Multithreading support

#include <libcork/threads.h>

libcork provides several functions for handling threads and writing thread-aware code in a portable way.

Thread IDs

unsigned int cork_thread_id

An identifier for a thread in the current process. This is a portable type; it is not based on the “raw” thread ID used by the underlying thread implementation. This type will always be equivalent to unsigned int, on all platforms. Furthermore, CORK_THREAD_NONE will always refer to an instance of this type that we guarantee will not be used by any thread.

cork_thread_id CORK_THREAD_NONE

A cork_thread_id value that will not be used as the ID of any thread. You can use this value to represent “no thread” in any data structures you create. Moreover, we guarantee that CORK_THREAD_NONE will have the value 0, which lets you zero-initialize a data structure containing a cork_thread_id, and have its initial state automatically represent “no thread”.

cork_thread_id cork_current_thread_get_id(void)

Returns the identifier of the currently executing thread. This function works correctly for any thread in the current proces — including the main thread, and threads that weren’t created by cork_thread_new().

Creating threads

The functions in this section let you create and start new threads in the current process. Each libcork thread is named and has a unique thread ID. Each thread also contains a run function, which defines the code that should be executed within the new thread.

Every thread goes through the same lifecycle:

  1. You create a new thread via cork_thread_new(). At this point, the thread is ready to execute, but isn’t automatically started. If you encounter an error before you start the thread, you must use cork_thread_free() to free the thread object.

    When you create the thread, you give it a cork_run_f function, which defines the code that will be executed in the new thread. You also provide a user_data value, which it gives you a place to pass data into and out of the thread.


    Any data passed into and out of the thread via the body instance is not automatically synchronized or thread-safe. You can safely pass in input data before calling cork_thread_new, and retrieve output data after calling cork_thread_join, all without requiring any extra synchronization effort. While the thread is executing, however, you must implement your own synchronization or locking to access the contents of the body from some other thread.

  2. You start the thread via cork_thread_start(). You must ensure that you don’t try to start a thread more than once. Once you’ve started a thread, you no longer have responsibility for freeing it; you must ensure that you don’t call cork_thread_free() on a thread that you’ve started.

  3. Once you’ve started a thread, you wait for it to finish via cork_thread_join(). Any thread can wait for any other thread to finish, although you are responsible for ensuring that your threads don’t deadlock. However, you can only join a particular thread once.

struct cork_thread

A thread within the current process. This type is opaque; you must use the functions defined below to interact with the thread.

struct cork_thread *cork_thread_new(const char *name, void *user_data, cork_free_f free_user_data, cork_run_f run)

Create a new thread with the given name that will execute run. The thread does not start running immediately.

When the thread is started, the run function will be called with user_data as its only parameter. When the thread finishes (or if it is freed via cork_thread_free() before the thread is started), we’ll use the free_user_data function to free the user_data value. You can provide NULL if user_data shouldn’t be freed, or if you want to free it yourself.


If you provide a free_user_data function, it will be called as soon as the thread finished. That means that if you use cork_thread_join() to wait for the thread to finish, the user_data value will no longer be valid when cork_thread_join() returns. You must either copy any necessary data out into more a more persistent memory location before the thread finishes, or you should use a NULL free_user_data function and free the user_data memory yourself once you’re sure the thread has finished.

void cork_thread_free(struct cork_thread *thread)

Free thread. You can only call this function if you haven’t started the thread yet. Once you start a thread, the thread is responsible for freeing itself when it finishes.

struct cork_thread *cork_current_thread_get(void)

Return the cork_thread instance for the current thread. This function returns NULL when called from the main thread (i.e., the one created automatically when the process starts), or from a thread that wasn’t created via cork_thread_new().

const char * cork_thread_get_name(struct cork_thread *thread)
cork_thread_id cork_thread_get_id(struct cork_thread *thread)

Retrieve information about the given thread.

int cork_thread_start(struct cork_thread *thread)

Start thread. After calling this function, you must not try to free thread yourself; the thread will automatically free itself once it has finished executing and has been joined.

int cork_thread_join(struct cork_thread *thread)

Wait for thread to finish executing. If the thread’s body’s run function an error condition, we will catch that error and return it ourselves. The thread is automatically freed once it finishes executing.

You cannot join a thread that has not been started, and once you’ve started a thread, you must join it exactly once. (If you don’t join it, there’s no guarantee that it will be freed.)

