C equivalent of a Pyrenean cucumber (object serialization)?

Question

What is C equivalent to this python code? Thank you

data = gather_me_some_data()
# where data = [ (metic, datapoints), ... ]
# and datapoints = [ (timestamp, value), ... ]

serialized_data = cPickle.dumps(data, protocol=-1)
length_prefix = struct.pack("!L", len(serialized_data))
message = length_prefix + serialized_data

Answer 1

C does not support the direct serialization mechanism, since in C you cannot get type information at runtime. You must enter the type information at runtime yourself, and then build the required object from this information. Therefore, identify all possible structures:

typedef struct {
  int myInt;
  float myFloat;
  unsigned char myData[MY_DATA_SIZE];
} MyStruct_1;

typedef struct {
  unsigned char myUnsignedChar;
  double myDouble;
} MyStruct_2;

Then, define an enumeration that collects information about what structures you have:

typedef enum {
  ST_MYSTRUCT_1,
  ST_MYSTRUCT_2
} MyStructType;

Define an auxiliary function that allows you to determine any size of the structure:

int GetStructSize(MyStructType structType) {
      switch (structType) {
          case ST_MYSTRUCT_1:
              return sizeof(MyStruct_1);
          case ST_MYSTRUCT_2:
              return sizeof(MyStruct_2);
          default:
              // OOPS no such struct in our pocket
              return 0;
      }
}

Then define the serialization function:

void BinarySerialize(
    MyStructType structType,
    void * structPointer,
    unsigned char * serializedData) {

  int structSize = GetStructSize(structType);

  if (structSize != 0) {
    // copy struct metadata to serialized bytes
    memcpy(serializedData, &structType, sizeof(structType));
    // copy struct itself
    memcpy(serializedData+sizeof(structType), structPointer, structSize);
  }
}

And de-serialization function:

void BinaryDeserialize(
    MyStructType structTypeDestination,
    void ** structPointer,
    unsigned char * serializedData)
{
    // get source struct type
    MyStructType structTypeSource;
    memcpy(&structTypeSource, serializedData, sizeof(structTypeSource));

    // get source struct size
    int structSize = GetStructSize(structTypeSource);

    if (structTypeSource == structTypeDestination && structSize != 0) {
      *structPointer = malloc(structSize);
      memcpy(*structPointer, serializedData+sizeof(structTypeSource), structSize);
    }
}

An example of using serialization:

MyStruct_2 structInput = {0x69, 0.1};
MyStruct_1 * structOutput_1 = NULL;
MyStruct_2 * structOutput_2 = NULL;
unsigned char testSerializedData[SERIALIZED_DATA_MAX_SIZE] = {0};

// serialize structInput
BinarySerialize(ST_MYSTRUCT_2, &structInput, testSerializedData);
// try to de-serialize to something
BinaryDeserialize(ST_MYSTRUCT_1, &structOutput_1, testSerializedData);
BinaryDeserialize(ST_MYSTRUCT_2, &structOutput_2, testSerializedData);
// determine which object was de-serialized
// (plus you will get code-completion support about object members from IDE)
if (structOutput_1 != NULL) {
   // do something with structOutput_1 
   free(structOutput_1);
}
else if (structOutput_2 != NULL) {
   // do something with structOutput_2
   free(structOutput_2);
}

, C. :

• struct , , / .
• endianess - , - , [ char * , enum] (... ).

Answer 2

++, PicklingTools