C code to reverse a string using pointer - c

Question

#include <stdio.h>
main()
{
char* str;
char* strrev;
int i = 0;
int j = 0;
int c;
printf("Enter the string\n");
scanf("%[^\n]%*c", str);
while (*(str + i) != '\n')
{
i++;
}
for (c = i; c >= 0; c--)
{
*(strrev + j) = *(str + c);
j++;
}
}
This is my code to reverse a string. When I compile the code , it gives an error segmentation fault . Someone plz help me understand the error and realize my mistake.
Thanks

Answer 1

There are many issues:
You never allocate memory for str or strrev. This is why it crashes; writing to uninitialized pointers invokes undefined behavior, often resulting in a seg fault.
You're using scanf() with a complex conversion when you could just use fgets().
You should use strlen() rather than looping to find the end of the string.

Answer 2

char *str,*strrev;
first your char buffers where never initialized.
http://www.cplusplus.com/reference/cstring/strlen/
second use strlen to determine string len
regards

Answer 3

In addition to #unwind, even if you used str[100], the loop to find the length will fail. Since str doens not have a \n, the while() loop never ends.
// bad
char str[100];
scanf("%[^\n]%*c", str);
while (*(str + i) != '\n') {
i++;
}
// better
char str[100];
scanf("%99[^\n]%*c", str);
while (*(str + i) != '\0') { // look for \0
i++;
}

Answer 4

Following (some of) #unwinds pointers, here is some code that will reverse a string:
(you can use scanf(), but do not need such a complicated format string)
#include<stdio.h>
main()
{
char str[260],strrev[260];
int i=0, len;
printf("Enter the string\n");
scanf("%s",str);
len = strlen(str);
for(i=1;i<len;i++)
{
strrev[i-1] = str[len-i];
}
strrev[len]=0;
;
}
If you would like to avoid using scanf, then replace it with something like:
char str[260], strrev[260];
//...
fgets(buf, sizeof(str), stdin);
[EDIT-per comment] If still want to use a char *, do something like this:
#include<stdio.h>
main()
{
char *str, *strrev;
int i=0, len;
printf("Enter the string\n");
str = malloc(260);
strrev = malloc(260);
scanf("%s",str);
len = strlen(str);
for(i=1;i<len;i++)
{
strrev[i-1] = str[len-i];
}
strrev[len]=0;
free(str);
free(strrev);
}
Note: Other than keystroke counting, I am not sure how you would determine at run-time just how many characters will be entered by a user. So in this example, 260 bytes is assumed.

Answer 5

#include<stdio.h>
void main()
{
void mystrrev(char *);
char str[100];
char *p1;
p1=str;
printf("Enter the string..?\n");
scanf("%s",str);
mystrrev(p1);
printf("Reverse is =%s",p1);
}
void mystrrev(char *p1)
{
char rev[100];
char *r;
r=rev;
int i=-1,j=0;
while(*p1!='\0')
{
p1++;
i++;
}
while(i>=0)
{
p1--;
*r=*p1;
r++;
j++;
i--;
}
*p1='\0';
*r='\0';
while(j-1>=0)
{
r--;
j--;
}
while(*r!='\0')
{
*p1=*r;
p1++;
r++;
}
*p1='\0';
}
simple code