20550e2018-03-06Pontus Östlund # Developer Documentation Custom functions are internally represented by `struct Sass_C_Function_Descriptor`. ## Sass_C_Function_Descriptor ```C struct Sass_C_Function_Descriptor { const char* signature; Sass_C_Function function; void* cookie; }; ``` - `signature`: The function declaration, like `foo($bar, $baz:1)` - `function`: Reference to the C function callback - `cookie`: any pointer you want to attach ### signature The signature defines how the function can be invoked. It also declares which arguments are required and which are optional. Required arguments will be enforced by LibSass and a Sass error is thrown in the event a call as missing an argument. Optional arguments only need to be present when you want to overwrite the default value. foo($bar, $baz: 2) In this example, `$bar` is required and will error if not passed. `$baz` is optional and the default value of it is 2. A call like `foo(10)` is therefore equal to `foo(10, 2)`, while `foo()` will produce an error. ### function The callback function needs to be of the following form: ```C union Sass_Value* call_sass_function( const union Sass_Value* s_args, void* cookie ) { return sass_clone_value(s_args); } ``` ### cookie The cookie can hold any pointer you want. In the `perl-libsass` implementation it holds the structure with the reference of the actual registered callback into the perl interpreter. Before that call `perl-libsass` will convert all `Sass_Values` to corresponding perl data types (so they can be used natively inside the perl interpretor). The callback can also return a `Sass_Value`. In `perl-libsass` the actual function returns a perl value, which has to be converted before `libsass` can work with it again! ## Sass_Values ```C // allocate memory (copies passed strings) union Sass_Value* make_sass_boolean (int val); union Sass_Value* make_sass_number (double val, const char* unit); union Sass_Value* make_sass_color (double r, double g, double b, double a); union Sass_Value* make_sass_string (const char* val); union Sass_Value* make_sass_list (size_t len, enum Sass_Separator sep); union Sass_Value* make_sass_map (size_t len); union Sass_Value* make_sass_null (); union Sass_Value* make_sass_error (const char* msg); // Make a deep cloned copy of the given sass value union Sass_Value* sass_clone_value (const union Sass_Value* val); // deallocate memory (incl. all copied memory) void sass_delete_value (const union Sass_Value* val); ``` ## Example main.c ```C #include <stdio.h> #include <stdint.h> #include "sass/context.h" union Sass_Value* call_fn_foo(const union Sass_Value* s_args, void* cookie) { // we actually abuse the void* to store an "int" return sass_make_number((size_t)cookie, "px"); } int main( int argc, const char* argv[] ) { // get the input file from first argument or use default const char* input = argc > 1 ? argv[1] : "styles.scss"; // create the file context and get all related structs struct Sass_File_Context* file_ctx = sass_make_file_context(input); struct Sass_Context* ctx = sass_file_context_get_context(file_ctx); struct Sass_Options* ctx_opt = sass_context_get_options(ctx); // allocate a custom function caller Sass_C_Function_Callback fn_foo = sass_make_function("foo()", call_fn_foo, (void*)42); // create list of all custom functions Sass_C_Function_List fn_list = sass_make_function_list(1); sass_function_set_list_entry(fn_list, 0, fn_foo); sass_option_set_c_functions(ctx_opt, fn_list); // context is set up, call the compile step now int status = sass_compile_file_context(file_ctx); // print the result or the error to the stdout if (status == 0) puts(sass_context_get_output_string(ctx)); else puts(sass_context_get_error_message(ctx)); // release allocated memory sass_delete_file_context(file_ctx); // exit status return status; } ``` ## Compile main.c ```bash gcc -c main.c -o main.o gcc -o sample main.o -lsass echo "foo { margin: foo(); }" > foo.scss ./sample foo.scss => "foo { margin: 42px }" ```