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copied!<h2>Version 0 — Critique of Question's Code</h2> <p>The posted code does not compile.</p> <h3>api.h</h3> <pre><code>typedef void* hidden_my_type; void do_something(my_type x); </code></pre> <p>This defines <code>hidden_my_type</code> but not the <code>my_type</code> that is passed to <code>do_something()</code>. Presumably, you intended:</p> <pre><code>typedef void *my_type; void do_something(my_type x); </code></pre> <h3>core.c</h3> <pre><code>struct _my_type { int a; } </code></pre> <p>As noted below too, there is a semi-colon missing after the structure definition.</p> <pre><code>void do_something(hidden_my_type void_x) { struct *_my_type x = void_x; printf("Value: %d\n", x->a); } </code></pre> <p>You have the <code>hidden_my_type</code> vs <code>my_type</code> problem again. You have the <code>*</code> of the pointer where it cannot go; it must go after the <code>struct _my_type</code>. You probably intended something like:</p> <pre><code>void do_something(my_type void_x) { struct _my_type *x = void_x; printf("Value: %d\n", x->a); } </code></pre> <p>This is now syntactically correct (I think; I haven't actually run it past a compiler). You have not shown how it is used; indeed, since the user code has no way to generate a pointer to a valid structure, there is no way for this code to be used safely.</p> <p>Your test code (unshown — why don't you show your test code) might look something like this:</p> <pre><code>#include "api.h" int main(void) { my_type x = 0; do_something(x); return 0; } </code></pre> <p>Alternatively, it might not have the <code>= 0</code> initializer in place. Either way, your code is unable to function sanely, and a core dump is almost inevitable. When you hide the structure from the user, you have to provide them with a mechanism to get hold of a valid (pointer to) the structure, and you've not done that.</p> <h2>Version 1</h2> <p>This is a better way to do it, because it is more nearly type-safe:</p> <h3>api.h version 1</h3> <pre><code>typedef struct _my_type *my_type; void do_something(my_type x); </code></pre> <h3>core.c version 1</h3> <pre><code>#include "api.h" struct _my_type { int a; }; </code></pre> <p>Note the added semi-colon, and the include of the <code>api.h</code> file.</p> <pre><code>void do_something(my_type x) { // Now you don't have to do casting here! //struct *_my_type x = void_x; /*Don't understand is that correct way to do, as I'm getting segmentation fault error */ printf("Value: %d\n", x->a); } </code></pre> <hr> <h2>Version 2</h2> <p>Actually, we can debate the wisdom of hiding the pointer; I would prefer not to do so:</p> <h3>api.h version 2</h3> <pre><code>#ifndef API_H_INCLUDED #define API_H_INCLUDED typedef struct my_type my_type; extern void do_something(my_type *x); extern my_type *my_type_initializer(void); extern void my_type_release(my_type *x); #endif /* API_H_INCLUDED */ </code></pre> <h3>core.c version 2</h3> <pre><code>#include "api.h" #include <stdio.h> #include <stdlib.h> struct my_type { int a; }; void do_something(my_type *x) { printf("Value: %d\n", x->a); } my_type *my_type_initializer(void) { my_type *x = malloc(sizeof(*x)); x->a = 57; // More plausibly, this would be 0 return x; } void my_type_release(my_type *x) { free(x); } </code></pre> <h3>main.c</h3> <pre><code>#include "api.h" int main(void) { my_type *x = my_type_initializer(); do_something(x); my_type_release(x); return 0; } </code></pre> <p>That's nice and clean. Of course, the user cannot allocate a <code>struct my_type</code> (only a pointer to it), so you need a function to allocate the structure for them. Think of the Standard C Library, and the <code>FILE</code> type, and <code>fopen()</code> to allocate and <code>fclose()</code> to release and <code>fprintf()</code> etc to manipulate the type. The <code>my_type_initializer()</code> is functioning as an analogue to <code>fopen()</code>, <code>my_type_release()</code> as an analogue to <code>fclose()</code>, and <code>do_something()</code> as an analogue to <code>fprintf()</code>.</p>

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