ref: b1ed9087223b78b86fdbb90ffc858c2c99b02833
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 diagnostic 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).