Locking

FRR ships two small wrappers around pthread_mutex_lock() / pthread_mutex_unlock. Use #include "frr_pthread.h" to get these macros.

frr_with_mutex(pthread_mutex_t *mutex)

Begin a C statement block that is executed with the mutex locked. Any exit from the block (break, return, goto, end of block) will cause the mutex to be unlocked:

int somefunction(int option)
{
    frr_with_mutex(&my_mutex) {
        /* mutex will be locked */

        if (!option)
            /* mutex will be unlocked before return */
            return -1;

        if (something(option))
            /* mutex will be unlocked before goto */
            goto out_err;

        somethingelse();

        /* mutex will be unlocked at end of block */
    }

    return 0;

out_err:
    somecleanup();
    return -1;
}

This is a macro that internally uses a for loop. It is explicitly acceptable to use break to get out of the block. Even though a single statement works correctly, FRR coding style requires that this macro always be used with a { ... } block.

frr_mutex_lock_autounlock(pthread_mutex_t *mutex)

Lock mutex and unlock at the end of the current C statement block:

int somefunction(int option)
{
    frr_mutex_lock_autounlock(&my_mutex);
    /* mutex will be locked */

    ...
    if (error)
      /* mutex will be unlocked before return */
      return -1;
    ...

    /* mutex will be unlocked before return */
    return 0;
}

This is a macro that internally creates a variable with a destructor. When the variable goes out of scope (i.e. the block ends), the mutex is released.

Warning

This macro should only used when frr_with_mutex() would result in excessively/weirdly nested code. This generally is an indicator that the code might be trying to do too many things with the lock held. Try any possible venues to reduce the amount of code covered by the lock and move to frr_with_mutex().