Program to reverse a string using pointers in C

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i am trying to reverse a string using pointers by making a function

What I have tried:

#include<stdio.h>
char reversestring(char* t);
int main(){
    char s[100],r[100];
    char* t;

    printf("enter the string");
    gets(s);
    t=s;
   r[100]=reversestring(t);
    puts(r);

}

now this is my funtion which i created to reverse the string can anyone tell me where is the error in this function

char reversestring(char* t)
{
    int count=0,end,begin;

    char r[100];

 while(*(t+count)!='\0')
    {
        count++;
}
     end=count-1;


     for(begin=0;begin<count;begin++){
     r[begin]=*(t+end);
     end--;
}

return r;
}

i m new to programming please help me

Posted 10-Jun-18 21:07pm

kavinderrana121

Updated 10-Jun-18 21:48pm

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CHill60 11-Jun-18 3:16am

What actually happens when you run/debug it?

kavinderrana121 11-Jun-18 3:19am

it is showing some garbage value(weird symbols) as output when i am running this program

2 solutions

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OriginalGriff · Accepted Answer · 2018-06-10T21:47:00

Look at your code.

char reversestring(char* t)
{
...
    char r[100];
...
return r;
}

And

    char s[100],r[100];
...
   r[100]=reversestring(t);

So what - exactly - is reversestring returning? Is it a char? No - it's an array of chars, which is a pointer-to-char.
And where do you store this? In a pointer-to-char? No, you try to put a pointer into an array at index 100, which is not even inside the array because C arrays start from index zero.

Start by correcting the code so reversestring returns the right type: pointer-to-char.
Then stop what you are doing and learn about dangling references - which is what returning r would be: it's a local array, so it only exists as long as your function is running. if your return a pointer to local memory, you will get problems later. So change your reversestring function to take two parameters: the input array and the output array, and then return the output array.

Finally, don't store the result in a single element of the array! Store it in a pointer-to-char variable instead!

User 7429338 · Accepted Answer · 2018-06-10T21:50:00

Solution 2

There's a few things wrong:

- reversestring returns a single character rather than a char*
- the result is assigned to r[100] instead of to r
- r in reversestring is only on the stack within the function and cannot be returned like that

You can fix it for example like so:

#include <stdio.h>

void reversestring(char* input, char* output);

int main() {
	char s[100], t[100];

	printf("enter the string");
	gets(s);
	reversestring(s, t);
	puts(t);
}

void reversestring(char* input, char* output)
{
	int count = 0, begin;

	while (*(input + count) != '\0')
	{
		count++;
	}

	for (begin = 0; begin<count; begin++)
	{
		output[begin] = *(input + count - 1 - begin);
	}
	output[count] = '\0';
}

Note that you will have a buffer overflow if the entered string doesn't fit in s.

Posted 10-Jun-18 21:50pm

User 7429338

Comments

Maciej Los 11-Jun-18 4:06am

5ed!

kavinderrana121 11-Jun-18 7:00am

to avoid buffer overflow how will i use memory allocation using malloc for this problem

[no name] 11-Jun-18 7:02am

You can use gets_s instead of gets to make sure the input fits in s[]. Malloc doesn't help with an overflow in this case.

kavinderrana121 11-Jun-18 7:05am

but if i want to use malloc then i will use that in main function or reversestring function??

[no name] 11-Jun-18 7:07am

In main:
char* s = (char*)malloc(100);
char* t = (char*)malloc(100);

The function just uses the provided buffers, it doesn't need to do any allocation.

kavinderrana121 11-Jun-18 7:08am

sorry i asked wrong question i am doing this problem now by returning pointer to char from reversestring in that case how will i use malloc please it would be really helpful if you can type that code
i m new brother working hard all day and night pls help me