ref: 4dca54568291a99779209f6b381435a6a62a232d
dir: /README/
Derivations from standard C =========================== This compiler is aimed to be being fully compatible with the C99 standard, but it will have some differences: - Type qualifiers are accepted but ignored. ----------------------------------------- Type qualifers make the type system ugly, and their uselessness add unnecessary complexity to the compiler (and increased compilation time): - const: The definition of const is not clear in the standard. If a const value is modified the behaviour is implementation defined. It seems it was defined in order to be able to allocate variables in ROM rather than error detection. This implememtation will not warn about these modifications and the compiler will treat them like normal variables (the standard specifies that a diagnosic message must be printed). - volatile: The definition of volatile is not concrete, because it is defined as 'remove all optimizations applied to the variable', which of course depends on the kind of optimizations applied to the variable. This qualifier was added to the standard to be able to deal with longjmp (local variables that are not volatile have undefined state) and for memory mapped registers or variables whose values are modified asynchronously. This can be achieved with special pragma values though. In the first case, this is non-portable code by definition (depending on the register mapped), so it is better to deal with it using another solution (compiler extensions or direct assembler). In the second case, it generates a lot of problems with modern processors and multithreading, where not holding the value in a register is not good enough (an explicit memory barrier is needed). - restrict: This qualifer can only be applied to pointers to mark that the pointed object has no other alias. This qualifer was introduced to be able to fix some performance problems in numerical algorithms, where FORTRAN could achieve a better performance (and in fact even with this specifier FORTRAN has a better performance in this field). Ignoring it doesn't make the code non-standard and in almost all applications the performance will be the same. - Function type names ------------------- C99 allows you to define type names of function types and write something like: int f(int (int)); Accepting function types in typenames (or abstract declarators) makes the grammar ambiguous because it is impossible to differentiate between: (int (f)) -> function returning int with one parameter of type f (int (f)) -> integer variable f Function type names are stupid, because they are used as an alias of the function pointer types, but it is stupid that something like sizeof(int (int)) is not allowed (because here it should be understood as the size of a function), but f(int (int)) is allowed because it is understood as a parameter of function pointer type. This complexity is not needed at all as function pointers fix all these problems without this complexity (and they are the more usual way of writing such code).