auto indent does not match tab stop size

when I press RET in, say a struct (in C) it will indent next line with 2 spaces instead of a 4 spaces tab, so I have to unindent it then reindent it with a tab each time, sorry if this is simple to solve and thanks in advance

You need to set your coding style. The default is "gnu", which makes sense, since Emacs is a Gnu project, but is also quite annoying since the gnu style is so quirky! A quick fix is to customize `c-default-style` and change "gnu" to "stroustrup" or "k&r". (Using customize is the easiest option since `c-default-style` is an alist.) Customizing a single option can be done via `M-x customize-option` or via the menu with Options->Customize Emacs->Specific Option. In either case, you will then be prompted for the option to customize, and can enter `c-default-style RET`. Eventually, you may want to create your own style, possibly inheriting from one of the existing ones, but that's an exercise for later.

actually I found it at `M-x c-set-style` but it's the same thing, thanks for your help, now auto-indent works the way I want c: