CODEWITHSUNDEEP
javascriptc++cwebassemblyemscripten
Zack Lee
Zack Lee

Reputation: 3044

EM_JS: How to convert int argc, char **argv to a JS array of strings?

I'm trying to call JS function from C/C++ with an array of strings as an argument.

Here's my example code:

main.c:

#include <stdio.h>
#include <emscripten.h>

EM_JS(void, call_myFunc, (int argc, char **argv), {
     
    var arr = [];
    for (let i = 0; i < argc; i++) {
      arr.push(UTF8ToString(argv[i]));
    }
    myFunc(arr);
});

int main()
{
    int argc = 4;
    char *argv[5] = { "ab", "cd", "ef", "gh" };
    call_myFunc(argc, argv);
}

index.html:

<!DOCTYPE html>
<html>

<head>
</head>

<body>
  <script>
  function myFunc(args) {
    console.log(args);
  }
  </script>
  <script async src="main.js"></script>
</body>

</html>

build:

emcc main.c -o main.html

result I get:

["", "", "", ""]

result I want:

["ab", "cd", "ef", "gh"]

How can I properly convert char **argv to a JS array of strings?

Upvotes: 2

Views: 938

Answers (1)

Azeem
Azeem

Reputation: 14607

How can I properly convert char **argv to a JS array of strings?

From the documentation of emscripten.h:

Null-terminated C strings can also be passed into EM_JS functions, but to operate on them, they need to be copied out from the heap to convert to high-level JavaScript strings.

EM_JS(void, say_hello, (const char* str), {
  console.log('hello ' + UTF8ToString(str));
}

In the same manner, pointers to any type (including void *) can be passed inside EM_JS code, where they appear as integers like char * pointers above did. Accessing the data can be managed by reading the heap directly.

EM_JS(void, read_data, (int* data), {
  console.log('Data: ' + HEAP32[data>>2] + ', ' + HEAP32[(data+4)>>2]);
});

int main() {
  int arr[2] = { 30, 45 };
  read_data(arr);
  return 0;
}

So, you can use HEAP32 along with UTF8ToString like this:

main.c

#include <stdio.h>
#include <emscripten.h>

EM_JS(void, call_myFunc, (const int argc, const char** argv), {
    var arr = [];
    for (let i = 0; i < argc; i++) {
        const mem = HEAP32[(argv + (i * 4)) >> 2];
        const str = UTF8ToString(mem);
        arr.push(str);
    }
    console.log(arr);
});

int main() {
    const int argc = 4;
    const char* argv[] = { "ab", "cd", "ef", "gh" };
    call_myFunc(argc, argv);
    return 0;
}

Compile:

emcc main.c -o main.html

Run with node:

node main.js

Output:

[ 'ab', 'cd', 'ef', 'gh' ]

Upvotes: 4

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