Scheme 48 Manual | Contents | In Chapter: Mixing Scheme 48 and C
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## Calling Scheme functions from C

External code that has been called from Scheme can call back to Scheme procedures using the following function.

- `s48_value s48_call_scheme(s48_value p, long nargs, ...)`

This calls the Scheme procedure `p` on `nargs` arguments, which are passed as additional arguments to `s48_call_scheme`. There may be at most twelve arguments. The value returned by the Scheme procedure is returned by the C procedure. Invoking any Scheme procedure may potentially cause a garbage collection.

There are some complications that occur when mixing calls from C to Scheme with continuations and threads. C only supports downward continuations (via `longjmp()`). Scheme continuations that capture a portion of the C stack have to follow the same restriction. For example, suppose Scheme procedure `s0` captures continuation `a` and then calls C procedure `c0`, which in turn calls Scheme procedure `s1`. Procedure `s1` can safely call the continuation `a`, because that is a downward use. When `a` is called Scheme 48 will remove the portion of the C stack used by the call to `c0`. On the other hand, if `s1` captures a continuation, that continuation cannot be used from `s0`, because by the time control returns to `s0` the C stack used by `c0` will no longer be valid. An attempt to invoke an upward continuation that is closed over a portion of the C stack will raise an exception.

In Scheme 48 threads are implemented using continuations, so the downward restriction applies to them as well. An attempt to return from Scheme to C at a time when the appropriate C frame is not on top of the C stack will cause the current thread to block until the frame is available. For example, suppose thread `t0` calls a C procedure which calls back to Scheme, at which point control switches to thread `t1`, which also calls C and then back to Scheme. At this point both `t0` and `t1` have active calls to C on the C stack, with `t1`'s C frame above `t0`'s. If thread `t0` attempts to return from Scheme to C it will block, as its frame is not accessible. Once `t1` has returned to C and from there to Scheme, `t0` will be able to resume. The return to Scheme is required because context switches can only occur while Scheme code is running. `T0` will also be able to resume if `t1` uses a continuation to throw past its call to C.

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