From be5b4c0cac83931bf67cc910ce21276f87bfbd16 Mon Sep 17 00:00:00 2001
From: Ariadne Conill <ariadne@dereferenced.org>
Date: Mon, 7 Dec 2020 18:14:35 -0700
Subject: [PATCH] add manual page

---
 doc/libucontext.scd | 174 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 174 insertions(+)
 create mode 100644 doc/libucontext.scd

diff --git a/doc/libucontext.scd b/doc/libucontext.scd
new file mode 100644
index 0000000..1f6c07a
--- /dev/null
+++ b/doc/libucontext.scd
@@ -0,0 +1,174 @@
+libucontext(3)
+
+# NAME
+
+libucontext - a library for userspace context swapping
+
+# SYNOPSIS
+
+*#include <libucontext/libucontext.h>*
+
+```
+typedef struct {
+  /* depends on target architecture */
+} libucontext_mcontext_t;
+
+typedef struct {
+  void *ss_sp;
+  int ss_flags;
+  size_t ss_size;
+} libucontext_stack_t;
+
+typedef struct libucontext_ucontext {
+  unsigned int uc_flags;
+  struct libucontext_ucontext *uc_link;
+  libucontext_stack_t uc_stack;
+  libucontext_mcontext_t uc_mcontext;
+} libucontext_ucontext_t;
+```
+
+*int libucontext_getcontext(libucontext_ucontext_t* \*_ucp_*);*
+
+*int libucontext_setcontext(const libucontext_ucontext_t* \*_ucp_*);*
+
+*void libucontext_makecontext(libucontext_ucontext_t* \*_ucp_*, void* _(\*func)()_*, int* _argc_*,* _..._*);*
+
+*int libucontext_swapcontext(libucontext_ucontext_t* \*_oucp_*, const libucontext_ucontext_t* \*_ucp_*);*
+
+# DESCRIPTION
+
+The *libucontext* library provides an implementation of the SysV ucontext functions.  These
+are traditionally used to implement user-space context swapping.  This is achieved by using
+the *libucontext_getcontext*, *libucontext_setcontext*, *libucontext_makecontext* and
+*libucontext_swapcontext* functions as appropriate.
+
+The *libucontext_getcontext* function initializes a structure pointed to by _ucp_ with the
+current user context.
+
+The *libucontext_setcontext* function sets the current user context to the structure pointed
+to by _ucp_.  It discards the current user context.
+
+The *libucontext_swapcontext* function saves the current user context in a structure pointed
+to by _oucp_ and then sets the current user context to the new context in a structure pointed
+to by _ucp_.
+
+The *libucontext_makecontext* function modifies a user context in a structure pointed to by
+_ucp_ to run a function pointed to by _func_ and sets up an argument list of _argc_ values.
+
+# CAVEATS
+
+In SysV, the ucontext functions save and restore signal masks.  The *libucontext* library,
+however, does not.  In practice, this does not usually matter, as users of these functions
+rarely change the signal mask between contexts.
+
+Other implementations may or may not save and restore additional processor registers that
+this implementation does not.  The *libucontext* library only saves and restores the general
+purpose registers.  In practice, this has proven sufficient.
+
+# EXAMPLE
+
+A practical example showing cooperative multithreading.  This program is intended for
+illustrative purpose only and has been written in a way favoring simplicity over performance
+and robustness:
+
+```
+#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;
+}
+```
+
+# AUTHORS
+
+Ariadne Conill <ariadne@dereferenced.org>
+