What is strrev() Function in C language?

In C programming, the strrev() function is used to reverse a string in-place. However, strrev() is not part of the standard C library ? it is a Microsoft-specific extension found in some compilers. For portable code, we need to implement string reversal manually.

Syntax

char* strrev(char* str);

Where str is the string to be reversed. The function returns a pointer to the reversed string.

Example 1: Manual String Reversal Using Two Pointers

Since strrev() is not standard, here's how to reverse a string using a custom function −

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

void reverseString(char str[]) {
    int start = 0;
    int end = strlen(str) - 1;
    
    while (start < end) {
        char temp = str[start];
        str[start] = str[end];
        str[end] = temp;
        start++;
        end--;
    }
}

int main() {
    char string[50];
    
    printf("Enter a string: ");
    fgets(string, sizeof(string), stdin);
    
    /* Remove newline if present */
    string[strcspn(string, "<br>")] = '\0';
    
    printf("Original string: %s<br>", string);
    reverseString(string);
    printf("Reversed string: %s<br>", string);
    
    return 0;
}

Enter a string: Hello World
Original string: Hello World
Reversed string: dlroW olleH

Example 2: Using Recursion

Here's another approach using recursive function to reverse a string −

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

void reverseRecursive(char str[], int start, int end) {
    if (start >= end) {
        return;
    }
    
    char temp = str[start];
    str[start] = str[end];
    str[end] = temp;
    
    reverseRecursive(str, start + 1, end - 1);
}

int main() {
    char text[] = "TutorialsPoint";
    
    printf("Before reversal: %s<br>", text);
    reverseRecursive(text, 0, strlen(text) - 1);
    printf("After reversal: %s<br>", text);
    
    return 0;
}

Before reversal: TutorialsPoint
After reversal: tnioPslairotpuT

Key Points

• Portability: strrev() is not part of standard C, so manual implementation is recommended.
• In-place Operation: String reversal modifies the original string without using extra memory.
• Null Terminator: Always ensure the string is properly null-terminated.
• Use fgets() instead of deprecated gets() for safe string input.

Conclusion

While strrev() exists in some compilers, implementing manual string reversal ensures code portability. The two-pointer approach is efficient with O(n/2) time complexity and O(1) space complexity.