generator: Generators for C

Generators are a limited form of co-routine, which people may be familiar
with from Python.  This module adds an implementation of generators for C.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

Makefile-ccan

@@ -64,6 +64,7 @@ MODS_WITH_SRC := aga \
 	err \
 	failtest \
 	foreach \
+	generator \
 	grab_file \
 	hash \
 	heap \

ccan/generator/LICENSE

@@ -0,0 +1 @@
+../../licenses/LGPL-2.1

ccan/generator/_info

@@ -0,0 +1,71 @@
+#include "config.h"
+#include <stdio.h>
+#include <string.h>
+
+/**
+ * generator - generators for C
+ *
+ * Generators are a limited form of coroutines, which provide a useful
+ * way of expressing certain problems, while being much simpler to
+ * understand than general coroutines.
+ *
+ * Instead of returning a single value, a generator can "yield" a
+ * value at various points during its execution.  Whenever it yields,
+ * the "calling" function resumes and obtains the newly yielded value
+ * to work with.  When the caller asks for the next value from the
+ * generator, the generator resumes execution from the last yield and
+ * continues onto the next.
+ *
+ * Example:
+ *	#include <stdio.h>
+ *	#include <ccan/generator/generator.h>
+ *
+ *	generator_def_static(simple_gen, int)
+ *	{
+ *      	generator_yield(1);
+ *		generator_yield(3);
+ *		generator_yield(17);
+ *	}
+ *
+ *	int main(int argc, char *argv[])
+ *	{
+ *		generator_t(int) gen = simple_gen();
+ *		int *ret;
+ *
+ *		while ((ret = generator_next(gen)) != NULL) {
+ *			printf("Generator returned %d\n", *ret);
+ *		}
+ *
+ *		return 0;
+ *	}
+ *
+ * Author: David Gibson <david@gibson.dropbear.id.au>
+ * License: LGPL (v2.1 or any later version)
+ *
+ * Ccanlint:
+ *      // We need several gcc extensions
+ *	objects_build_without_features FAIL
+ *      tests_compile_without_features FAIL
+ *	tests_helpers_compile_without_features FAIL
+ */
+int main(int argc, char *argv[])
+{
+	/* Expect exactly one argument */
+	if (argc != 2)
+		return 1;
+
+	if (strcmp(argv[1], "depends") == 0) {
+		printf("ccan/build_assert\n");
+		printf("ccan/ptrint\n");
+		printf("ccan/alignof\n");
+		printf("ccan/cppmagic\n");
+		return 0;
+	}
+
+	if (strcmp(argv[1], "testdepends") == 0) {
+		printf("ccan/str\n");
+		return 0;
+	}
+
+	return 1;
+}

ccan/generator/generator.c

@@ -0,0 +1,54 @@
+/* Licensed LGPLv2.1+ - see LICENSE file for details */
+#include <stdlib.h>
+#include <stdbool.h>
+#include <stddef.h>
+
+#include <ccan/alignof/alignof.h>
+
+#include <ccan/generator/generator.h>
+
+#define DEFAULT_STATE_SIZE	8192
+#define STATE_ALIGN		ALIGNOF(struct generator_)
+
+void *generator_new_(generator_wrapper_ *fn, size_t retsize)
+{
+	char *base;
+	size_t size = DEFAULT_STATE_SIZE;
+	void *ret;
+	struct generator_ *gen;
+
+	base = malloc(size);
+	if (!base)
+		abort();
+
+	retsize = (retsize + STATE_ALIGN) & ~(STATE_ALIGN - 1);
+	ret = base + size - retsize;
+	gen = (struct generator_ *)ret - 1;
+
+	gen->base = base;
+	gen->complete = false;
+
+	getcontext(&gen->gen);
+
+	gen->gen.uc_stack.ss_sp = gen->base;
+	gen->gen.uc_stack.ss_size = (char *)gen - base;
+
+	if (HAVE_POINTER_SAFE_MAKECONTEXT) {
+		makecontext(&gen->gen, (void *)fn, 1, ret);
+	} else {
+		ptrdiff_t si = ptr2int(ret);
+		ptrdiff_t mask = (1UL << (sizeof(int) * 8)) - 1;
+		int lo = si & mask;
+		int hi = si >> (sizeof(int) * 8);
+
+		makecontext(&gen->gen, (void *)fn, 2, lo, hi);
+	}
+
+	return ret;
+}
+
+void generator_free_(void *ret)
+{
+	struct generator_ *gen = generator_state_(ret);
+	free(gen->base);
+}

