Extending SpiderMonkey JavaScript on Windows

Originally published on: Thu, 29 Jul 2010

My goal for this afternoon was to be able to compile Mozilla's SpiderMonkey JavaScript engine on Windows. I had tried this a few weeks ago using an older version of Microsoft Visual C++, but I ran into some errors in the build. Once I got the build working, I wanted to try to add a Windows API function to the engine to determine the difficulty of adding further extensions.

Please note that any code snippets that I supply in this article fall under the following copyright:

Copyright 2010 by James K. Lawless. See MIT/X11 license at http://www.mailsend-online.com/wp/license.php.

I created a batch file that would invoke the compile process using the file js.mak.

compy.bat

call nmake -E "CFG=jsshell - Win32 Release" -f js.mak

A number of lines flew by the screen as each C source file was compiled ending in a link-time error message that began with:

link.exe @C:\DOCUME~1\Owner\LOCALS~1\Temp\nma03320. Creating library .\Release/js32.lib and object .\Release/js32.exp jsapi.obj : error LNK2001: unresolved external symbol _js_GeneratorClass jsapi.obj : error LNK2001: unresolved external symbol _js_IteratorClass jsapi.obj : error LNK2001: unresolved external symbol _js_StopIterationClass jsapi.obj : error LNK2001: unresolved external symbol _js_InitIteratorClasses jsobj.obj : error LNK2001: unresolved external symbol _js_InitIteratorClasses ...

This was the same error that I had seen when trying to perform the build from the IDE.

I found that these missing functions were defined in a file called jsiter.c. I could not find reference to jsiter anywhere in js.mak.

I manually compiled jsiter.c using an altered copy of the command-line used to compile most of the other C files:

cl.exe /nologo /MD /W3 /GX /O2 /D "NDEBUG" /D _X86_=1 /D "_WINDOWS" /D "WIN32" /D "XP_WIN" /D "JSFILE" /D "EXPORT_JS_API" /Fp".\Release/js.pch" /I".\Release" /YX /Fo".\Release/" /c "jsiter.c"

Once that was done, I added jsiter.obj by changing the line:

"$(INTDIR)\jsinterp.obj" \

...to...

"$(INTDIR)\jsiter.obj" \ "$(INTDIR)\jsinterp.obj" \

...in two places in js.mak. I then ran the make again. No errors were reported on this build.

In the Release directory, I found an executable named jsshell.exe. When I invoked the JS shell, I was greeted with a "js>" prompt. I tried a quick, one-liner JS snippet at the REPL prompt.

js> for(i=0;i<10;i++) { print(i);} 0 1 2 3 4 5 6 7 8 9 js>

It looked like all was working well.

At the prompt, I entered a call for the help() function to see what options were present in SpiderMonkey.

js> help() JavaScript-C 1.7.0 2007-10-03 Command Usage Description ======= ===== =========== version version([number]) Get or set JavaScript version number options options([option ...]) Get or toggle JavaScript options load load(['foo.js' ...]) Load files named by string arguments readline readline() Read a single line from stdin print print([exp ...]) Evaluate and print expressions help help([name ...]) Display usage and help messages quit quit() Quit the shell gc gc() Run the garbage collector trap trap([fun, [pc,]] exp) Trap bytecode execution untrap untrap(fun[, pc]) Remove a trap line2pc line2pc([fun,] line) Map line number to PC pc2line pc2line(fun[, pc]) Map PC to line number stringsAreUtf8 stringsAreUTF8() Check if strings are UTF-8 encoded testUtf8 testUTF8(mode) Perform UTF-8 tests (modes are 1 to 4) throwError throwError() Throw an error from JS_ReportError build build() Show build date and time clear clear([obj]) Clear properties of object intern intern(str) Internalize str in the atom table clone clone(fun[, scope]) Clone function object seal seal(obj[, deep]) Seal object, or object graph if deep getpda getpda(obj) Get the property descriptors for obj getslx getslx(obj) Get script line extent toint32 toint32(n) Testing hook for JS_ValueToInt32 evalcx evalcx(s[, o]) Evaluate s in optional sandbox object o if (s == '' && !o) return new o with eager standard classes if (s == 'lazy' && !o) return new o with lazy standard classes js>

With the functions documented in the list above, I could use the jsshell program to write filter programs and such using the readline() function to process input and could use print() to process output. I would have to redirect console input into the jsshell process in order to process the input and would likewise have to redirect the output into a file if I wanted to capture the data.

Rhino JavaScript has some very nice interoperability with the Java environment. One has access to most all of the Java API. SpiderMonkey, however, is meant to compile and run on a number of operating-systems, so no ties are provided for the Windows API.

I then tried to add a simple WINAPI function call to the list of built-in functions. I added a function named msgbox() which accepts a single string parameter and displays the string in a popup window via the MessageBoxA API function.

Under the body of the Print() function in js.c, I added the following:

#pragma comment(lib, "user32.lib") extern int __stdcall MessageBoxA(void *,char *,char*,int); static JSBool MsgBox(JSContext *cx, JSObject *obj, uintN argc, jsval *argv, jsval *rval) { JSString *str; str = JS_ValueToString(cx, argv[0]); if (!str) { return JS_FALSE; } MessageBoxA(NULL,JS_GetStringBytes(str),"JavaScript",0); return JS_TRUE; }

In the table of functions, I changed the line:

{"print", Print, 0,0,0},

...to...

{"print", Print, 0,0,0}, {"msgbox", MsgBox, 0,0,0},

I recompiled with no errors using the makefile. I then tested from the REPL using the new version of jsshell.exe:

js> msgbox("How are you?"); js>

After invoking the msgbox() function, I was greeted with the following popup window:

In later experiments, I hope to add a few functions that will allow one to invoke arbitrary WINAPI functions. I'd also like to see if I can embed a script directly into the EXE so that stand-alone programs can be created.

Unless otherwise noted, all code and text entries are Copyright ©2010 by James K. Lawless

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