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sizeof() operator in C

The sizeof() operator is commonly used in C. It determines the size of the expression or the data type specified in the number of char-sized storage units. The sizeof() operator contains a single operand which can be either an expression or a data typecast where the cast is data type enclosed within parenthesis. The data type cannot only be primitive data types such as integer or floating data types, but it can also be pointer data types and compound data types such as unions and structs.

Need of sizeof() operator

Mainly, programs know the storage size of the primitive data types. Though the storage size of the data type is constant, it varies when implemented in different platforms. For example, we dynamically allocate the array space by using sizeof() operator:

In the above example, we use the sizeof() operator, which is applied to the cast of type int. We use malloc() function to allocate the memory and returns the pointer which is pointing to this allocated memory. The memory space is equal to the number of bytes occupied by the int data type and multiplied by 10.

The output can vary on different machines such as on 32-bit operating system will show different output, and the 64-bit operating system will show the different outputs of the same data types.

The sizeof() operator behaves differently according to the type of the operand.

  • Operand is a data type
  • Operand is an expression

When operand is a data type.

In the above code, we are printing the size of different data types such as int, char, float with the help of sizeof() operator.


sizeof() operator in C

When operand is an expression

In the above code, we have created two variables 'i' and 'j' of type double and float respectively, and then we print the size of the expression by using sizeof(i+j) operator.


size of (i+j) expression is : 8

Handling Arrays and Structures

The sizeof() operator is highly helpful when working with arrays and structures in addition to the above usage cases. Contiguous blocks of memory are known as arrays, and understanding their size is crucial for a few tasks.

For example:


Size of the array arr is: 20
Number of elements in arr is: 5

Sizeof(arr) returns the array's overall size in bytes, whereas sizeof(arr[0]) returns the array's smallest element's size. The number of items in the array is determined by dividing the overall size by the size of a single element (arrSize). By using this technique, the code will continue to be flexible in the face of changing array sizes.

Similarly, you can use the sizeof() operator to figure out the size of structures:


Size of the structure Person is: 40 bytes

Allocation of dynamic memory and pointer arithmetic

Other applications of the sizeof() operator include pointer arithmetic and dynamic memory allocation. Knowing the size of data types becomes essential when working with arrays and pointers for correct memory allocation and element access.


Dynamic array elements: 1 2 3 4 5


In this example, a size numElements integer array has a memory that is dynamically allocated. numElements * sizeof(int) bytes represent the total amount of memory allocated. By doing this, the array is guaranteed to have enough room to accommodate the desired amount of integers.

Sizeof() for Unions

Unions and the sizeof() operator are compatible. Unions are comparable to structures, except only one member can be active at once, and all its members share memory.


Size of the union Data is: 20 bytes

The sizeof() operator is extremely important since it's essential for memory management, portability, and effective data handling. The sizeof() operator is crucial in C for the reasons listed in the list below:

Memory Allocation: When working with arrays and dynamic memory allocation, the sizeof() operator is frequently used in memory allocation. Knowing the size of data types when allocating memory for arrays or structures guarantees that the correct amount of memory is reserved, reducing memory overflows and improving memory utilization.

Portability: Since C is a popular programming language, code frequently has to operate on several systems with differing architectures and data type sizes. As it specifies the size of data types at compile-time, the sizeof() operator aids in designing portable code by enabling programs to adapt automatically to various platforms.

Pointer Arithmetic: When dealing with pointers, the sizeof() operator aids in figuring out memory offsets, allowing accurate movement within data structures, arrays, and other memory regions. It is extremely helpful when iterating across arrays or dynamically allocated memory.

Handling Binary Data: The sizeof() operator guarantees that the right amount of data is read or written when working with binary data or files, eliminating mistakes brought on by inaccurate data size assumptions.

Unions and Structures: The sizeof() operator is essential when managing structures and unions, especially when utilizing them to build complicated data structures. Memory allocation and access become effective and error-free when you are aware of the size of structures and unions.

Safe Buffer Management: The sizeof() operator helps make sure that the buffer is big enough to hold the data being processed while working with character arrays (strings), preventing buffer overflows and potential security flaws.

Data Serialization and Deserialization: The sizeof() operator guarantees that the right amount of data is handled, maintaining data integrity throughout data transfer or storage, in situations where data needs to be serialized (converted to a byte stream) or deserialized (retrieved from a byte stream).

Code Improvement: Knowing the size of various data formats might occasionally aid in code optimization. For instance, it enables the compiler to more effectively align data structures, reducing memory waste and enhancing cache performance.

Sizeof() Operator Requirement in C

The sizeof() operator is a key component in C programming due to its need in different elements of memory management and data processing. Understanding data type sizes is essential for effectively allocating memory, especially when working with arrays and dynamic memory allocation. By ensuring that the appropriate amount of memory is reserved, this information helps to avoid memory overflows and optimize memory use. The sizeof() operator is also essential for creating portable code, which may execute without error on several systems with differing architectures and data type sizes.

The program can adapt to many platforms without the need for manual modifications since it supplies the size of data types at compile-time. Additionally, the sizeof() operator makes it possible to navigate precisely around data structures and arrays while working with pointers, facilitating safe and effective pointer arithmetic. Another application for the sizeof() operator is handling unions and structures. It ensures precise memory allocation and access within intricate data structures, preventing mistakes and inefficiencies. The sizeof() operator is a basic tool that enables C programmers to develop effective, portable, and resilient code while optimizing performance and data integrity. It ensures safe buffer management and makes data serialization and deserialization easier.


In summary, the C sizeof() operator is a useful tool for calculating the size of many sorts of objects, including data types, expressions, arrays, structures, unions, and more. As it offers the size of data types at compile-time, catering to multiple platforms and settings, it enables programmers to create portable and flexible code. Developers may effectively handle memory allocation, pointer arithmetic, and dynamic memory allocation in their programs by being aware of the storage needs of various data types.

When working with arrays and structures, the sizeof() operator is very helpful since it ensures proper memory allocation and makes it simple to retrieve elements. Additionally, it facilitates pointer arithmetic, making it simpler to move between memory regions. However, because of operator precedence, programmers should be cautious when utilizing complicated expressions with sizeof() operator.

Overall, learning the sizeof() operator equips C programmers to create stable and adaptable software solutions by enabling them to write efficient, dependable, and platform-independent code.

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