Apple's iBooks app, if submitted by a third-party developer, wouldn't have passed the App Store submission process, one frustrated developer wrote Tuesday on his blog.

Marco Arment, lead developer of the Instapaper app and the Tumblr blogging service, wrote Tuesday that iBooks uses "tons of private APIs," which he said would get his app rejected if he tried to use it.

"I won’t be able to offer some features that iBooks has (such as a true brightness control), but my customers will expect them, making my app inferior to Apple’s in key areas," Arment wrote.

Arment said that iBooks' use of the private APIs — interfaces that let pieces of software interact with each other — was "very obvious."

App developers use APIs to let their apps hook into iPhone features. When an app asks to use your location information, it's talking to the device's location API. When it lets you choose different pieces of music from your iTunes library, to play, it's talking to the APIs that handle the library and playback. APIs make rapid application development possible, because developers don't have to reinvent wheels.

It's Apple's use of APIs that other developers can't use that has Arment frustrated.

"We’re going to start seeing ... arguments against Apple — rightfully so — as they move into more application markets if they choose to exert unfair advantages," Arment said.

A report at Gizmodo speculates that the APIs will be made available to developers at some point in the future, suggesting that the APIs were rushed and aren't yet documented for developers to use in their own apps.

We'd love for every iPhone and iPad app to have brightness control available (or rather, to be able to somehow adjust brightness without exiting an app.)