About the Objective-C category

This subcategory is for Objective-C programmers. Don’t be shy, no question is too simple or too complicated.

Please, do not post here if your question is about a puzzle, a contest or anything that relates to the CodinGame platform (there are other categories for that purpose).

Does anyone knows a good documentation about Objective-C ? Like other people I submit pure C in Objective-C language, but I have a segmentation fault with a solutions working in C.
Maybe the problem comes from strcmp, or maybe from strcat, I don’t know, I would like to read some documentation to see if they work the same as in C, but apple documentation seem to be … very light ! strcmp | Apple Developer Documentation

Thanks in advance.