strlen() vs. sizeof() in C: Understanding Their Differences and Proper Usage

What is strlen()

In C, strlen() returns the number of characters in a null‑terminated string, excluding the terminating '\0'. It counts alphabetic characters, digits, symbols, and spaces.

This article explains the role of strlen(), sizeof(), and why they behave differently.

• Definition of strlen()
• Definition of sizeof()
• Syntax examples
• Key differences

What is sizeof()

The sizeof operator yields the size, in bytes, of its operand. It is evaluated at compile time for static types and at run time for variable-length arrays. It applies to all C data types, including primitives, pointers, structs, and unions. The result is of type size_t, an unsigned integer type guaranteed to be large enough to hold the size of the largest object.

Because the size of types such as int or double can vary between 32‑bit and 64‑bit systems, sizeof ensures portability.

Key Differences

• strlen() measures the number of characters in a string; sizeof() measures the memory occupied by a variable or type.
• Only strlen() considers the string’s terminating '\0'; sizeof() ignores content.
• The return type of strlen() is size_t (commonly a 64‑bit unsigned integer on modern systems); sizeof() also returns size_t.
• strlen() is a runtime function defined in string.h; sizeof is a compile‑time unary operator.
• Only strlen() operates on character arrays; sizeof works with any type.

Syntax of strlen()

size_t length = strlen(my_string);

Here, my_string is a character array or pointer to a null‑terminated string.

Example of strlen()

#include <stdio.h>
#include <string.h>

int main() {
    char strng[] = "January";
    printf("Length of given string is: %zu\n", strlen(strng));
    return 0;
}

Output:

Length of given string is: 7

Syntax of sizeof()

1) sizeof(type)

sizeof(type) yields the size of the specified type.

#include <stdio.h>
#define my_sizeof(type) ((char*)(&type+1) - (char*)&type)

int main() {
    double x;
    printf("%zu", my_sizeof(x));
    return 0;
}

Output:

8

2) sizeof(variable)

Pass a variable name to obtain its memory footprint.

#include <stdio.h>

int main() {
    printf("sizeof(char) = %zu\n", sizeof(char));
    return 0;
}

Output:

sizeof(char) = 1

3) sizeof(expression)

Evaluate an expression’s type size without computing its value.

#include <stdio.h>

int main() {
    char p = 'S';
    double q = 4.65;
    printf("Size of variable p : %zu\n", sizeof(p));
    printf("Size of an expression : %zu\n", sizeof(p+q));
    int a = (int)(p+q);
    printf("Size of explicitly converted expression : %zu\n", sizeof(a));
    return 0;
}

Output:

Size of variable p : 1
Size of an expression : 8
Size of explicitly converted expression : 4

Visual Comparison

Below is a side‑by‑side summary of the two constructs.

Feature	strlen()	sizeof()
Header file	string.h	No header required (built‑in operator)
Purpose	Compute string length (excluding null)	Determine memory size of a type or variable
Return type	size_t	size_t
Evaluation time	Runtime (may traverse the string)	Compile‑time (except for VLA)
Applicable to	Only character arrays or pointers to null‑terminated strings	All data types and expressions

When to Use Which

• Use strlen() when you need the logical length of a string, such as for buffer allocation, substring extraction, or input validation.
• Use sizeof() when you need the physical size of a data type or variable, for example when copying memory with memcpy or allocating a block of memory for an array of structs.
• Remember that sizeof(char) is always 1 byte, which is the base unit of memory in C.
• For string literals, sizeof("hello") yields 6 (five characters plus the null terminator), while strlen("hello") yields 5.

Adhering to these guidelines ensures efficient, portable, and maintainable C code.