string reverse without new array

hi can anybody tell me the error in this?

#include<stdio.h>

int main()
{
    char a[]="abcdefgh";
    int i=0;
    int n=strlen(a);
    char *fi开发者_如何学运维rst;
    char *second;
    char *c;
    *first=a[0];
    *second=a[7];
    for(i=0;i<=n/2;i++)
    {
            *c=*first;
            *first=*second;
            *second=*c;
            first++;
            second--;
    }

    for(i=0;i<=7;i++)
    {
            printf("%c",a[i]);
    }
}

The problem is these lines:

*first=a[0];
*second=a[7];

I think what you want is to get first and second to point to the correct elements, which is:

first = &a[0]; // address of the first element
second = &a[7]; // address of the eighth element

What you have is assigning the value of a[0] to the address pointed to by first, which is not initialized. Also, you might as well use n - 1 instead of 7 here, so you don't hardcode the size. Also these lines:

*c=*first;
*first=*second;
*second=*c;

You see, the pointer c also hasn't been initialized. What you should do is not have c as a pointer:

char c;

Then use it just like a normal variable:

c = *first;
*first = *second;
*second = c;

And just a design note, you don't need the counter/for-loop. Rather, you know you're done with second is <= to first (that is, we're at or have crossed the half-way point):

while (second > first)

Lastly, through some spaces in there! Your code is very condensed and hard to read. Don't be afraid to space things out.

    *first=a[0];

The pointer first isn’t initialized, so you’re assigning to invalid memory.