Unfortunately the compiled C code can only access the single Emscripten HEAP, so all fat data should live there in the first place in order to avoid copies, like Brion also suggested. Your code already allocates memory from the HEAP and generates a view to it, so in JS side this would amount to reusing that view when generating the data in the original source.
2015-03-06 17:06 GMT+02:00 Brion Vibber <[email protected]>: > Depending on your needs, you might consider creating an aliased Uint8Array > view into the heap-allocated array and using that directly on the > JavaScript side. You'll need a way to control the lifetime of the pointer > on the C side so this can be more complex but saves you the copies. This > can make a big performance difference when pushing WebGL textures for > instance. > > -- brion > On Mar 6, 2015 3:14 AM, "Stefano Sabatini" <[email protected]> wrote: > >> Hi, >> >> I want to call a C function dealing with arrays from Javascript. >> >> The function is created with emscripten. I also want to create JS >> functions handling native JavaScript arrays, for example a >> Uint8Array. >> >> From my understanding I have to create a JS wrapper allocating an >> array in the heap, copy the data from the JS array, process the data >> through the emscripten generated function, and finally copy the >> processed data back to a JS array. >> >> Consider this example: >> >> C function: >> void multiply_array(uint8_t factor, uint8_t *buf, size_t buf_size) >> { >> for (int i = 0; i < buf_size; i++) { >> buf[i] = factor * buf[i]; >> } >> } >> >> JS wrapper: >> multiply_by_factor = function(u8array, factor) { >> // need to get the array pointer, u8array is Uint8Array >> >> // Get data byte size, allocate memory on Emscripten heap, and get >> pointer >> var dataPtr = Module._malloc(u8array.length); >> >> // allocate new Uint8Array in the Emscripten heap (directly accessed >> from Module.HEAPU8) >> var dataHeap = new Uint8Array(Module.HEAPU8.buffer, dataPtr, >> u8array.length); >> >> // copy data to the heap >> dataHeap.set(u8array); >> >> // Call function and process data inplace >> _multiply_array(factor, dataPtr, u8array.length); >> var result = new Uint8Array(u8array.length); >> result.set(dataHeap); >> >> // Free memory from emscripten heap >> Module._free(dataHeap.byteOffset); >> return result; >> } >> >> This function is working fine, but I wonder if it's possible to avoid >> to allocate data on the emscripten heap, in order to avoid the two >> memcpy calls. >> >> TIA >> >> -- >> You received this message because you are subscribed to the Google Groups >> "emscripten-discuss" group. >> To unsubscribe from this group and stop receiving emails from it, send an >> email to [email protected]. >> For more options, visit https://groups.google.com/d/optout. >> > -- > You received this message because you are subscribed to the Google Groups > "emscripten-discuss" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "emscripten-discuss" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. For more options, visit https://groups.google.com/d/optout.
