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## Calling C functions from Scheme

There are three different ways to call C functions from Scheme, depending on how the C function was obtained.

- `(call-imported-binding binding arg0 ...) -> value`
- `(call-external external arg0 ...) -> value`
- `(call-external-value value name arg0 ...) -> value`

Each of these applies its first argument, a C function, to the rest of the arguments. For `call-imported-binding` the function argument must be an imported binding. For `call-external` the function argument must be an external bound in the current process (see the section on Dynamic Loading). For `call-external-value` `value` must be a byte vector whose contents is a pointer to a C function and `name` should be a string naming the function. The `name` argument is used only for printing error messages.

For all of these, the C function is passed the `argi` values and the value returned is that returned by C procedure. No automatic representation conversion occurs for either arguments or return values. Up to twelve arguments may be passed. There is no method supplied for returning multiple values to Scheme from C (or vice versa) (mainly because C does not have multiple return values).

Keyboard interrupts that occur during a call to a C function are ignored until the function returns to Scheme (this is clearly a problem; we are working on a solution).

- `(import-lambda-definition `name` (`formal` ...))` | syntax

- `(import-lambda-definition `name` (`formal` ...) `c-name`)` | syntax

These macros simplify importing functions from C. They define `name` to be a function with the given formals that applies those formals to the corresponding C binding. `C-name`, if supplied, should be a string. These expand into

>     (define temp (lookup-imported-binding c-name))
>     (define name
>       (lambda (formal ...)
>         (external-apply temp formal ...)))

If `c-name` is not supplied, it is derived from `name` by converting all letters to lowercase and replacing ``-`' with ``_`'.

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