Скачать или смотреть Understanding Why addrinfo Struct Values Disappear After Function Call

Discover how to effectively handle the `addrinfo` struct in C to retain values after function calls and avoid common pitfalls.
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Understanding Why addrinfo Struct Values Disappear After Function Call

In C programming, particularly when dealing with sockets, mismanaging memory can lead to frustrating bugs. A common issue arises when working with the addrinfo struct, which is utilized to store address information. Many developers encounter a problem where the addrinfo struct seems to lose its values after a function call.

The Problem

Imagine you have a static addrinfo struct that you intend to populate with results from the getaddrinfo() function. Everything appears to be correct within the function; however, upon returning to the calling function, values seem to vanish. The code snippet below illustrates the issue:

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

The output shows that the addrlen results in 0, indicating that the information you set is not retained after returning from the function.

The Resolution

The root of this problem lies in how the addrinfo pointer is being passed to the init_socket function. In your original code, you are passing a pointer to a structure, but the function needs to modify this structure's memory, so you should pass a pointer to a pointer.

Updated Code Explanation

Change the Function Signature: Modify the init_socket function to accept a pointer to a pointer (struct addrinfo **resInfo).

Update the Call to getaddrinfo: Pass resInfo directly, which allows getaddrinfo to allocate the necessary memory.

Here’s the corrected code:

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

Key Changes Explained

Use of struct addrinfo **resInfo: This adjustment allows the function to modify the addrinfo structure in a way that persists after returning.

Dereferencing in getaddrinfo: When we call getaddrinfo, we need to dereference resInfo to get the actual address where the memory is allocated.

Conclusion

This issue can be common among those new to memory management in C. By understanding how pointers and memory allocation work, you can avoid losing data when making function calls. Always remember to free any dynamically allocated memory once it is no longer needed to prevent memory leaks.

With these corrections, you will retain the values in your addrinfo struct and ensure that your socket programming endeavors run smoothly.