Getting wrong output while executing sizeof(char) - ios

Question

My question is sizeof(char) is 1 byte but while executing below code why I am getting wrong output. Kindly help me. Thank you
typedef struct {
int x;
int y;
char a;
}Point2D;
main() {
Point2D *myPoint=malloc(sizeof(Point2D));
NSLog(#"sizeof(Point2D): %zu", sizeof(Point2D));
}
Output: sizeof(Point2D) : 12 //But it should return 9 [int + int + char / 4 + 4 + 1]
Note: While running char individually , I am getting correct output
Example:
typedef struct {
char a;
char b;
}Point2D;
main() {
Point2D *myPoint=malloc(sizeof(Point2D));
NSLog(#"sizeof(Point2D): %zu", sizeof(char));
}
output: sizeof(char) : 2

Answer 1

You are not getting "wrong" output, when an (Objective-)C compiler lays out a struct it is allowed to use internal padding so that the fields start at the best memory alignment for their type.
If you need the size of a struct to be exactly the sum of its field sizes you can use __attribute__((__packed__)). E.g:
typedef struct
{
int x;
int y;
char a;
} __attribute__((__packed__)) Point2D;
has a size of 9. However access to the fields may be slower due to the CPU having to deal with values not having optimal storage alignment.