How to use unpacked data msgpack-c

Question

I know how to unpack data but don't know how to use specific data from array or map, Here's the code :

#include <msgpack.h>
#include <stdio.h>

int main(void) {
  /* msgpack::sbuffer is a simple buffer implementation. */
  msgpack_sbuffer sbuf;
  msgpack_sbuffer_init(&sbuf);

  /* serialize values into the buffer using msgpack_sbuffer_write callback
   * function. */
  msgpack_packer pk;
  msgpack_packer_init(&pk, &sbuf, msgpack_sbuffer_write);

  msgpack_pack_array(&pk, 3);
  msgpack_pack_int(&pk, 1);
  msgpack_pack_true(&pk);
  msgpack_pack_str(&pk, 7);
  msgpack_pack_str_body(&pk, "example", 7);

  msgpack_unpacked result;
  size_t off = 0;
  msgpack_unpack_return ret;
  int i = 0;

  msgpack_unpacked_init(&result);
  ret = msgpack_unpack_next(&result, sbuf.data, sbuf.size, &off);
  while (ret == MSGPACK_UNPACK_SUCCESS) {
    msgpack_object obj = result.data;
    printf("Object on %d:\n", ++i);
    msgpack_object_print(stdout, obj);
    printf("\n");
    ret = msgpack_unpack_next(&result, sbuf.data, sbuf.size, &off);
  }
  msgpack_unpacked_destroy(&result);

  return 0;
}

How can I store the first index of the array in a variable ?

Answer 1

msgpack-c (C version) document is here: https://github.com/msgpack/msgpack-c/wiki/v2_0_c_overview#object

In your case, the variable obj's type is msgpack_object.

typedef struct msgpack_object {
    msgpack_object_type type;
    msgpack_object_union via;
} msgpack_object;

msgpack_object_union is defined as follows:

typedef union {
    bool boolean;
    uint64_t u64;
    int64_t  i64;
#if defined(MSGPACK_USE_LEGACY_NAME_AS_FLOAT)
    double   dec; /* obsolete*/
#endif /* MSGPACK_USE_LEGACY_NAME_AS_FLOAT */
    double   f64;
    msgpack_object_array array;
    msgpack_object_map map;
    msgpack_object_str str;
    msgpack_object_bin bin;
    msgpack_object_ext ext;
} msgpack_object_union;

msgpack_object_array is defined as follows:

typedef struct {
    uint32_t size;
    struct msgpack_object* ptr;
} msgpack_object_array;

obj.type is MSGPACK_OBJECT_ARRAY. You can access array elements using the member via. For example, you can access the first index of the array obj.via.array.ptr[0].

When you pack as follows:

  msgpack_pack_array(&pk, 3);
  msgpack_pack_int(&pk, 1);
  msgpack_pack_true(&pk);
  msgpack_pack_str(&pk, 7);
  msgpack_pack_str_body(&pk, "example", 7);

obj has the following values:

---

• obj.via.array.size is 3.

---

• obj.via.array.ptr[0].type is MSGPACK_OBJECT_POSITIVE_INTEGER.
• obj.via.array.ptr[0].via.u64 is 1.

---

• obj.via.array.ptr[1].type is MSGPACK_OBJECT_BOOLEAN.
• obj.via.array.ptr[1].via.bool is true.

---

• obj.via.array.ptr[2].type is MSGPACK_OBJECT_STR.
  • obj.via.array.ptr[2].via.str.ptr is "example" (The address of the memory that the first charactor e is stored).
  • obj.via.array.ptr[2].via.str.size is 7.

---

How can I store the first index of the array in a variable ?

So you can do that as follows:

uint64_t store_value = obj.via.array.ptr[0].via.u64;