callcc.c File Reference

First-class continuation primitives (call/cc via stack copying). More...

#include "x-prim.h"
#include "x-eval.h"
#include "x-env.h"
#include "x-heap.h"
#include <setjmp.h>
#include <string.h>
#include "x-type/list.h"
#include "x-type/prim.h"
#include "x-type/ptr.h"
#include "x-type/procedure.h"
#include "x-type/symbol.h"

Data Structures
struct	x_callcc_cont_t

Macros
#define	X_CALLCC_NO_ASAN

Functions
static X_CALLCC_NO_ASAN void	x_callcc_copy (char *dst, const char *src, size_t n)
X_CALLCC_NO_ASAN void	x_callcc_init (void)
static X_CALLCC_NO_ASAN void	x_callcc_sp (void **pp_sp)
static void	x_callcc_restore (x_callcc_cont_t *cont)
static x_obj_t *	x_prim_cc_invoke (x_obj_t *p_base, x_obj_t *p_args)
static X_CALLCC_NO_ASAN x_obj_t *	x_prim_callcc (x_obj_t *p_base, x_obj_t *p_args)
x_obj_t *	x_prim_callcc_register (x_obj_t *p_base, x_obj_t *p_args)

Variables
static void(*volatile	x_callcc_sp_fn )(void **) = x_callcc_sp
static void(*volatile	x_callcc_restore_fn )(x_callcc_cont_t *)
Continuation Data
Stack-copying continuations: capture the C call stack segment from a known base (set in main) to the current frame. Invocation restores the stack and longjmps back to the capture point. Interpreter state (env, save_stack, error_handler) is saved as a GC-visible x-lang list in the continuation procedure's closure env.
static void *	g_stack_base = NULL

Detailed Description

First-class continuation primitives (call/cc via stack copying).

Author

Jon Ruttan (jonru.nosp@m.ttan.nosp@m.@gmai.nosp@m.l.co.nosp@m.m)

Copyright

2026 Jon Ruttan @license MIT No Attribution (MIT-0)

Macro Definition Documentation

X_CALLCC_NO_ASAN

#define X_CALLCC_NO_ASAN

Function Documentation

x_callcc_copy()

static X_CALLCC_NO_ASAN void x_callcc_copy ( char *  dst, const char *  src, size_t  n  )

static

Copy a captured stack segment. Under ASan this must be a plain byte loop in an exempt function – see the truce note above.

x_callcc_init()

X_CALLCC_NO_ASAN void x_callcc_init

(

void

)

Establish the stack base address for continuation capture.

Must be called once at interpreter startup, from a frame close to the bottom of the C call stack (typically main). The address of a local variable is recorded as the upper bound of all future stack captures.

Note

Uses a volatile local to prevent the compiler from eliminating the anchor variable.

x_callcc_restore()

static X_CALLCC_NO_ASAN void x_callcc_restore ( x_callcc_cont_t *  cont )

static

Restore a captured continuation by replacing the current stack.

Recursively grows the C stack (via a 2 KiB pad per frame) until the stack pointer is below the lower bound of the captured segment, then copies the saved bytes back and longjmps to the capture point.

Parameters

cont	Continuation whose stack segment and jmp_buf to restore.

Note

This function never returns.

x_callcc_sp()

static X_CALLCC_NO_ASAN void x_callcc_sp ( void **  pp_sp )

static

Store an address strictly below the caller's entire stack frame.

The capture in x_prim_callcc() must include EVERY slot of its own frame: locals the compiler spills below the address taken (gcc puts p_base/cont there) would otherwise be restored as garbage after longjmp – the crash class that only clang's register allocation hid. A callee's local is below the caller's whole frame by construction, so its address is a safe lower bound.

Note

Out-parameter, not a return value: gcc's -Wreturn-local-addr pass REPLACES a returned local address with NULL at -O; a store through a pointer (same idiom as x_callcc_init) is only warned about, never rewritten.

x_prim_callcc()

static X_CALLCC_NO_ASAN x_obj_t * x_prim_callcc ( x_obj_t *  p_base, x_obj_t *  p_args  )

static

Capture the current continuation and pass it to a procedure. x-lang: (call/cc proc)

Allocates a continuation struct with a copy of the C stack segment from the current frame up to g_stack_base, saves interpreter state in a GC-visible list, and constructs a variadic closure k that invokes cc-invoke when called. Then applies proc to k.

When k is later invoked, the saved stack is restored via x_callcc_restore() and control returns here through longjmp, yielding the value passed to k.

Parameters

p_base	Interpreter base context.
p_args	Unevaluated argument list: (call/cc proc).

Returns

The result of proc, or the value passed to the continuation.

See also

x_prim_cc_invoke, x_callcc_restore

x_prim_callcc_register()

x_obj_t * x_prim_callcc_register	(	x_obj_t *	p_base,
		x_obj_t *	p_args
	)

Register continuation primitives.

Binds: cc-invoke, call/cc.

Parameters

p_base	Interpreter base context.
p_args	Unused.

Returns

p_base.

x_prim_cc_invoke()

static x_obj_t * x_prim_cc_invoke ( x_obj_t *  p_base, x_obj_t *  p_args  )

static

Invoke a captured continuation, restoring its stack and interpreter state. x-lang: (cc-invoke ptr state args-list)

This is the internal entry point called by the closure k that call/cc hands to its procedure. args-list is the variadic argument collector: () for zero arguments, (val) for one.

Parameters

p_base	Interpreter base context.
p_args	Unevaluated argument list: (cc-invoke ptr state args-list).

Returns

Does not return; transfers control via longjmp.

See also

x_prim_callcc, x_callcc_restore

Variable Documentation

g_stack_base

void* g_stack_base = NULL

static

Stack base pointer, set once at startup via x_callcc_init().

x_callcc_restore_fn

void(*volatile x_callcc_restore_fn) (x_callcc_cont_t *) ( x_callcc_cont_t *  )

static

Initial value:

=
    x_callcc_restore

Volatile call path for the recursion: sibling-call optimization would reuse the frame, so the pad would never advance and the descent would spin forever. A call through a volatile pointer cannot be folded.

x_callcc_sp_fn

void(*volatile x_callcc_sp_fn) (void **) ( void **  ) = x_callcc_sp

static

Volatile call path to x_callcc_sp(): inlining would hoist the anchor back into the caller's frame and re-create the under-capture; a call through a volatile pointer cannot be inlined or sibling-call folded.