docs.sheetjs.com/docz/docs/03-demos/31-engines/01_duktape.md

---
title: C + Duktape
pagination_prev: demos/cli
pagination_next: demos/clipboard
---

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](/docs/getting-started/installation/standalone) 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:

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

/* duktape does not expose a standard "global" by default */
// highlight-next-line
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:

```c
/* 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; }

  // highlight-start
  /* 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);
  // highlight-end
  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:

```c
  /* 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:

```c
/* 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:

```c
/* 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](#flow-diagram)

0) Download and extract Duktape:

```bash
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:

<ul>
<li><a href={`https://cdn.sheetjs.com/xlsx-latest/package/dist/shim.min.js`}>shim.min.js</a></li>
<li><a href={`https://cdn.sheetjs.com/xlsx-latest/package/dist/xlsx.full.min.js`}>xlsx.full.min.js</a></li>
<li><a href="https://sheetjs.com/pres.numbers">pres.numbers</a></li>
</ul>

```bash
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`](pathname:///duk/sheetjs.duk.c):

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

3) Compile standalone `sheetjs.duk` binary

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

4) Run the demo:

```bash
./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

```mermaid
sequenceDiagram
  participant F as Filesystem
  participant C as C Code
  participant D as Duktape
  activate C
  opt
    Note over F,D: ~ Prepare Duktape ~
    C->>+D: Initialize
    deactivate C
    D->>-C: Done
    activate C
    C->>F: Need SheetJS
    F->>C: SheetJS Code
    C->>+D: Load SheetJS Code
    deactivate C
    D->>-C: Loaded
    activate C
    C->>+D: Execute Code
    deactivate C
    Note over D: Eval SheetJS Code
    D->>-C: Done
    activate C
    Note over D: XLSX<br/>ready to rock
  end
  opt
    Note over F,D: ~ Parse File ~
    C->>F: Read Spreadsheet
    F->>C: Spreadsheet File
    C->>+D: Load Data
    deactivate C
    D->>-C: Loaded
    activate C
    C->>+D: eval `var workbook = XLSX.read(...)`
    deactivate C
    Note over D: Parse File
    D->>-C: Done
    activate C
    Note over D: `workbook`<br/>can be used later
  end
  opt
    Note over F,D: ~ Print CSV to screen ~
    C->>+D: eval `XLSX.utils.sheet_to_csv(...)`
    deactivate C
    Note over D: Generate CSV
    D->>-C: CSV Data
    activate C
    Note over C: Print to standard output
  end
  opt
    Note over F,D: ~ Write XLSB File ~
    C->>+D: eval `XLSX.write(...)`
    deactivate C
    Note over D: Generate File
    D->>-C: done
    activate C
    C->>+D: get file bytes
    deactivate C
    D->>-C: binary data
    activate C
    C->>F: Write File
  end
  deactivate C
```