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# 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 interpreter). 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* sass_make_null (void);
union Sass_Value* sass_make_boolean (bool val);
union Sass_Value* sass_make_string (const char* val);
union Sass_Value* sass_make_qstring (const char* val);
union Sass_Value* sass_make_number (double val, const char* unit);
union Sass_Value* sass_make_color (double r, double g, double b, double a);
union Sass_Value* sass_make_list (size_t len, enum Sass_Separator sep, bool is_bracketed);
union Sass_Value* sass_make_map (size_t len);
union Sass_Value* sass_make_error (const char* msg);
union Sass_Value* sass_make_warning (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 }"
```
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