The sizeof operator yeilds the number of bytes required to store an object of the type of its operand.

	sizeof(char) 		// = 1
	sizeof(my_array) 		// = size of the array in bytes
	sizeof(my_structure) 	// = size of the structure in bytes
	sizeof(my_union) 		// = size of the union in bytes

	for count = 0; count < (sizeof(my_array) / sizeof(my_array_tye)); count++)
	{
		//Do each entry in the array of a not necessarily known object size

Note that 16 and 32bit compilers may add padd bytes and these will be included in the sizeof – this can be a source of bugs!

Getting array length from sizeof

int Prices[5] = { 1, 2, 3, 4, 5 };

int Length = sizeof(Prices) / sizeof(Prices[0]);

Avoiding Padding

You can used the packed variable attribute. For example in the following code, type c_t will have a size of 3 bytes and there will be no wasted space between a and b:

typedef struct
{
  unsigned char a[1];
} __attribute__ ((packed)) a_t;

typedef struct
{
  unsigned char b[2];
} __attribute__ ((packed)) b_t;

typedef struct
{
  a_t a;
  b_t b;
} __attribute__ ((packed)) c_t;