Remove chars from one string which are present in another string - iphone

Question

this is the code to remove the characters from one string, say p, which are there in the other string, say s, and finally concatenate the two strings and print the final string.
for ex: if inputs are
stringP= "Hello"
StringS= "fellow"
output: Hfellow
another EX
input
stringP= "Android"
StringS= "Google"
output
AndridGoogle
the comparison of the characters is case sensitive.
Currently i am getting segmentation fault after entering the first string. Can anyone please help me in correcting this code? and also why segmentation fault occurs, in which scenarios it occurs?
Thanks in advance.
#include <stdio.h>
#include<string.h>
void remove_char(int *len, char *string1);
int main()
{
char *p,*s,*t;
int len1,len2,i,j;
printf("enter the two strings\n");
scanf("%s",p);
scanf("%s",s);
len1=strlen(p);
len2=strlen(s);
for(i=0;i<len1;i++)
{
for(j=0;j<len2;j++)
{
if(*p==*(s+j))
{
remove_char(&len1,p);
}
if(*p=='\0'||*(s+j)=='\0')
{
break;
}
}
p++;
}
strcat(p,s);
strcat(t,p);
printf("%s",t);
return 0;
}
void remove_char(int *len, char* string1)
{
int a;
for(a=0;a<*len;a++)
{
*string1=*(string1+1);
string1++;
}
len--;
}

Answer 1

The declaration char* p says that p is a variable that contains the address of a character (or potentially the address of a character array).
In your code, you do create storage space for the character (or the array). Think of it this was, p is an alias for a location in the local stack frame (say a four-byte area), and it's contents are interpreted as an address in the heap.
Now, no where in your code to you actually allocate memory, to do this you would need to do something like this:
p = malloc(15*sizeof(char));
This is assuming that you want the user to enter a string of no more than 14 characters (don't forget the null-terminator). Now p contains the address of an area on the heap that can be used to store characters.
So, to answer your question, you are getting a segmentation fault after entering the first string because you failed to allocate memory to store the string in.
Also, note that scanf is insecure in that it will take a string as long as the user feels like typing in and this could result in heap-based buffer overflow. If you want to read in a string of at most 14 character, try this;
#define STR_LEN 14
char* p = malloc(STR_LEN*sizeof(char));
scanf("%STR_LENs", p);
Notice that I have put a length in the specifier so that scanf will limit itself to reading in at most STR_LEN characters. This will help prevent over-flows, not entirely prevent it.