Echo Server

To an Internet Computer app, incoming HTTP requests appear as a Candid blob. This echo server responds to an HTTP request by showing the blob in hex:

https://fchos-eaaaa-aaaae-aaaoq-cai.raw.icp0.io/

The source:

#define WASM_IMPORT(m,n) __attribute__((import_module(m))) __attribute__((import_name(n)));
#define WASM_EXPORT(n) asm(n) __attribute__((visibility("default")))
typedef unsigned u32;
u32 arg_size(void) WASM_IMPORT("ic0", "msg_arg_data_size");
void arg_copy(void *, u32, u32) WASM_IMPORT("ic0", "msg_arg_data_copy");
void reply_append(void *, u32) WASM_IMPORT("ic0", "msg_reply_data_append");
void reply(void) WASM_IMPORT("ic0", "msg_reply");
char hexdig(u32 i) { return i < 10 ? i + '0' : i + 'a' - 10; }
void go() WASM_EXPORT("canister_query http_request");
void go() {
  enum { max = 16384 };
  unsigned char buf[max];
  u32 sz = arg_size();
  sz = sz > max ? max : sz;
  arg_copy(buf, 0, sz);
  char msg[max*2 + 32];
  char *p = msg;
  u32 i;
  for (i = 0; i < sz; i++) {
    *p++ = hexdig(buf[i] / 16);
    *p++ = hexdig(buf[i] % 16);
  }
  char hdr[] = "DIDL\x03\x6c\x03"
    "\xa2\xf5\xed\x88\x04\x01"
    "\xc6\xa4\xa1\x98\x06\x02"
    "\x9a\xa1\xb2\xf9\x0c\x7a\x6d"
    "\x7b\x6d\x6f\x01\x00";
  char ftr[] = "\x00\xc8\x00";
  reply_append(hdr, sizeof(hdr) - 1);
  char leb[5];
  u32 n = 2*sz;
  for (i = 0; i < 4; i++) {
    leb[i] = (n & 127) | 128;
    n >>= 7;
  }
  leb[i] = n;
  reply_append(leb, 5);
  reply_append(msg, 2*sz);
  reply_append(ftr, sizeof(ftr) - 1);
  reply();
}

Download echo.c.

To compile:

$ clang --target=wasm32 -c echo.c
$ wasm-ld --export-dynamic --allow-undefined --no-entry -o echo.wasm echo.c

To deploy:

$ touch nothing
$ echo '{"canisters":{"echo":{"type":"custom","candid":"nothing","wasm":"echo.wasm","build":""}}}' > dfx.json
$ dfx deploy

Echo Server

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