ccan/generator/generator.h

@@ -0,0 +1,202 @@
+/* Licensed under LGPLv2.1+ - see LICENSE file for details */
+#ifndef CCAN_GENERATOR_H
+#define CCAN_GENERATOR_H
+#include "config.h"
+
+#if !HAVE_UCONTEXT
+#error Generators require working ucontext.h functions
+#endif
+
+#if !HAVE_TYPEOF
+#error Generators require typeof
+#endif
+
+#if !HAVE_STATEMENT_EXPR
+#error Generators require statement expressions
+#endif
+
+#include <assert.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <ucontext.h>
+
+#include <ccan/ptrint/ptrint.h>
+#include <ccan/build_assert/build_assert.h>
+#include <ccan/cppmagic/cppmagic.h>
+
+/*
+ * Internals - included just for the use of inlines and macros
+ */
+
+struct generator_ {
+	ucontext_t gen;
+	ucontext_t caller;
+	bool complete;
+	void *base;
+};
+
+static inline struct generator_ *generator_state_(const void *ret)
+{
+	return (struct generator_ *)ret - 1;
+}
+
+struct generator_incomplete_;
+
+#define generator_rtype_(gen_)			\
+	typeof((*(gen_))((struct generator_incomplete_ *)NULL))
+
+#if HAVE_POINTER_SAFE_MAKECONTEXT
+#define generator_wrapper_args_()	void *ret
+#else
+#define generator_wrapper_args_()	int lo, int hi
+#endif
+typedef void generator_wrapper_(generator_wrapper_args_());
+
+void *generator_new_(generator_wrapper_ *fn, size_t retsize);
+void generator_free_(void *ret);
+
+/*
+ * API
+ */
+
+/**
+ * generator_t - type for an in-progress generator
+ * @rtype: type of values the generator yield
+ */
+#define generator_t(rtype_)			\
+	typeof(rtype_ (*)(struct generator_incomplete_ *))
+
+/**
+ * generator_declare - declare (but don't define) a generator function
+ * @name: name for the generator
+ * @rtype: return type for the generator
+ *
+ * Declares (as an extern) a generator function named @name, which
+ * will yield return values of type @rtype.
+ *
+ * Example:
+ *	generator_declare(count_to_3, int);
+ */
+#define generator_declare(name_, rtype_)	\
+	generator_t(rtype_) name_(void)
+
+/**
+ * generator_def - define a generator function
+ * @name: name for the generator
+ * @rtype: return type for the generator
+ *
+ * Define a generator function named @name yielding return values of
+ * type @rtype.  The generator_def() line is followed immediately by a
+ * block containing the generator's code.
+ *
+ * Example:
+ *	generator_def(count_to_3, int)
+ *	{
+ *		generator_yield(1);
+ *		generator_yield(2);
+ *		generator_yield(3);
+ *	}
+ */
+#define generator_def_(name_, rtype_, storage_)				\
+	static void name_##_generator_(rtype_ *ret_);			\
+	static void name_##_generator__(generator_wrapper_args_())	\
+	{								\
+		struct generator_ *gen;					\
+		CPPMAGIC_IFELSE(HAVE_POINTER_SAFE_MAKECONTEXT)		\
+			()						\
+			(ptrdiff_t hilo = ((ptrdiff_t)hi << (8*sizeof(int))) \
+			 	+ (ptrdiff_t)lo;			\
