Question

gcc 4.7.2 c89

Hello,

I have the following function:

void module_param_destroy(module_param_t *param)
{
    param = NULL;

    /* Stop the compiler from complaining */
    APR_UNUSED(param);
}

And I am calling it like this:

module_param_destroy(module->call_param);

Would the parameter param be set to NULL. As I am passing in a local copy, I don’t think it is doing anything useful here.

Would it be better to do this:

void module_param_destroy(module_t *md)
{
    md->param = NULL;

    /* Stop the compiler from complaining */
    APR_UNUSED(md->param);
}

And calling it like this:

module_param_destroy(md);

The structures have globally allocated memory from malloc.

Many thanks for any suggestions,

Answer 1

If param points to memory obtained from malloc, then setting it to NULL is not enough to properly cleanup.

There are two common ways to use module_param_destroy:

void module_param_destroy(module_param_t *param)
{
    free(param);
}

or

void module_param_destroy(module_param_t **param)
{
    free(*param);
    *param = NULL;
}

The main difference is that with the second variant, param gets set to a testable value to indicate it has been cleaned up. With the first variant, you have to remember yourself that the pointer passed to module_param_destroy is unusable after the function has returned.

You call them respectively as

module_param_destroy(module->call_param);

and

module_param_destroy(&module->call_param);