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I need to create and implement usage of the struct that contains an array inside it. How to do proper initialization and allocation of the struct and array? Here's my current implementation that seems to work. However, not sure is it proper way and where is the struct allocated (on stack or on heap memory). IMHO a call to malloc makes to store an array on heap, however not sure about remaining struct members. Here are my structs:

typedef struct {

  uint8_t objectSize;
  double objectDepth; 

} ObjectData;

typedef struct {

  ObjectData *objectList;
  double startMark;
  double endMark;

} MyDataPacket;

Here is the function that fills and returns the struct:

void getMyPacket(MyDataPacket *myDataPacket, uint8_t objectNum)
{
  myDataPacket->startMark = 10.0; 
  myDataPacket->endMark = 60.0; 
  myDataPacket->objectList = malloc(objectNum * sizeof(MyDataPacket));

  uint8_t x;
  for (x = 0; x < objectNum; x++)
  { 
    myDataPacket->objectList[x].objectSize = x; // just test values
    myDataPacket->objectList[x].objectDepth = x; // just test values
  }
}

Here's function calling part:

uint8_t objectNum = 10;
MyDataPacket myDataPacket;
getMyPacket(&myDataPacket, objectNum);

Does the struct data is mixed and placed both on stack and heap, or everything resides at one place?

Centurion
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3 Answers3

4

If myDataPacket is allocated on the heap, then it will all be on the heap:

MyDataPacket *myDataPacket = malloc(sizeof(MyDataPacket));

If myDataPacket is an automatic variable, it will all live on the stack, including the objectList pointer itself, the memory it points to however, will be on the heap:

MyDataPacket myDataPacket; //this is all on the stack
myDataPacket->objectList = malloc(...) //memory allocated here is on the heap

If it's a global variable, it depends on whether or not it's initialized, and whether it's initialized to zero or not, but generally speaking uninitialized variables go to the .bss section and initialized variables go to the data section, and of course the dynamically allocated memory will be on the heap as usual.

If you're worried about efficiency check this SO question:

Which is faster: Stack allocation or Heap allocation

Community
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iabdalkader
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2

To answer your question: If myDataPacket is a local variable inside a function, then it will be placed on the stack, while myDataPacket.objectList will be placed on the heap (and also needs to be free'd).

Some programmer dude
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  • So, the struct is just a set of "variables-members" that might be placed on separate places. Just heard that structs are always placed on stack. It seems that it depends on how members are initialized and might have different places. – Centurion Nov 12 '12 at 13:34
  • @Centurion That's correct. If `myDataPacket` was a global variable instead of a local, then it would be stored in yet another segment of memory. And if you had it as a pointer and allocated it with `malloc` then it would be on the heap too, but most likely disjoint from the memory allocated for `objectList`. – Some programmer dude Nov 12 '12 at 13:38
1

Change

myDataPacket->objectList = malloc(objectNum * sizeof(MyDataPacket));

to 

myDataPacket->objectList = malloc(objectNum * sizeof(ObjectData));

That should fix your problem. You are currently allocating a number of MyDataPackets instead of ObjectData's.

Daan Timmer
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