Difference between char s[] and char *s in C

In C programming, there are two common ways to declare strings: char s[] and char *s. While both can hold strings, they have fundamental differences in memory allocation, mutability, and behavior.

Syntax

char s[] = "string literal";     // Array declaration
char *s = "string literal";      // Pointer declaration

Key Differences

Example 1: Memory Size Difference

This example demonstrates the difference in memory allocation −

#include <stdio.h>

int main() {
    char s1[] = "Hello World";
    char *s2 = "Hello World";
    
    printf("sizeof(s1): %lu bytes<br>", sizeof(s1));
    printf("sizeof(s2): %lu bytes<br>", sizeof(s2));
    
    return 0;
}

sizeof(s1): 12 bytes
sizeof(s2): 8 bytes

Example 2: Modifying Array String (char s[])

Arrays created with char s[] are modifiable −

#include <stdio.h>

int main() {
    char s[] = "Hello World";
    printf("Original: %s<br>", s);
    
    s[6] = 'x';  // Modify character at position 6
    printf("Modified: %s<br>", s);
    
    return 0;
}

Original: Hello World
Modified: Hello xorld

Example 3: Attempting to Modify Pointer String (char *s)

Trying to modify a string literal through a pointer results in undefined behavior −

#include <stdio.h>

int main() {
    char *s = "Hello World";
    printf("Original: %s<br>", s);
    
    s[6] = 'x';  // Undefined behavior - may cause segmentation fault
    printf("Modified: %s<br>", s);
    
    return 0;
}

Note: The above code will likely cause a segmentation fault or runtime error because string literals are stored in read-only memory.

Key Points

• char s[] creates a local array copy that can be modified safely.
• char *s points to a string literal in read-only memory.
• Use char s[] when you need to modify the string content.
• Use char *s for read-only string references to save memory.

Conclusion

Understanding the difference between char s[] and char *s is crucial for proper memory management and avoiding runtime errors. Choose arrays for modifiable strings and pointers for read-only string literals.