scc

simple c99 compiler
git clone git://git.simple-cc.org/scc
Log | Files | Refs | README | LICENSE

README (3500B)


      1 Deviations from standard C
      2 ===========================
      3 This compiler aims to be fully compatible with the C99 standard, but
      4 it will have some differences:
      5 
      6 - Type qualifiers are accepted but ignored.
      7   -----------------------------------------
      8 
      9 Type qualifiers make the type system ugly, and their uselessness adds
     10 unnecessary complexity to the compiler (and increased compilation time):
     11 	- const: The definition of const is not clear in the standard.
     12 	  If a const value is modified the behaviour is undefined
     13 	  behaviour. It seems it was defined in order to be able to
     14 	  allocate variables in ROM rather than error detection. This
     15 	  implememtation will not warn about these modifications and
     16 	  the compiler will treat them like normal variables (the standard
     17 	  specifies that a diagnostic message must be printed).
     18 
     19 	- volatile: This qualifier was added to the standard
     20 	  to be able to deal with longjmp (local variables that are not
     21 	  volatile have undefined state) and for memory mapped registers
     22 	  or variables whose values are modified asynchronously. This can
     23 	  be achieved with special pragma values though.
     24 	  In the first case, it generates a lot of problems with modern
     25 	  processors and multithreading, when not holding the value in a
     26 	  register is not good enough (an explicit memory barrier is needed).
     27 	  In the second case, this is non-portable code by definition
     28 	  (depending on the register mapped), so it is better to deal with
     29 	  it using another solution (compiler extensions or direct
     30 	  assembly).
     31 
     32 	- restrict: This qualifier can only be applied to pointers to
     33 	  mark that the pointed object has no other alias. This qualifier
     34 	  was introduced to be able to fix some performance problems in
     35 	  numerical algorithms, where FORTRAN could achieve a better
     36 	  performance (and in fact even with this specifier FORTRAN has a
     37 	  better performance in this field). Ignoring it doesn't make the
     38 	  compiler non-standard and in almost all applications the performance
     39 	  will be the same.
     40 
     41 - Function type names
     42   -------------------
     43 
     44 C99 allows you to define type names of function types and write something
     45 like:
     46 
     47 int f(int (int));
     48 
     49 Accepting function types in type names (or abstract declarators) makes the
     50 grammar ambiguous because it is impossible to differentiate between:
     51 
     52         (int (f))  -> function returning int with one parameter of type f
     53         (int (f))  -> integer variable f
     54 
     55 If you don't believe me try this code:
     56 
     57 int
     58 f(int g())
     59 {
     60 	return g();
     61 }
     62 
     63 Function type names are stupid, because they are used as an alias
     64 of the function pointer types, but it is stupid that something
     65 like sizeof(int (int)) is not allowed (because here it should be
     66 understood as the size of a function), but f(int (int)) is allowed
     67 because it is understood as a parameter of function pointer type.
     68 
     69 This complexity is not needed at all as function pointers fix all these
     70 problems without this complexity (and they are the more usual
     71 way of writing such code).
     72 
     73 - Definition of variables with incomplete type
     74   ---------------------------------------------
     75 
     76 C89 allows the definition of variables with incomplete type that
     77 have external linkage and file scope. The type of the variable
     78 is the composition of all the definitions find in the file. The exact
     79 rules are a bit complex (3.7.2) so SCC ignores them at this moment
     80 and it does not allow any definition of variables with incomplete type.
     81 
     82 If you don't believe me try this code:
     83 
     84 struct foo x;
     85 
     86 struct foo {
     87 	int i;
     88 };