Cannot execute standalone webassembly file with wasmer - c

Question

I wrote a matrix multiplication program in C and compiled it using Emscripten with the following command
emcc matrix.c -o matrix.wasm -s STANDALONE_WASM
And the C program is as follows,
#include <stdlib.h>
#include <time.h>
#include <stdio.h>
int matrix() {
int a[101][101];
int b[101][101];
int r[101][101];
for(int i = 0; i<101; i++) {
for(int j = 0; j<101; j++) {
a[i][j] = rand()%1000+1;
b[i][j] = rand()%1000+1;
}
}
for(int i = 0; i<101; i++) {
for(int j = 0; j<101; j++) {
r[i][j] = 0;
for(int k = 0; k<101; k++) {
r[i][j] += a[i][k] * b[k][j];
}
}
}
return 0;
}
int main(){
clock_t start, finish;
double duration;
start = clock();
matrix();
finish = clock();
duration = (double)(finish - start) / CLOCKS_PER_SEC;
printf("computing duration: %fs\n", duration);
return 0;
}
Then I used wasmer to directly run this webassembly file:
wasmer matrix.wasm.
It returned the expected result. Then I wanted to execute this file with a specific function, that is the export function in webassembly file.
I used wasm2wat to translate this executable file into a wat file.Then I found (export "_start" (func 6)). By the way, I did not found any code about export the matrix function. Then I executed the matrix.wasm with the command:
wasmer matrix.wasm -i _start
However, the error appeared. It said:
error: failed to run `matrix.wasm`
╰─> 1: Error while importing "wasi_snapshot_preview1"."clock_time_get": unknown import. Expected Function(FunctionType { params: [I32, I64, I32], results: [I32] })
Then I tried Rust to write a simple program, which just contained a main function and a add function. I used cargo to compile it to two kinds of targets, which is wasm32-unknown-unknown and wasm32-wasi. I compiled them into wat file. This time I found (export "add" (func $add.command_export)). When I executed the wasm32-wasi programm with
wasmer add.wasm -i add
There was also errors appearing. It said:
error: failed to run `hello.wasm`
╰─> 1: Error while importing "wasi_snapshot_preview1"."args_get": unknown import. Expected Function(FunctionType { params: [I32, I32], results: [I32] })
I could execute the file whose target was wasm32-unknown-unknown correctly but I could not use lib function in this kind of target.
I think there are something wrong with my wasm32-wasi file but I do not know why it is and how to deal with it. Could you please tell me how I can call an export function in wasm32-wasi file and how I can call a lib function in wasm32-unknown-unknown file. Also I have some questions about why I use Emscripten compile the C file but the matrix function does not export in wat file. Thank you!

Answer 1

Compilers will often inline functions and remove code that isn't used, this is why your C program ends up with everything inside a _start function. As explained in the FAQ you may list functions to export using emcc -s EXPORTED_FUNCTIONS=_main,_matrix in order to prevent them from being inlined or removed. Adding this results in a wasm module with the function correctly exported.
As for running functions directly, the source code for wasmer run has logic to determine which runtime environment should be exposed to the module. However, if you pass -i function, it entirely skips the environment setup and runs your function directly. In this case, the modules fails to initialize because it imports functions from WASI (in order to write things to the console, and get the current clock time).
I believe the reason why wasm32-unknown-unknown works is that it doesn't link to any runtime, and implements dummy interfaces for things that it can't simulate (all filesystem calls result in errors, etc.)
In summary, wasmer run -i function isn't meant to run functions from modules that have imports, it might be possible to patch wasmer-cli for that, but I'm not sure if it would work across all runtime environments.