Here is simple C program to swap two string is given below.It takes two string using gets function and using a build in function strcpy() ,which is use to copy the string in variable.
C program to swap two string
#include <stdio.h>
#include <string.h>
#include <malloc.h>
int main()
{
char String1[100], String2[100], *temp;
printf("Enter the first string\n");
gets(String1);
printf("Enter the second string\n");
gets(String2);
printf("\nBefore Swapping\n");
printf("First string: %s\n",String1);
printf("Second string: %s\n\n",String2);
temp = (char*)malloc(100);
strcpy(temp,String1);
strcpy(String1,String2);
strcpy(String2,temp);
printf("After Swapping\n");
printf("First string: %s\n",String1);
printf("Second string: %s\n",String2);
return 0;
}