WebAssembly LinkError: _sprintf function import requires a callable

Question

Creating the WebAssembly.Instance in JS from a Emscripten compiled WASM which included a call to sprintf, results in this error:

Uncaught (in promise) LinkError: WebAssembly.Instance(): Import #1 module="env" function="_sprintf" error: function import requires a callable...

Is sprintf not included by Emscripten as part of libc?

The code:

#include <stdio.h>

extern "C" {
    int main() {
        char buffer [50];
        int n, a=5, b=3;
        n=sprintf (buffer, "%d plus %d is %d", a, b, a+b);

        return 0;
    }
}

The compilation command:

emcc src/test.cpp -O3 -s WASM=1 -s SIDE_MODULE=1 -o out/test.wasm

The emcc compilation runs without error.

Commenting out the sprintf line runs without error returning 0 as expected.

What's the reason for this error and how can it be avoided when using sprintf?

Answer 1

Is sprintf not included by Emscripten as part of libc?

You're compiling with SIDE_MODULE=1 which by definition does not link in system libraries.

You can provide your own implementation of sprintf or stop compiling as a side module and allow emscripten to take care of it for you.