Pragma in C

C/C++ is used for various purposes starting for application development, GUI to embedded systems but C has a very small set of commands and constructs. So how do you give information to compiler which are not provided by the language.

for example if you want to keep some part of code in CACHE (application development) or want a portion of code to be 16bit aligned or want to add an assembly code to a C code ( embedded) or want to write a optimised code yourself and do not want compiler to do any changes (optimisation) in your code. So how do you let compiler know you are making a request to him