- Sizeof:
- The sizeof operator can be used with any user-defined data type to find out how many bytes of memory space the variable will need.
Example:
#include <stdio.h> /* ------------------------ Structure definition ------------------------ */ struct BankAccount { int accNum; double balance; }; int main(int argc, char *argv[]) { printf("sizeof(struct BankAccount) = %d\n", sizeof(struct BankAccount) ); }Output:
sizeof(struct BankAccount) = 16
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc sizeof1.c
- To run: ./a.out
- $64,000 question:
- Wait a minute...
didn't we learned that:
sizeof(int) = 4 sizeof(double) = 8
- How is it that
sizeof(struct BankAccount) = 16
???
struct BankAccount { int accNum; // 4 bytes double balance; // 8 bytes }; ---------------- 12 bytes !!!Should a struct BanAccount variable use 12 bytes ????
Answer:
- There are memory address
alignment requirements in the computer hardware
(the manifacturer set these
requirements to
make the memory operations
efficient)
- Commonly used requirments:
- A double typed variable
must be located at a
memory address that
is divisible by 8
- An int typed variable
(and a float typed variable)
must be located at a
memory address that
is divisible by 4
- A short typed variable
must be located at a
memory address that
is divisible by 2
- A char typed variable can be located at any memory address
- A double typed variable
must be located at a
memory address that
is divisible by 8
- In order to store
a struct BankAccount,
the C compiler must
use the following storage format:
- Wait a minute...
didn't we learned that:
- Experiment that show the
memory address alignment requirement:
struct s1 { char a; int d; }; struct s2 { char a; char b; int d; }; struct s3 { char a; char b; char c; int d; }; int main(int argc, char *argv[]) { printf("sizeof(struct s1) = %d\n", sizeof(struct s1) ); printf("sizeof(struct s2) = %d\n", sizeof(struct s2) ); printf("sizeof(struct s3) = %d\n", sizeof(struct s3) ); }Output:
sizeof(struct s1) = 8 sizeof(struct s2) = 8 sizeof(struct s3) = 8
Explanation:
- The variables of these
3 structures are stored
in the following manner:
That's why all 3 structures will use 8 bytes to store their variables.
- The variables of these
3 structures are stored
in the following manner:
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link(s) and
save in a scratch directory
- To compile: gcc sizeof2.c
- To run: ./a.out
- Teaching Note:
- I will delay the discussion
of reference (= pointer) variables
to struct variables
- This topic will be discusses
after I discussed the
other kinds of
user defined types
in C
(You can defined reference variables that point to variables of any of the user-defined data types.)
- I will delay the discussion
of reference (= pointer) variables
to struct variables