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The sizeof operator

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• Size of a data type
  • Size of a data type

    Size of a data type = the number of bytes used to store a value of that data type.

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• The sizeof operator
  • C has a special operator (sizeof) that returns the size of a data type

    Syntax:

    sizeof( dataType ) returns the size of a variable of type "dataType" or sizeof( varName ) returns the size of the variable "varName"

    Comment:

    The sizeof operator is unusual in that: The sizeof operator is represented by an English-like word ("sizeof") Other operators in C are represented by symbols such as +, =, &&, ||, etc

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  • Example:

    int main( int argc, char* argv[] ) { printf("sizeof(char) = %lu\n", sizeof(char) ); printf("sizeof(short) = %lu\n", sizeof(short) ); printf("sizeof(int) = %lu\n", sizeof(int) ); printf("sizeof(long) = %lu\n", sizeof(long) ); printf("sizeof(long long) = %lu\n", sizeof(long long) ); printf("\n"); printf("sizeof(unsigned char) = %lu\n", sizeof(unsigned char) ); printf("sizeof(unsigned short) = %lu\n", sizeof(unsigned short) ); printf("sizeof(unsigned int) = %lu\n", sizeof(unsigned int) ); printf("sizeof(unsigned long) = %lu\n", sizeof(unsigned long) ); printf("sizeof(unsigned long long) = %lu\n", sizeof(unsigned long long) ); printf("\n"); printf("sizeof(float) = %lu\n", sizeof(float) ); printf("sizeof(double) = %lu\n", sizeof(double) ); printf("sizeof(long double) = %lu\n", sizeof(long double) ); printf("\n"); int x; printf("sizeof(x) = %lu\n", sizeof(x) ); }

    Output:

    // gcc on Ubuntu 18 sizeof(char) = 1 sizeof(short) = 2 sizeof(int) = 4 sizeof(long) = 8 sizeof(long long) = 8 sizeof(unsigned char) = 1 sizeof(unsigned short) = 2 sizeof(unsigned int) = 4 sizeof(unsigned long) = 8 sizeof(unsigned long long) = 8 sizeof(float) = 4 sizeof(double) = 8 sizeof(long double) = 16 sizeof(x) = 4

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/C/set1/sizeof1.c

    How to run the program:

    To compile:   gcc sizeof1.c          To run: ./a.out

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• Typical use of sizeof
  • The sizeof operator is commonly used to find the size of user defined data types (= Java's "classes")

  • Later, we will learn hot to define user-defined data types.

    Here I will give you a preview:

    #include <stdio.h> /* ---------------------------------------------- User-defined data type (like a class in Java) ---------------------------------------------- */ struct BankAccount { int accNum; // 4 bytes double balance; // 8 bytes }; int main( int argc, char* argv[] ) { struct BankAccount x; printf("sizeof(BankAccount) = %lu\n", sizeof(struct BankAccount) ); printf("sizeof(x) = %lu\n", sizeof(x) ); }

    Output:

    sizeof(BankAccount) = 16 sizeof(x) = 16

    Comment:

    A BankAccount object requires 16 bytes to store because: Alignment requirement for the double typed variable balance. Graphically explained: There is a gap of 4 bytes between the 2 components in the BankAccount object

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• Experiment to show the gap in the BankAccount object
  • This program shows that there is a gap in the BankAccount object:

    #include <stdio.h> /* ---------------------------------------------- User-defined data type (like a class in Java) ---------------------------------------------- */ struct BankAccount { int accNum; // 4 bytes int FILLER; // Gap Filler of 4 bytes double balance; // 8 bytes }; int main( int argc, char* argv[] ) { struct BankAccount x; printf("sizeof(BankAccount) = %lu\n", sizeof(struct BankAccount) ); printf("sizeof(x) = %lu\n", sizeof(x) ); }

    Output:

    sizeof(BankAccount) = 16 same size !!! sizeof(x) = 16

    You can insert an int variable in the BankAccount object without increasing its size !!!

    The FILLER variable occupied the 4 byte gap between accNum and balance.