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C + Duktape

Duktape is an embeddable JS engine written in C. It has been ported to a number of exotic architectures and operating systems.

The Standalone scripts can be parsed and evaluated in a Duktape context.

Integration Details​

Initialize Duktape

Duktape does not provide a global variable. It can be created in one line:

/* initialize */
duk_context *ctx = duk_create_heap_default();

/* duktape does not expose a standard "global" by default */
duk_eval_string_noresult(ctx, "var global = (function(){ return this; }).call(null);");

Load SheetJS Scripts

The shim and main libraries can be loaded by reading the scripts from the file system and evaluating in the Duktape context:

/* simple wrapper to read the entire script file */
static duk_int_t eval_file(duk_context *ctx, const char *filename) {
  size_t len;
  /* read script from filesystem */
  FILE *f = fopen(filename, "rb");
  if(!f) { duk_push_undefined(ctx); perror("fopen"); return 1; }
  long fsize; { fseek(f, 0, SEEK_END); fsize = ftell(f); fseek(f, 0, SEEK_SET); }
  char *buf = (char *)malloc(fsize * sizeof(char));
  len = fread((void *) buf, 1, fsize, f);
  fclose(f);
  if(!buf) { duk_push_undefined(ctx); perror("fread"); return 1; }

  /* load script into the context */
  duk_push_lstring(ctx, (const char *)buf, (duk_size_t)len);
  /* eval script */
  duk_int_t retval = duk_peval(ctx);
  /* cleanup */
  duk_pop(ctx);
  return retval;
}

// ...
  duk_int_t res = 0;

  if((res = eval_file(ctx, "shim.min.js")) != 0) { /* error handler */ }
  if((res = eval_file(ctx, "xlsx.full.min.js")) != 0) { /* error handler */ }

To confirm the library is loaded, XLSX.version can be inspected:

  /* get version string */
  duk_eval_string(ctx, "XLSX.version");
  printf("SheetJS library version %s\n", duk_get_string(ctx, -1));
  duk_pop(ctx);

Reading Files​

Duktape supports Buffer natively but should be sliced before processing. Assuming buf is a C byte array, with length len, this snippet parses data:

/* load C char array and save to a Buffer */
duk_push_external_buffer(ctx);
duk_config_buffer(ctx, -1, buf, len);
duk_put_global_string(ctx, "buf");

/* parse with SheetJS */
duk_eval_string_noresult("workbook = XLSX.read(buf.slice(0, buf.length), {type:'buffer'});");

workbook will be a variable in the JS environment that can be inspected using the various SheetJS API functions.

Writing Files​

duk_get_buffer_data can pull Buffer object data into the C code:

/* write with SheetJS using type: "array" */
duk_eval_string(ctx, "XLSX.write(workbook, {type:'array', bookType:'xlsx'})");

/* pull result back to C */
duk_size_t sz;
char *buf = (char *)duk_get_buffer_data(ctx, -1, sz);

/* discard result in duktape */
duk_pop(ctx);

The resulting buf can be written to file with fwrite.

Complete Example​

note

This demo was tested with Duktape 2.7.0 (darwin-x64) on 2023 February 12.

This program parses a file and prints CSV data from the first worksheet. It also generates an XLSB file and writes to the filesystem.

The flow diagram is displayed after the example steps

0) Download and extract Duktape:

mkdir sheetjs-duk
cd sheetjs-duk
curl -LO https://duktape.org/duktape-2.7.0.tar.xz
tar -xJf duktape-2.7.0.tar.xz
mv duktape-2.7.0/src/*.{c,h} .

1) Download the standalone script, shim and test file:

curl -LO https://cdn.sheetjs.com/xlsx-latest/package/dist/shim.min.js
curl -LO https://cdn.sheetjs.com/xlsx-latest/package/dist/xlsx.full.min.js
curl -LO https://sheetjs.com/pres.numbers

2) Download sheetjs.duk.c:

curl -LO https://docs.sheetjs.com/duk/sheetjs.duk.c

3) Compile standalone sheetjs.duk binary

gcc -std=c99 -Wall -osheetjs.duk sheetjs.duk.c duktape.c -lm

4) Run the demo:

./sheetjs.duk pres.numbers

If the program succeeded, the CSV contents will be printed to console and the file sheetjsw.xlsb will be created. That file can be opened with Excel.

Flow Diagram​

Bindings​

Duktape is easily embeddable. Bindings exist for many languages.

Perl​

The Perl binding for Duktape is available on CPAN:

cpan install JavaScript::Duktape

The Perl binding does not have raw Buffer ops, so Base64 strings are used. With the Extendscript build:

# usage: perl SheetJSDuk.pl path/to/file
use JavaScript::Duktape;
use File::Slurp;
use MIME::Base64 qw( encode_base64 decode_base64 );

# Initialize
my $js = JavaScript::Duktape->new( max_memory => 1024 * 1024 * 1024 );
$js->eval("var global = (function(){ return this; }).call(null);");

# Load the ExtendScript build
my $src = read_file('xlsx.extendscript.js', { binmode => ':raw' });
$src =~ s/^\xEF\xBB\xBF//;
my $XLSX = $js->eval($src);

# Print version number
$js->set('log' => sub { print $_[0], "\n"; });
$js->eval("log('SheetJS library version ' + XLSX.version);");

# Parse File
my $raw_data = encode_base64(read_file($ARGV[0], { binmode => ':raw' }), "");
$js->set("b64", $raw_data);
$js->eval(qq{
  global.wb = XLSX.read(b64, {type: "base64"});
  global.ws = wb.Sheets[wb.SheetNames[0]];
});

# Print first worksheet CSV
my $csv = $js->eval('XLSX.utils.sheet_to_csv(global.ws)');
print $csv

# Write XLSB file
my $xlsb = $js->eval("XLSX.write(global.wb, {type:'base64', bookType:'xlsb'})");
write_file("SheetJSDuk.xlsb", decode_base64($xlsb));