+			rtype_ *ret = (rtype_ *)int2ptr(hilo);		\
+			BUILD_ASSERT(sizeof(struct generator_ *)	\
+				     <= 2*sizeof(int));)		\
+		gen = generator_state_(ret);				\
+		name_##_generator_(ret);				\
+		gen->complete = true;					\
+		setcontext(&gen->caller);				\
+		assert(0);						\
+	}								\
+	storage_ generator_t(rtype_) name_(void)			\
+	{								\
+		return generator_new_(name_##_generator__,		\
+				      sizeof(rtype_));			\
+	}								\
+	static void name_##_generator_(rtype_ *ret_)
+#define generator_def(name_, rtype_)		\
+	generator_def_(name_, rtype_, )
+
+/**
+ * generator_def_static - define a private / local generator function
+ * @name: name for the generator
+ * @rtype: return type for the generator
+ *
+ * As generator_def, but the resulting generator function will be
+ * local to this module.
+ */
+#define generator_def_static(name_, rtype_)	\
+	generator_def_(name_, rtype_, static)
+
+/**
+ * generator_yield - yield (return) a value from a generator
+ * @val: value to yield
+ *
+ * Invoke only from within a generator.  Yield the given value to the
+ * caller.  This will stop execution of the generator code until the
+ * caller next invokes generator_next(), at which point it will
+ * continue from the generator_yield statement.
+ */
+#define generator_yield(val_)						\
+	do {								\
+		struct generator_ *gen_ = generator_state_(ret_);	\
+		int rc;							\
+		*(ret_) = (val_);					\
+		rc = swapcontext(&gen_->gen, &gen_->caller);		\
+		assert(rc == 0);					\
+	} while (0)
+
+/**
+ * generator_next - get next value from a generator
+ * @gen: a generator state variable
+ *
+ * Returns a pointer to a (correctly typed) buffer containing the next
+ * value yielded by @gen, or NULL if @gen is finished.  The buffer
+ * contents is only valid until the next time @gen is called or
+ * manipulated.
+ */
+static inline void *generator_next_(void *ret_)
+{
+	struct generator_ *gen = generator_state_(ret_);
+	int rc;
+
+	if (gen->complete)
+		return NULL;
+
+	rc = swapcontext(&gen->caller, &gen->gen);
+	assert(rc == 0);
+
+	return gen->complete ? NULL : ret_;
+}
+#define generator_next(gen_)				\
+	((generator_rtype_(gen_) *)generator_next_(gen_))
+
+/**
+ * generator_next_val - store next value from a generator
+ * @val: a variable of type suitable to store the generator's return
+ *       type (lvalue)
+ * @gen: a generator state variable
+ *
+ * Returns 'true' if @gen yielded a new value, false if @gen is
+ * complete.  If a new value was yielded, it is stored in @val.
+ */
+#define generator_next_val(val_, gen_)			\
+	({						\
+		generator_rtype_(gen_) *ret;		\
+		ret = generator_next(gen_);		\
+		if (ret)				\
+			(val_) = *ret;			\
+		!!ret;					\
+	})
+
+#define generator_free(gen_)					\
+	generator_free_((generator_rtype_(gen_) *)(gen_))
+
+#endif /* CCAN_GENERATOR_H */

