How to write a "thread safe" function in C?

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

Standard C doesn't "know" anything about threads. Anything threading related as thread themselves, synchronization primitives, atomic operations are not part of the language or the standard library. They are always part of system libraries as POSIX or the Windows API.

Therefore it is not possible to protect your data structure against a race condition when it is used by multiple threads using only standard C.

If an instance of your structure is only used by a single thread, it can be used in a multi-thread scenario as you e.g. don't use static variables inside your functions.

Answer 2

Your code could cause a race condition. You should use a mutex for this.