add cooperative threading example

Makefile

@@ -37,6 +37,8 @@ LIBUCONTEXT_STATIC_PATH = ${LIBDIR}/${LIBUCONTEXT_STATIC_NAME}
 LIBUCONTEXT_HEADERS = \
 	include/libucontext/libucontext.h \
 	include/libucontext/bits.h
+LIBUCONTEXT_EXAMPLES = \
+	examples/cooperative_threading
 
 all: ${LIBUCONTEXT_SONAME} ${LIBUCONTEXT_STATIC_NAME} ${LIBUCONTEXT_PC}
 
@@ -98,6 +100,10 @@ check: test_libucontext ${LIBUCONTEXT_SONAME}
 test_libucontext: test_libucontext.c ${LIBUCONTEXT_NAME}
 	$(CC) -std=c99 -D_BSD_SOURCE ${CFLAGS} ${CPPFLAGS} $@.c -o $@ -L. -lucontext
 
+examples: ${LIBUCONTEXT_EXAMPLES}
+examples/cooperative_threading: examples/cooperative_threading.c ${LIBUCONTEXT_NAME}
+	$(CC) -std=c99 -D_BSD_SOURCE ${CFLAGS} ${CPPFLAGS} $@.c -o $@ -L. -lucontext
+
 ifeq ($(FREESTANDING),no)
 
 include/libucontext/bits.h: arch/common/bits.h

examples/cooperative_threading.c

@@ -0,0 +1,94 @@
+#include <assert.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <libucontext/libucontext.h>
+
+libucontext_ucontext_t mainctx = {};
+libucontext_ucontext_t *curthr = &mainctx;
+libucontext_ucontext_t *threads = NULL;
+size_t thrcount = 0;
+
+void
+yieldto(libucontext_ucontext_t *target)
+{
+	libucontext_ucontext_t *oldthr = curthr;
+	curthr = target;
+
+	libucontext_swapcontext(oldthr, curthr);
+}
+
+void
+yield(void)
+{
+	libucontext_ucontext_t *newthr;
+
+	/* we set uc_flags to non-zero to signal thread completion. */
+	do
+		newthr = &threads[random() % thrcount];
+	while (newthr == curthr || newthr->uc_flags);
+
+	srandom(time(NULL));
+
+	yieldto(newthr);
+}
+
+void
+worker(size_t multiple)
+{
+	size_t accum = 1;
+
+	for (size_t i = 0; i < 10; i++)
+	{
+		accum += (multiple * i);
+
+		printf("[%p] accumulated %zu\n", curthr, accum);
+		yield();
+	}
+
+	/* mark thread as completed, so we don't return here */
+	curthr->uc_flags = 1;
+}
+
+void
+create(size_t multiple)
+{
+	libucontext_ucontext_t *cursor;
+
+	thrcount += 1;
+	threads = realloc(threads, sizeof(*threads) * thrcount);
+
+	cursor = &threads[thrcount - 1];
+	memset(cursor, '\0', sizeof *cursor);
+
+	/* initialize the new thread's values to our current context */
+	libucontext_getcontext(cursor);
+
+	/* set up uc_link */
+	cursor->uc_link = thrcount > 1 ? &threads[thrcount - 2] : &mainctx;
+
+	/* set up a stack */
+	cursor->uc_stack.ss_size = 8192;
+	cursor->uc_stack.ss_sp = calloc(1, cursor->uc_stack.ss_size);
+
+	/* set up the function call */
+	libucontext_makecontext(cursor, worker, 1, multiple);
+}
+
+int
+main(int argc, const char *argv[])
+{
+	srandom(time(NULL));
+
+	libucontext_getcontext(&mainctx);
+
+	for (size_t i = 1; i < 4; i++)
+		create(i);
+
+	/* start the threads off by yielding to the last one */
+	yieldto(&threads[thrcount - 1]);
+
+	return EXIT_SUCCESS;
+}