data structures in C, and I’ve realized that their associated functions aren’t thread safe. The i am writing code uses only standard C, and I want to achieve some sort of ‘synchronization’.

I was thinking to do something like this:

enum sync_e { TRUE, FALSE };
typedef enum sync_e sync;


struct list_s {
//Other stuff
    struct list_node_s *head;
    struct list_node_s *tail;
    enum sync_e locked;
};
typedef struct list_s list;

, to include a “boolean” field in the list structure that indicates the structures state: locked, unlocked.

For example an insertion function will be rewritten this way:

int list_insert_next(list* l, list_node *e, int x){
    while(l->locked == TRUE){
        /* Wait */
    }
    l->locked = TRUE;
    /* Insert element */
    /* -------------- */
    l->locked = FALSE;
    return (0);
}

While operating on the list the ‘locked’ field will be set to TRUE, not allowing any other alterations. After operation completes the ‘locked’ field will be again set to ‘TRUE’.

Is this approach good ? Do you know other approaches (using only standard C).

Answer 1

There is no guarantee that

l->locked = TRUE;

will happen instantly within a single atomic operation. It might be compiled into several CPU instructions (that depends on CPU architecture, compiler, operating system etc.)

You should use synchronization mechanisms that are provided by your environment. Those are beyond the scope of the C standard.