Atomic operations

We provide several platform-agnostic macros for implementing common atomic operations.


int cork_int_atomic_add(volatile int *var, int delta)
unsigned int cork_uint_atomic_add(volatile unsigned int *var, unsigned int delta)
size_t cork_size_atomic_add(volatile size_t *var, size_t delta)

Atomically add delta to the variable pointed to by var, returning the result of the addition.

int cork_int_atomic_pre_add(volatile int *var, int delta)
unsigned int cork_uint_atomic_pre_add(volatile unsigned int *var, unsigned int delta)
size_t cork_size_atomic_pre_add(volatile size_t *var, size_t delta)

Atomically add delta to the variable pointed to by var, returning the value from before the addition.


int cork_int_atomic_sub(volatile int *var, int delta)
unsigned int cork_uint_atomic_sub(volatile unsigned int *var, unsigned int delta)
size_t cork_size_atomic_sub(volatile size_t *var, size_t delta)

Atomically subtract delta from the variable pointed to by var, returning the result of the subtraction.

int cork_int_atomic_pre_sub(volatile int *var, int delta)
unsigned int cork_uint_atomic_pre_sub(volatile unsigned int *var, unsigned int delta)
size_t cork_size_atomic_pre_sub(volatile size_t *var, size_t delta)

Atomically subtract delta from the variable pointed to by var, returning the value from before the subtraction.


int cork_int_cas(volatile int_t *var, int old_value, int new_value)
unsigned int cork_uint_cas(volatile uint_t *var, unsigned int old_value, unsigned int new_value)
size_t cork_size_cas(volatile size_t *var, size_t old_value, size_t new_value)
TYPE *cork_ptr_cas(TYPE * volatile *var, TYPE *old_value, TYPE *new_value)

Atomically check whether the variable pointed to by var contains the value old_value, and if so, update it to contain the value new_value. We return the value of var before the compare-and-swap. (If this value is equal to old_value, then the compare-and-swap was successful.)

Executing something once

The functions in this section let you ensure that a particular piece of code is executed exactly once, even if multiple threads attempt the execution at roughly the same time.


Declares a barrier that can be used with the cork_once() macro.

cork_once(barrier, call)
cork_once_recursive(barrier, call)

Ensure that call (which can be an arbitrary statement) is executed exactly once, regardless of how many times control reaches the call to cork_once. If control reaches the cork_once call at roughly the same time in multiple threads, exactly one of them will be allowed to execute the code. The call to cork_once won’t return until call has been executed.

If you have multiple calls to cork_once that use the same barrier, then exactly one call will succeed. If the call statements are different in those cork_once invocations, then it’s undefined which one gets executed.

If the function that contains the cork_once call is recursive, then you should call the _recursive variant of the macro. With the _recursive variant, if the same thread tries to obtain the underlying lock multiple times, the second and later calls will silently succeed. With the regular variant, you’ll get a deadlock in this case.

These macros are usually used to initialize a static variable that will be shared across multiple threads:

static struct my_type  shared_value;

static void
    /* do something to initialize shared_value */


struct my_type *
    cork_once(shared_value_once, expensive_initialization());
    return &shared_value;

Each thread can then call get_shared_value to retrieve a properly initialized instance of struct my_type. Regardless of how many threads call this function, and how often they call it, the value will be initialized exactly once, and will be guaranteed to be initialized before any thread tries to use it.

Thread-local storage

The macro in this section can be used to create thread-local storage in a platform-agnostic manner.

cork_tls(TYPE type, SYMBOL name)

Creates a static function called [name]_get, which will return a pointer to a thread-local instance of type. This is a static function, so it won’t be visible outside of the current compilation unit.

When a particular thread’s instance is created for the first time, it will be filled with 0 bytes. If the actual type needs more complex initialization before it can be used, you can create a wrapper struct that contains a boolean indiciating whether that initialization has happened:

struct wrapper {
    bool  initialized;
    struct real_type  val;

cork_tls(struct wrapper, wrapper);

static struct real_type *
    struct wrapper * wrapper = wrapper_get();
    struct real_type * real_val = &wrapper->val;
    if (CORK_UNLIKELY(!wrapper->initialized)) {
    return real_val;

It’s also not possible to provide a finalization function; if your thread-local variable acquires any resources or memory that needs to be freed when the thread finishes, you must make a “thread cleanup” function that you explicitly call at the end of each thread.


On some platforms, the number of thread-local values that can be created by any given process is limited (i.e., on the order of 128 or 256 values). This means that you should limit the number of thread-local values you create, especially in a library.