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POStatic allocation of opaque data types
 

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    <p>Very often malloc() is absolutely not allowed when programming for embedded systems. Most of the time I'm pretty able to deal with this, but one thing irritates me: it keeps me from using so called 'opaque types' to enable data hiding. Normally I'd do something like this:</p> <pre><code>// In file module.h typedef struct handle_t handle_t; handle_t *create_handle(); void operation_on_handle(handle_t *handle, int an_argument); void another_operation_on_handle(handle_t *handle, char etcetera); void close_handle(handle_t *handle); // In file module.c struct handle_t { int foo; void *something; int another_implementation_detail; }; handle_t *create_handle() { handle_t *handle = malloc(sizeof(struct handle_t)); // other initialization return handle; } </code></pre> <p>There you go: create_handle() performs a malloc() to create an 'instance'. A construction often used to prevent having to malloc() is to change the prototype of create_handle() like this:</p> <pre><code>void create_handle(handle_t *handle); </code></pre> <p>And then the caller could create the handle this way:</p> <pre><code>// In file caller.c void i_am_the_caller() { handle_t a_handle; // Allocate a handle on the stack instead of malloc() create_handle(&amp;a_handle); // ... a_handle is ready to go! } </code></pre> <p>But unfortunately this code is obviously invalid, the size of handle_t isn't known!</p> <p>I never really found a solution to solve this in a proper way. I'd very like to know if anyone has a proper way of doing this, or maybe a complete different approach to enable data hiding in C (not using static globals in the module.c of course, one must be able to create multiple instances).</p>
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    1. COMaybe I'm missing something. Why isn't the size of handle_t known? "create_handle" takes an argument of type "handlet_t*" so it should have knowledge about the size of it. I think it would be a different matter if you passed an array though.
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    2. CO@onemasse The size of handle_t isn't known in caller.c, only a pointer to handle_t can be used. The size of handle_t is only known in module.c
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    3. CO@onemasse Forward declaration and pointers allows use of opaque types so that only the implementation knows the size, not the client.
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