The standard C/C++ editor is provided by the CDT plug-in that provides C and C++ extensions to Eclipse. It provides syntax highlighting, formatting of code and content assistance when editing C/C++ code. Embedded assembler in C/C++ files is supported by the ARM Compiler but this editor does not support it and so an error is displayed. This type of code is ARM-specific and accepted Eclipse behavior so you can ignore the syntax error.

If this is not the default editor, right-click on a source file in the Project Explorer view and select Open With → C/C++ Editor from the context menu.

See the C/C++ Development User Guide for further information. Select Help → Help Contents from the main menu.