Thread: help "wrapping" data types in char array

Hi, I've been working on a problem of mine, and I'm currently tearing my hair out...... basically what I'm trying to do is implement a simple client / server protocol in C. My problem lies in the fact that I have a char* (char array) which I use for my buffer to pass to a C function to send it across the socket to the server/client. What I want to do is to fill this char array with structs OR other data types such as floats. Now I thought this would be possible by filling every 4th char in the array with a float (a float is 4 bytes), and then casting it at the other end, but apparently this isn't working. Can anyone help me before I start stomping on my computer?
Many Thanks
Tom

If that is what you have done then it shouldn't be any problem. Did you cast it as a float or a float pointer that you later dereferenced? I can see the possibility of the compiler trying to convert it from an int to a float when casting.
But I can't tell for sure if you're not posting any code.

hmmm, right something very odd is going on... i'll post the important part of my code:

Code:

typedef struct enemyType
{
  char header;
  char playerNo;
  float eyex, eyey, eyez;
} enemyType;

enemyType enemy[MAX_PLAYERS];
char noPlayers;


void writeStuff(char *writeBuffer) {
  char playerCount;
  int currentByte;
  
  writeBuffer[0] = (char) 2;
  currentByte = 1;
  for (playerCount = 0; playerCount < noPlayers; playerCount++) {
    writeBuffer[currentByte++] = playerCount;
    writeBuffer[currentByte]   = enemy[playerCount].eyex;
    currentByte += 4;
    writeBuffer[currentByte]   = enemy[playerCount].eyey;
    currentByte += 4;
    writeBuffer[currentByte]   = enemy[playerCount].eyez;
    currentByte += 4;
  }
}

now it seems in the function which i call this function, i can read the values of writeBuffer[2 / 6 / 10] using the %f notation in printf, and I will get a print value of 0.0000000; however trying this using the %d notation will get the correct truncated, increased by a power of ten!?!? i am really not sure what's going on. Here's I'll put in a initialisation function for you to try

Code:

void testInit(void) {
  noPlayers = 2;
  enemy[0].eyex = 1.0;
  enemy[0].eyey = 1.0;
  enemy[0].eyez = 65.34;
  enemy[0].header = 2;
  enemy[0].playerNo = 0;
  
  enemy[1].eyex = 2.0;
  enemy[1].eyey = 2.0;
  enemy[1].eyez = 2.0;
  enemy[0].header = 2;
  enemy[0].playerNo = 1;
  printf("Test data plugged in\n");
}

Code:

writeBuffer[currentByte]   = enemy[playerCount].eyex;

This should cause a problem, the float is casted to a char. Try outputting the raw data of what you're inserting to the buffer and then outputting the raw data of the buffer and see how it differs.
I think that

Code:

*((float*) &writeBuffer[currentByte])   = enemy[playerCount].eyex;

would do the trick

Code:

currentByte += 4;

Don't do this, even if float always will be 4 bytes it's better to use sizeof(float) instead to eliminate the use of magic numbers.

I don't think that would work would it? Passing the pointer to the array of structures will just be passing a pointer to a pointer which wouldn't transfer the actual data across the network. Correct me if I'm wrong, but that's only my thinking...