Скачать или смотреть Understanding the Memory Semantics of dladdr() Out Parameter

Dive into the specifics of the memory semantics of the `dladdr()` out parameter, including details on string management and lifecycle in C dynamic linking.
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Understanding the Memory Semantics of dladdr() Out Parameter

When working with dynamic linking in C, you might encounter a function known as dladdr(). This function is pivotal for retrieving information about symbol addresses within shared objects, but it can raise questions about how memory is managed within its output parameters. Let's delve deeper into the specifics to illuminate this topic.

The Purpose of dladdr()

The dladdr() function allows developers to obtain details about a specific address in memory. Here's how the function is defined:

[[See Video to Reveal this Text or Code Snippet]]

When you call dladdr(), it fills a Dl_info structure with critical information about the address you provide. But one major question needs addressing: what happens to the strings returned in dli_fname and dli_sname? Can we free them?

Memory Semantics of dli_fname and dli_sname

The confusion surrounding whether you should manage (i.e., free) the memory of dli_fname and dli_sname can be clarified with the following points:

Validity Duration:

The strings returned in both dli_fname and dli_sname remain valid until the corresponding shared object is unloaded through the dlclose() function—either directly or through other means. This means that as long as the shared object is loaded in memory, you can safely use these strings.

Const Qualification:

The const char * type for both fields indicates that these pointers point to read-only strings. Importantly, as developers, we should not attempt to modify or free these strings. The reason behind this is that free() expects a void *, and since the strings are marked as const, passing them to free() would be incorrect and could lead to undefined behavior.

No User Management Required:

You do not need to manage the memory of these strings. They are not allocated from the heap, which would necessitate user management. Instead, they are typically managed by the dynamic loader, meaning the user is not responsible for freeing them.

Summary

In summary, when you use the dladdr() function, recall these critical points:

dli_fname and dli_sname are safe to use until the shared object is unloaded.

Do not free these strings as they are not allocated with user responsibility in mind. They're managed by the dynamic linking library, ensuring reentrancy and safety in multi-threaded scenarios.

By understanding these aspects of dladdr(), you can harness the power of dynamic linking in C without getting tangled in memory management pitfalls.