ccan/generator/test/api.c

@@ -0,0 +1,62 @@
+#include <ccan/generator/generator.h>
+#include <ccan/tap/tap.h>
+#include <ccan/str/str.h>
+
+#include "example-gens.h"
+
+generator_def_static(genx, const char *)
+{
+	generator_yield("one");
+	generator_yield("two");
+	generator_yield("three");
+	generator_yield("four");
+}
+
+static void test1(void)
+{
+	generator_t(int) state1 = gen1();
+	int *ret;
+
+	ok1((ret = generator_next(state1)) != NULL);
+	ok1(*ret == 1);
+	ok1((ret = generator_next(state1)) != NULL);
+	ok1(*ret == 3);
+	ok1((ret = generator_next(state1)) != NULL);
+	ok1(*ret == 17);
+	ok1((ret = generator_next(state1)) == NULL);
+
+	/* Test that things don't go bad if we try to invoke an
+	 * already completed generator */
+	ok1((ret = generator_next(state1)) == NULL);
+
+	generator_free(state1);
+}
+
+static void testx(void)
+{
+	generator_t(const char *) statex = genx();
+	const char *val;
+
+	ok1(generator_next_val(val, statex));
+	ok1(streq(val, "one"));
+	ok1(generator_next_val(val, statex));
+	ok1(streq(val, "two"));
+	ok1(generator_next_val(val, statex));
+	ok1(streq(val, "three"));
+	ok1(generator_next_val(val, statex));
+	ok1(streq(val, "four"));
+	ok1(!generator_next_val(val, statex));
+	generator_free(statex);
+}
+
+int main(void)
+{
+	/* This is how many tests you plan to run */
+	plan_tests(8 + 9);
+
+	test1();
+	testx();
+
+	/* This exits depending on whether all tests passed */
+	return exit_status();
+}

ccan/generator/test/compile_fail-1.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ccan/generator/generator.h>
+
+#include <ccan/generator/generator.c>
+
+#include "example-gens.h"
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int *g = gen1();
+#else
+	generator_t(int) g = gen1();
+#endif
+
+	printf("%d", *generator_next(g));
+
+	exit(0);
+}

ccan/generator/test/compile_fail-2.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ccan/generator/generator.h>
+
+#include <ccan/generator/generator.c>
+
+#include "example-gens.h"
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	generator_t(char) g = gen1();
+#else
+	generator_t(int) g = gen1();
+#endif
+
+	printf("%d", *generator_next(g));
+
+	exit(0);
+}

ccan/generator/test/compile_fail-3.c

@@ -0,0 +1,24 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ccan/generator/generator.h>
+
+#include <ccan/generator/generator.c>
+
+generator_def_static(intgen, int)
+{
+#ifdef FAIL
+	generator_yield("a");
+#else
+	generator_yield(1);
+#endif
+}
+
+int main(int argc, char *argv[])
+{
+	generator_t(int) g = intgen();
+
+	printf("%d", *generator_next(g));
+
+	exit(0);
+}

ccan/generator/test/compile_fail-4.c

@@ -0,0 +1,23 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ccan/generator/generator.h>
+
+#include <ccan/generator/generator.c>
+
+#include "example-gens.h"
+
+int main(int argc, char *argv[])
+{
+	generator_t(int) g = gen1();
+#ifdef FAIL
+	char *val;
+#else
+	int val;
+#endif
+
+	generator_next_val(val, g);
+	printf("%d", val);
+
+	exit(0);
+}

ccan/generator/test/compile_fail-5.c

@@ -0,0 +1,21 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+#include <ccan/generator/generator.h>
+
+#include <ccan/generator/generator.c>
+
+#include "example-gens.h"
+
+int main(int argc, char *argv[])
+{
+#ifdef FAIL
+	int *g = NULL;
+#else
+	generator_t(int) g = gen1();
+#endif
+
+	generator_free(g);
+
+	exit(0);
+}

ccan/generator/test/example-gens.c

@@ -0,0 +1,10 @@
+#include <ccan/generator/generator.h>
+
+#include "example-gens.h"
+
+generator_def(gen1, int)
+{
+	generator_yield(1);
+	generator_yield(3);
+	generator_yield(17);
+}

ccan/generator/test/example-gens.h

@@ -0,0 +1,8 @@
+#ifndef _EXAMPLE_GENS_H
+#define _EXAMPLE_GENS_H
+
+#include <ccan/generator/generator.h>
+
+generator_declare(gen1, int);
+
+#endif /* _EXAMPLE_GENS_H */