Add Emscripten backend for WebAssembly + WebUSB support

Fixes libusb#501
Closes libusb#1057

.gitignore

@@ -6,6 +6,9 @@ Makefile.in
 *.la
 *.lo
 *.o
+*.js
+*.wasm
+*.html
 libtool
 ltmain.sh
 missing

README

@@ -5,7 +5,8 @@
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/2180/badge.svg)](https://scan.coverity.com/projects/libusb-libusb)
 
 libusb is a library for USB device access from Linux, macOS,
-Windows, OpenBSD/NetBSD, Haiku and Solaris userspace.
+Windows, OpenBSD/NetBSD, Haiku, Solaris userspace, and WebAssembly
+via WebUSB.
 It is written in C (Haiku backend in C++) and licensed under the GNU
 Lesser General Public License version 2.1 or, at your option, any later
 version (see [COPYING](COPYING)).

configure.ac

@@ -83,6 +83,11 @@ case $host in
  backend=haiku
  platform=posix
  ;;
+wasm32-**)
+ AC_MSG_RESULT([Emscripten])
+ backend=emscripten
+ platform=posix
+ ;;
 *-linux* | *-uclinux*)
  dnl on Android Linux, some functions are in different places
  case $host in
@@ -138,7 +143,11 @@ esac
 if test "x$platform" = xposix; then
  AC_DEFINE([PLATFORM_POSIX], [1], [Define to 1 if compiling for a POSIX platform.])
  AC_CHECK_TYPES([nfds_t], [], [], [[#include <poll.h>]])
- AC_CHECK_FUNCS([pipe2])
+ if test "x$backend" != xemscripten; then
+ # pipe2 is detected as present on Emscripten, but it isn't actually ported and always
+ # returns an error. https://github.com/emscripten-core/emscripten/issues/14824
+ AC_CHECK_FUNCS([pipe2])
+ fi
  dnl Some compilers do not support the '-pthread' option so check for it here
  saved_CFLAGS="${CFLAGS}"
  CFLAGS="-Wall -Werror -pthread"
@@ -217,6 +226,11 @@ windows)
  AC_DEFINE([_WIN32_WINNT], [_WIN32_WINNT_VISTA], [Define to the oldest supported Windows version.])
  LT_LDFLAGS="${LT_LDFLAGS} -avoid-version -Wl,--add-stdcall-alias"
  ;;
+emscripten)
+ AC_SUBST(EXEEXT, [.html])
+ # Note: LT_LDFLAGS is not enough here because we need link flags for executable.
+ AM_LDFLAGS="${AM_LDFLAGS} --bind -s ASYNCIFY -s ASSERTIONS -s ALLOW_MEMORY_GROWTH -s INVOKE_RUN=0 -s EXPORTED_RUNTIME_METHODS=['callMain']"
+ ;;
 *)
  dnl no special handling required
  ;;
@@ -372,6 +386,7 @@ AM_CONDITIONAL([OS_NULL], [test "x$backend" = xnull])
 AM_CONDITIONAL([OS_OPENBSD], [test "x$backend" = xopenbsd])
 AM_CONDITIONAL([OS_SUNOS], [test "x$backend" = xsunos])
 AM_CONDITIONAL([OS_WINDOWS], [test "x$backend" = xwindows])
+AM_CONDITIONAL([OS_EMSCRIPTEN], [test "x$backend" = xemscripten])
 AM_CONDITIONAL([PLATFORM_POSIX], [test "x$platform" = xposix])
 AM_CONDITIONAL([PLATFORM_WINDOWS], [test "x$platform" = xwindows])
 AM_CONDITIONAL([USE_UDEV], [test "x$use_udev" = xyes])
@@ -399,6 +414,7 @@ AM_CXXFLAGS="-std=${c_dialect}++11 ${EXTRA_CFLAGS} ${SHARED_CFLAGS} -Wmissing-de
 AC_SUBST(AM_CXXFLAGS)
 
 AC_SUBST(LT_LDFLAGS)
+AC_SUBST(AM_LDFLAGS)
 
 AC_SUBST([EXTRA_LDFLAGS])
 

libusb/Makefile.am

@@ -20,6 +20,7 @@ OS_DARWIN_SRC = os/darwin_usb.h os/darwin_usb.c
 OS_HAIKU_SRC = os/haiku_usb.h os/haiku_usb_backend.cpp \
  os/haiku_pollfs.cpp os/haiku_usb_raw.h os/haiku_usb_raw.cpp
 OS_LINUX_SRC = os/linux_usbfs.h os/linux_usbfs.c
+OS_EMSCRIPTEN_SRC = os/emscripten_webusb.cpp
 OS_NETBSD_SRC = os/netbsd_usb.c
 OS_NULL_SRC = os/null_usb.c
 OS_OPENBSD_SRC = os/openbsd_usb.c
@@ -48,6 +49,12 @@ OS_SRC += os/linux_netlink.c
 endif
 endif
 
+if OS_EMSCRIPTEN
+noinst_LTLIBRARIES = libusb_emscripten.la
+libusb_emscripten_la_SOURCES = $(OS_EMSCRIPTEN_SRC)
+libusb_1_0_la_LIBADD = libusb_emscripten.la
+endif
+
 if OS_NETBSD
 OS_SRC = $(OS_NETBSD_SRC)
 endif