Can I read files from disk using Webassembly?

Question

I followed the website assembly tutorial http://webassembly.org/getting-started/developers-guide/

It worked great and displayed "Hello world!" message in the browser.

Then I tried a small C ++ code that opens a text file and performs the calculation (10 * 20) after reading the file.

emcc compiled the file just fine, no errors.

But when I serve the file via HTTP by running emrun, it cannot open the file.

This is what I see on the emrun web console:

Unable to open file 200 

Are there any restrictions on opening files from a local drive?

  [ thiago@terra hello]$ cat pfile.cpp #include <iostream> #include <fstream> #include <string> using namespace std; int main() { string line; int a, b, c; ifstream myfile("test.txt"); if (myfile.is_open()) { while (getline (myfile, line)) { cout << line << endl; } myfile.close(); } else cout << "Unable to open file" << endl; a = 10; b = 20; c = a * b; cout << c << endl; return 0; } [ thiago@terra hello]$ emcc pfile.cpp -s WASM=1 -o pfile.html -v INFO:root:(Emscripten: Running sanity checks) clang version 4.0.0 (https://github.com/kripken/emscripten-fastcomp-clang.git c7c210fee24e0227f882337521b25b1ed9c36d5b) (https://github.com/kripken/emscripten-fastcomp.git 90b726ede4acf47c1bca089de6c79a0b8f2c5d9a) (emscripten 1.37.18 : 1.37.18) Target: asmjs-unknown-emscripten Thread model: posix InstalledDir: /home/thiago/Downloads/emsdk/clang/fastcomp/build_incoming_64/bin "/home/thiago/Downloads/emsdk/clang/fastcomp/build_incoming_64/bin/clang-4.0" -cc1 -triple asmjs-unknown-emscripten -emit-llvm-bc -emit-llvm-uselists -disable-free -main-file-name pfile.cpp -mrelocation-model static -mthread-model posix -mdisable-fp-elim -no-integrated-as -mconstructor-aliases -v -dwarf-column-info -debugger-tuning=gdb -coverage-notes-file /tmp/tmpV3VHOz/pfile_0.gcno -nostdsysteminc -nobuiltininc -resource-dir /home/thiago/Downloads/emsdk/clang/fastcomp/build_incoming_64/bin/../lib/clang/4.0.0 -D __EMSCRIPTEN_major__=1 -D __EMSCRIPTEN_minor__=37 -D __EMSCRIPTEN_tiny__=18 -D _LIBCPP_ABI_VERSION=2 -Werror=implicit-function-declaration -std=c++03 -fdeprecated-macro -fno-dwarf-directory-asm -fdebug-compilation-dir /home/thiago/hello -ferror-limit 19 -fmessage-length 164 -fobjc-runtime=gnustep -fcxx-exceptions -fexceptions -fdiagnostics-show-option -nobuiltininc -nostdsysteminc -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include/libcxx -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/lib/libcxxabi/include -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include/compat -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include/SSE -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include/libc -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/lib/libc/musl/arch/emscripten -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/local/include -isystem/home/thiago/Downloads/emsdk/emscripten/incoming/system/include/SDL -o /tmp/tmpV3VHOz/pfile_0.o -x c++ pfile.cpp clang -cc1 version 4.0.0 based upon LLVM 4.0.0 default target x86_64-unknown-linux-gnu #include "..." search starts here: #include <...> search starts here: /home/thiago/Downloads/emsdk/emscripten/incoming/system/include/libcxx /home/thiago/Downloads/emsdk/emscripten/incoming/system/lib/libcxxabi/include /home/thiago/Downloads/emsdk/emscripten/incoming/system/include/compat /home/thiago/Downloads/emsdk/emscripten/incoming/system/include /home/thiago/Downloads/emsdk/emscripten/incoming/system/include/SSE /home/thiago/Downloads/emsdk/emscripten/incoming/system/include/libc /home/thiago/Downloads/emsdk/emscripten/incoming/system/lib/libc/musl/arch/emscripten /home/thiago/Downloads/emsdk/emscripten/incoming/system/local/include /home/thiago/Downloads/emsdk/emscripten/incoming/system/include/SDL End of search list. [ thiago@terra hello]$ emrun --no_browser --port 8080 . 

Answer 1

Keep Safe - WebAssembly is specified to run in a secure, isolated runtime. Like other web codes, it will have policies of the same origin and browser permissions.

So, the short answer is yes, there are limitations. You do not have access to files on disks. You only have a block of memory, WASM code can be called from JS, and WASM can call JS functions.

But there is one interesting feature in Emscripten - in WASM you can have your own "virtual" file system with files. You can use it to “attach” some const files at compile time and read them at run time. See https://kripken.imtqy.com/emscripten-site/docs/api_reference/Filesystem-API.html