What is wrong with my code? It says segmentation fault - c

Question

What I am trying to do is, take the message from the user input and reverse the string. So, I am using 2 pointers, one is pointing at the first element of the message array and the other is pointing to the last element. And then I am doing the swap until the two pointers meet.
#include <stdio.h>
#include <string.h>
void reverse(char* message);
int main()
{
char message[100];
printf("Enter a message: ");
scanf("%s",message);
printf("\nok");
reverse(message);
printf("ok \n");
printf("%s",message);
return 0;
}
void reverse(char* message){
char* p = message;
char* temp = message;
char* q = (message + (strlen(message)); // pointing to the last element before
// null character
printf("\ncoming");
while( p != q){
// why am I getting a segmentation fault? whats happening?
*p = *q;
*q = *temp;
p++;
q--;
temp = p;
}// while loop
}// reverse

Answer 1

This line
char* q = (message + (strlen(message));
Does it point to the last character.
Consider a message of one character - strlen(message) will by one. But the first character would be char * q = message
So the line should read
char* q = (message + strlen(message) - 1;
Also the swap is a be awry
Try
char temp;
Instead then instead of
*p = *q;
*q = *temp;
p++;
q--;
temp = p;
Use
temp = *p; // We got the temp
*p = *q; // Old p is safe - it is now temp.
*q = temp;
p++;
q++;
With this code the null character is not reversed! Hence in the right place!
Finalluy,
while( p != q){
Should be
while( p < q){

Answer 2

regarding:
p++;
q--;
After the above statements, the pointers can have passed each other. Then the loop keeps running until the seg. fault occurs

Answer 3

I fixed the while loop condition. Now i gave while(p < q) but the swapping isnt happening, can anyone explain why?
Both pointers p and temp are pointing at message:
char* p = message;
char* temp = message;
When *p = *q; is executed in the while loop, the character pointed by "p" and the character pointed by "temp", get updated with the character at "q" (because of the above declarations).
The next line *q = *temp; is essentially copying the newly updated character("*q") back to "q".
In addition, as pointed out by "M Oehm", the line
message + strlen(message), points at the NULL character, instead of the last character before NULL.
Also note the comment posted by "user3629249" regarding the missing parentheses.
Correcting these your code should look like...
void reverse(char* message){
char* p = message;
char* temp = message;
char* q = (message + strlen(message)-1); // pointing to the last element
char c; //to store the value of "temp"
printf("\ncoming");
while( p < q){
c = *temp;
*p = *q;
*q = c;
p++;
q--;
temp = p;
}// while loop
}// reverse
Having said this, since "C" passes parameters by value, you could implement the same function, like so....
void reverse(char* message){
char* q = (message + strlen(message)-1);
char c;
while( message < q){
c = *message;
*message = *q;
*q = c;
message++;
q--;
}
}

Answer 4

A segmentation fault is caused by a program trying to read or write an illegal memory location. With both p++ and q-- the while (p != q) can be always true, for instance if p and q point at 2 consecutive characters. Check this approach for string reversing:
#include <string.h>
void reverse(char *str)
{
//skip null string
if (str == 0)
{
return;
}
// skip empty string
if (*str == 0)
{
return;
}
// get range
char *start = str;
char *end = start + strlen(str) - 1; // -1 for the end \0
char temp;
// reverse
while (end > start)
{
// swap chars
temp = *start;
*start = *end;
*end = temp;
// move to next position
++start;
--end;
}
}
int main(void)
{
char s1[] = "Stack Overflow is GREAT!";
reverse(s1);
printf("%s\n", s1);
return 0;
}