What kind of features in C language an embedded systems engineer must?
It varies from vendor to vendor, chip to chip, and architecture to architecture. For example, you may have to put certain data structures at particular memory location during compile time. You may have to make structures which have bit fields. Etc etc. The one thing I personally detest dealing with are ISRs (Interrupt Service Routine) because they usually require some compile time settings and some run time settings, and the documentation is shoddy. Another thing you may have to deal with are writing specific routines of specific length.