# SDL: emscripten: Let SDL hints be set by URL parameters.

**URL:** <https://discourse.libsdl.org/t/sdl-emscripten-let-sdl-hints-be-set-by-url-parameters/52486>\
**Category:** SDL Commits\
**Created:** [July 4, 2024, 3:10pm UTC](https://discourse.libsdl.org/t/sdl-emscripten-let-sdl-hints-be-set-by-url-parameters/52486 "2024-07-04T15:10:02Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![SDLPushPostBot](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/sdlpushpostbot/32/4370_2.png) [@SDLPushPostBot](https://discourse.libsdl.org/u/SDLPushPostBot)\
**Post date:** [July 4, 2024, 3:10pm UTC](https://discourse.libsdl.org/t/sdl-emscripten-let-sdl-hints-be-set-by-url-parameters/52486/1 "2024-07-04T15:10:02Z")

</div>

> <https://github.com/libsdl-org/SDL/commit/f338fa20dd8114d0a656305a8566a74c7f63d3db>
>
> Any parameters (key/value pairs after the '?' in a URL) that have a keyname
> that… starts with \`SDL\_\` will be put into Emscripten's environment variable
> emulation table at startup, before SDL\_main runs.
> 
> This lets users set hints the same way they might set them from a shell's
> command line on a desktop platform:
> 
> For example:
> 
> \`https://example.com/my\_sdl3\_application.html?SDL\_RENDER\_DRIVER=software\`
> 
> Fixes #10154.

```diff
From f338fa20dd8114d0a656305a8566a74c7f63d3db Mon Sep 17 00:00:00 2001
From: "Ryan C. Gordon" <[EMAIL REDACTED]>
Date: Wed, 3 Jul 2024 20:05:43 -0400
Subject: [PATCH] emscripten: Let SDL hints be set by URL parameters.

Any parameters (key/value pairs after the '?' in a URL) that have a keyname
that starts with `SDL_` will be put into Emscripten's environment variable
emulation table at startup, before SDL_main runs.

This lets users set hints the same way they might set them from a shell's
command line on a desktop platform:

For example:

`https://example.com/my_sdl3_application.html?SDL_RENDER_DRIVER=software`

Fixes #10154.
---
 CMakeLists.txt | 2 +
 include/SDL3/SDL_main.h | 10 +++++
 src/core/emscripten/SDL_emscripten.c | 56 ++++++++++++++++++++++++++++
 3 files changed, 68 insertions(+)
 create mode 100644 src/core/emscripten/SDL_emscripten.c

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0472fd8dc7d4b..a7f342579d07d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1480,6 +1480,8 @@ elseif(EMSCRIPTEN)
   # project. Uncomment at will for verbose cross-compiling -I/../ path info.
   sdl_compile_options(PRIVATE "-Wno-warn-absolute-paths")
 
+ sdl_glob_sources("${SDL3_SOURCE_DIR}/src/core/emscripten/*.c")
+
   sdl_glob_sources("${SDL3_SOURCE_DIR}/src/main/emscripten/*.c")
   set(HAVE_SDL_MAIN_CALLBACKS TRUE)
 
diff --git a/include/SDL3/SDL_main.h b/include/SDL3/SDL_main.h
index e293cc5054715..cd6a0b332f35a 100644
--- a/include/SDL3/SDL_main.h
+++ b/include/SDL3/SDL_main.h
@@ -94,6 +94,16 @@
         /* We need to export SDL_main so it can be launched from Java */
         #define SDLMAIN_DECLSPEC SDL_DECLSPEC
 
+ #elif defined(SDL_PLATFORM_EMSCRIPTEN)
+ /* On Emscripten, SDL provides a main function that converts URL
+ parameters that start with "SDL_" to environment variables, so
+ they can be used as SDL hints, etc.
+
+ This is 100% optional, so if you don't want this to happen, you may
+ define SDL_MAIN_HANDLED
+ */
+ #define SDL_MAIN_AVAILABLE
+
     #elif defined(SDL_PLATFORM_PSP)
         /* On PSP SDL provides a main function that sets the module info,
            activates the GPU and starts the thread required to be able to exit
diff --git a/src/core/emscripten/SDL_emscripten.c b/src/core/emscripten/SDL_emscripten.c
new file mode 100644
index 0000000000000..6eda160962579
--- /dev/null
+++ b/src/core/emscripten/SDL_emscripten.c
@@ -0,0 +1,56 @@
+/*
+ Simple DirectMedia Layer
+ Copyright (C) 1997-2024 Sam Lantinga <slouken@libsdl.org>
+
+ This software is provided 'as-is', without any express or implied
+ warranty. In no event will the authors be held liable for any damages
+ arising from the use of this software.
+
+ Permission is granted to anyone to use this software for any purpose,
+ including commercial applications, and to alter it and redistribute it
+ freely, subject to the following restrictions:
+
+ 1. The origin of this software must not be misrepresented; you must not
+ claim that you wrote the original software. If you use this software
+ in a product, an acknowledgment in the product documentation would be
+ appreciated but is not required.
+ 2. Altered source versions must be plainly marked as such, and must not be
+ misrepresented as being the original software.
+ 3. This notice may not be removed or altered from any source distribution.
+*/
+#include "SDL_internal.h"
+
+#ifdef SDL_PLATFORM_EMSCRIPTEN
+
+#include <emscripten/emscripten.h>
+
+EM_JS_DEPS(sdlrunapp, "$dynCall,$stringToNewUTF8");
+
+int SDL_RunApp(int argc, char* argv[], SDL_main_func mainFunction, void * reserved)
+{
+ (void)reserved;
+
+ // Move any URL params that start with "SDL_" over to environment
+ // variables, so the hint system can pick them up, etc, much like a user
+ // can set them from a shell prompt on a desktop machine. Ignore all
+ // other params, in case the app wants to use them for something.
+ MAIN_THREAD_EM_ASM({
+ var parms = new URLSearchParams(window.location.search);
+ for (const [key, value] of parms) {
+ if (key.startsWith("SDL_")) {
+ var ckey = stringToNewUTF8(key);
+ var cvalue = stringToNewUTF8(value);
+ if ((ckey != 0) && (cvalue != 0)) {
+ //console.log("Setting SDL env var '" + key + "' to '" + value + "' ...");
+ dynCall('iiii', $0, [ckey, cvalue, 1]);
+ }
+ _free(ckey); // these must use free(), not SDL_free()!
+ _free(cvalue);
+ }
+ }
+ }, SDL_setenv);
+
+ return mainFunction(argc, argv);
+}
+
+#endif

```