ref: f441e1db75872457deb4aacf39500f3cc6185554
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. - LIBPREFIX: Prefix of the path where scc searchs for headers and libraries when scc is executed. $PREFIX by default. - DESTDIR: Temporary directory prepend to PREFIX used in the install path. It is mainly intended to help package maintainers to install in a specific directory used as base for the package generation. - 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 - i386-linux - TOOL: Specify the toolchain type to be used. Possible supported values are: - unix (by default) - gnu - gnu-darwin - clang The main targets of the Makefile are: - all: Compile the toolchain and the libc. It automatically determines what is the best value for HOST. It sets the value of CONF for the toolchain that is used by the toolchain as the default target. It also compiles the libc for all the available configurations based in the host architecture. - config Generate headers supposed to be customized by the user. - toolchain Compile the toolchain with the default configuration specified in CONF. - libc: Compile the libc for the target specified in CONF. - install: Installs scc in PREFIX. - clean: Remove all the generated files except the one supposed to be edited by the user. - distclean Remove all the generated files. Musl libc support ================= The Scc libc is a C99 library and cannot used to compile POSIX compliant programs. Scc includes a template that can be used to use a musl libc compiled by gcc: $ make LIBPROFILE=musl config It will generate the files sysld.h and sysincludes.h configured to be used with a musl libc. Beware that it is likely that those files have to be customized to fit your system because the macro GCCLIBPATH depends heavily of the toolchain used to compile musl. As the musl libc is likely will be installed in a different prefix the scc compilation must be modified to: $ make LIBPREFIX=/usr/local/musl If the helper scc shell script is used instead of scc-cc then the environment variable SCCLIBPREFIX must be set: $ SCCLIBPREFIX=/usr/local/musl scc hello.c 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 more complex and they 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 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 (ANSI 3.7.2, or ISO C99 6.2.5p22) 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)