[wasm2c] Add rot13 example (#1384)

This example demonstrates how to use imported functions. The `rot13`
program takes each command line argument, and rot13-encodes it.

The exported `rot13` function has no arguments, and instead calls back
into the program (via `fill_buf`) with a buffer to fill in. When the
function finishes it calls `buf_done`.

(rot13.wat is the same as in src/test-interp.cc.)

1 files changed, 93 insertions, 0 deletions

diff --git a/wasm2c/examples/rot13/main.c b/wasm2c/examples/rot13/main.c
new file mode 100644
index 00000000..f99ee014
--- /dev/null
+++ b/wasm2c/examples/rot13/main.c
@@ -0,0 +1,93 @@
+/* Entry point for the rot13 example.
+ *
+ * This example shows how you can fulfill wasm module imports in your C
+ * program, and access linear memory.
+ *
+ * The program reads arguments from the command line, and [rot13] encodes them,
+ * e.g.:
+ *
+ * ```
+ * $ rot13 foo bar
+ * foo -> sbb
+ * bar -> one
+ * ```
+ *
+ * [rot13]: https://en.wikipedia.org/wiki/ROT13
+ */
+#include <stdio.h>
+#include <stdlib.h>
+
+/* Uncomment this to define rot13_init rot13_Z_rot13Z_vv instead. */
+/* #define WASM_RT_MODULE_PREFIX rot13_ */
+
+#include "rot13.h"
+
+/* Define the imports as declared in rot13.h. */
+wasm_rt_memory_t (*Z_hostZ_mem);
+u32 (*Z_hostZ_fill_bufZ_iii)(u32, u32);
+void (*Z_hostZ_buf_doneZ_vii)(u32, u32);
+
+/* Define the implementations of the imports. */
+static wasm_rt_memory_t s_memory;
+static u32 fill_buf(u32 ptr, u32 size);
+static void buf_done(u32 ptr, u32 size);
+
+/* The string that is currently being processed. This needs to be static
+ * because the buffer is filled in the callback. */
+static const char* s_input;
+
+int main(int argc, char** argv) {
+  /* Initialize the rot13 module. Since we didn't define WASM_RT_MODULE_PREFIX,
+  the initialization function is called `init`. */
+  init();
+
+  /* Allocate 1 page of wasm memory (64KiB). */
+  wasm_rt_allocate_memory(&s_memory, 1, 1);
+
+  /* Provide the imports expected by the module: "host.mem", "host.fill_buf"
+   * and "host.buf_done". Their mangled names are `Z_hostZ_mem`,
+   * `Z_hostZ_fill_bufZ_iii` and `Z_hostZ_buf_doneZ_vii`. */
+  Z_hostZ_mem = &s_memory;
+  Z_hostZ_fill_bufZ_iii = &fill_buf;
+  Z_hostZ_buf_doneZ_vii = &buf_done;
+
+  /* Call `rot13` on each argument, using the mangled name. */
+  while (argc > 1) {
+    /* Move to next arg. Do this first, so the program name is skipped. */
+    argc--; argv++;
+
+    s_input = argv[0];
+    Z_rot13Z_vv();
+  }
+  return 0;
+}
+
+/* Fill the wasm buffer with the input to be rot13'd.
+ *
+ * params:
+ *   ptr: The wasm memory address of the buffer to fill data.
+ *   size: The size of the buffer in wasm memory.
+ * result:
+ *   The number of bytes filled into the buffer. (Must be <= size).
+ */
+u32 fill_buf(u32 ptr, u32 size) {
+  for (size_t i = 0; i < size; ++i) {
+    if (s_input[i] == 0) {
+      return i;
+    }
+    s_memory.data[ptr + i] = s_input[i];
+  }
+  return size;
+}
+
+/* Called when the wasm buffer has been rot13'd.
+ *
+ * params:
+ *   ptr: The wasm memory address of the buffer.
+ *   size: The size of the buffer in wasm memory.
+ */
+void buf_done(u32 ptr, u32 size) {
+  /* The output buffer is not necessarily null-terminated, so use the %*.s
+   * printf format to limit the number of characters printed. */
+  printf("%s -> %.*s\n", s_input, (int)size, &s_memory.data[ptr]);
+}