ref: 589eb3a61a8ee46e857aa2e20d9f7014923dc00e
dir: /README/
Compiling ========= SCC is a portable toolchain that can be compiled on any UNIX system out of the box. It supports four main configuration options that can be passed to the command line: - PREFIX: Prefix of the path where scc toolchain is going to be installed. /usr/local by default. - CROSS_COMPILE: Specify a prefix name for the tools called by the Makefile. - HOST: Specify the host system to be used. Possible supported values are: - unix (by default) - bsd - plan9 - CONF: Specify which version of libc to build. Once the build process completes only the target specified in CONF will be built. Supported values are: - amd64-linux (default) - amd64-darwin - amd64-openbsd - arm64-linux - amd64-dragonfly - amd64-netbsd - arm32-linux - TOOL: Specify the toolchain type to be used. Possible supported values are: - unix (by default) - gnu - gnu-darwin - clang Deviations from standard C =========================== This compiler aims to be fully compatible with the C99 standard, but it will have some differences: - Type qualifiers are accepted but ignored. ----------------------------------------- Type qualifiers make the type system ugly, and their uselessness adds 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 behavior is undefined behaviour. 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: 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 by using special pragma values though. In the first case, it generates a lot of problems with modern processors and multithreading, when not holding the value in a register is not good enough (an explicit memory barrier is needed). In the second 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 assembly). Since it is needed for the correct behaviour of portable programs that use longjmp, this specifier eventually will be implemented. - restrict: This qualifier can only be applied to pointers that mark the pointed object and has no other alias. This qualifier 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 compiler 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 type names (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 If you don't believe me try this code: int f(int g()) { return g(); } 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 common way of writing such code). - Definition of variables with incomplete type --------------------------------------------- C89 allows the definition of variables with incomplete type that have external linkage and file scope. The type of the variable is the composition of all the definitions found in the file. The exact rules are a bit complex (3.7.2) so SCC ignores them at this moment by simply not allowing any definition of variables with incomplete type. If you don't believe me try this code: struct foo x; struct foo { int i; }; - Variadic function alike macros ------------------------------ The standard (C99 6.10.3 c 4) forces passing more parameters than the number of parameters present in the variadic argument list (excluding ...). SCC accepts a parameter list with the same number of arguments. #define P(a, ...) a P(1)