C89, Mixing Variable Declarations and Code

Question

I\'m very curious to know why exactly C89 compilers will dump on you when you try to mix variable declarations and code, like this for example:

rutski@imac:~         


        

Answer 1

It is much easier to write a compiler for language which requires all variables to be declared at the start of function. Some languages even require variables to be declared in specific clause outside of function code (Pascal and Smalltalk come to mind).

The reason is it's easier to map this variables to stack (or registers if your compiler is smart enough) if they are known and don't change.

Any other statements (esp. function calls) may modify stack/registers, making variable mapping more complex.