Is there a way to use libMobileGestalt.tbd in an Apple-approved manner?

Are there official header files for libMobileGestalt.tbd … ?

No. Mobile Gestalt is an implementation detail, not an API.

are there any substitutes for its functionality?

What are you trying to do?

Mobile Gestalt does a lot of different things, so I need more details before I can give you a definitive answer.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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I'm trying to get a list of detailed hardware and software information, similar to what /System/Applications/Utilities/System Information.app does.

What platform are you targeting?

Mobile Gestalt is mostly relevant on iOS (hence the Mobile bit) but the System Information app is a macOS thing. And this matters because your options in this space are very different on the two platforms.

Also, what do you intend to use this “list of detailed hardware and software information”?

That matters because the answers are different depending on the common use cases (showing it to the user, logging and analytics, and so on).

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
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It’s better to reply as a reply, rather than in the comments; see Quinn’s Top Ten DevForums Tips for this and other titbits.

The information is used both for user-level presentation and adjusting app behavior accordingly.

Presenting information to the user is rarely a compatibility issue, but “adjusting app behaviour” is more more of a concern. What behaviours are you adjusting? And why?

ps Things would go a lot quicker if you offered more details up front. I only pass through forums once a day, so every time I have to ask you another clarifying question it’s another day of delay. That delay doesn’t bother me — I’m not in a hurry here — but if you want to resolve this more quickly then it’s in your interests to be more forthcoming.

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Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"