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copied!<p>To understand what <code>objc_msgSend_fixup</code> is and what it does it's necessary to know exactly how message sending is performed in Objective-C. All the ObjC programmers have heard one day that the compiler transforms <code>[obj message]</code> statements into <code>objc_msgSend(obj, sel_registerName("message"))</code> calls. However, that's not entirely accurate.</p> <p>To better ilustrate my explanation, consider the following ObjC snippet:</p> <pre><code>[obj mesgA]; [obj mesgB]; [obj mesgA]; [obj mesgB]; </code></pre> <p>In this snippet, two messages are sent to <code>obj</code>, each of which is sent twice. So, you might imagine that the following code is generated:</p> <pre><code>objc_msgSend(obj, sel_registerName("mesgA")); objc_msgSend(obj, sel_registerName("mesgB")); objc_msgSend(obj, sel_registerName("mesgA")); objc_msgSend(obj, sel_registerName("mesgB")); </code></pre> <p>However <code>sel_registerName</code> may be too costly and call it whenever a specific method is called is not a smart thing to do. Then, the compiler generates structures like this for each message to be sent:</p> <pre><code>typedef struct message_ref { id (*trampoline) (id obj, struct message_ref *ref, ...); union { const char *str; SEL sel; }; } message_ref; </code></pre> <p>So, in the example above, when the program starts, we have something like this:</p> <pre><code>message_ref l_objc_msgSend_fixup_mesgA = { &objc_msgSend_fixup, "mesgA" }; message_ref l_objc_msgSend_fixup_mesgB = { &objc_msgSend_fixup, "mesgB" }; </code></pre> <p>When these messages need to be sent to <code>obj</code>, the compiler generates code equivalent to the following:</p> <pre><code>l_objc_msgSend_fixup_mesgA.trampoline(obj, &l_objc_msgSend_fixup_mesgA, ...); // [obj mesgA]; l_objc_msgSend_fixup_mesgB.trampoline(obj, &l_objc_msgSend_fixup_mesgB, ...); // [obj mesgB]; </code></pre> <p>At the program startup, the message reference trampolines are pointers to the <code>objc_msgSend_fixup</code> function. For each <code>message_ref</code>, when its <code>trampoline</code> pointer is invoked for the first time, <code>objc_msgSend_fixup</code> gets called receiving the <code>obj</code> to which the message's got to be sent and the <code>message_ref</code> structure from which it was called. So, what <code>objc_msgSend_fixup</code> must do is get the selector for the message to be called. Since, this has to be done only once for each message reference, <code>objc_msgSend_fixup</code> must also replace the <code>trampoline</code> field of the ref by a pointer to another function that doesn't fix the message's selector. This function is called <code>objc_msgSend_fixedup</code> (the selector has been fixed up). Now that the message selector has been set and this doesn't have to be done again, <code>objc_msgSend_fixup</code> just calls <code>objc_msgSend_fixedup</code> and this just calls <code>objc_msgSend</code>. After that, if a message ref's <code>trampoline</code> is called again, its selector is already fixed, and <code>objc_msgSend_fixedup</code> is the one that gets called.</p> <p>In short, we could write <code>objc_msgSend_fixup</code> and <code>objc_msgSend_fixedup</code> like this:</p> <pre><code>id objc_msgSend_fixup(id obj, struct message_ref *ref, ...) { ref->sel = sel_registerName(ref->str); ref->trampoline = &objc_msgSend_fixedup; objc_msgSend_fixedup(obj, ref, ...); } id objc_msgSend_fixedup(id obj, struct message_ref *ref, ...) { objc_msgSend(obj, ref->sel, ...); } </code></pre> <p>This makes message sending a lot faster, since the appropriate selector is discovered only at the first time the message is called (by <code>objc_msgSend_fixup</code>). On later calls, the selector will have been already found and the message is called directly with <code>objc_msgSend</code> (by <code>objc_msgSend_fixedup</code>).</p> <p>In the question's assembly code, <code>l_objc_msgSend_fixup_alloc</code> is the <code>alloc</code> method's <code>message_ref</code> structure and the segmentation fault may have been caused by a problem in its first field (maybe it's not pointing to <code>objc_msgSend_fixup</code>...)</p>

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