Getting wrong output while executing sizeof(char)

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

possible duplicate of stackoverflow.com/questions/1841863/size-of-struct-in-c — Aris
– Aris, Commented Jun 19, 2017 at 9:50

CRD · Accepted Answer · 2017-06-19 16:56:59Z

2

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.

edited Jun 19, 2017 at 16:56

answered Jun 19, 2017 at 10:10

CRD

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