Segmentation Fault error in method C programming - c

Question

Below is the code for a method i made called inputReader it reads input from a textfile and copies it to a struct wordz and from that struct i retrieve the 3 most frequent words which is what is displayed below.
I am trying to concatenate all 3 words as one pointer so i can return it to the main method but whenever i use any method to do with w1, w2, w3, that copies to a new struct or array or pointer i ALWAYS get this error "Segmentation fault(core dumped)"
Any idea why this happens or how I can work around it?
Struct Code:
#define maxLetters 101
typedef struct {
char word[maxLetters];
int freq;
} WordArray; //struct type
Code:
char * w1; // most frequent word
char * w2; // second most frequent word
char * w3; // third most frequent word
// finds w1
for(j = 0; j < uniqueWords; j++)
if(wordz[j].freq == freqz[uniqueWords-2]+1)//excludes whitespace frequency
w1 = wordz[j].word;
// finds w2
for(j = 0; j < uniqueWords; j++)
if(wordz[j].freq == freqz[uniqueWords-3]+1)//excludes whitespace frequency
w2 = wordz[j].word;
// finds w3
for(j = 0; j < uniqueWords; j++)
if(wordz[j].freq == freqz[uniqueWords-4]+1)//excludes whitespace frequency
w3 = wordz[j].word;
char *p;
// if i dont include strcat methods the method runs fine and outputs fine
strcat(p, w1); // once this operation is executed i get the error
strcat(p, " ");
strcat(p, w2);
strcat(p, " ");
strcat(p, w3);

Answer 1

You're trying to concatenate to an uninitialized pointer. Allocate memory to 'p'.
char *p = malloc(size)

Answer 2

It's a good idea to read the documentation of strcat thoroughly to check you are using it properly. You can look here http://man7.org/linux/man-pages/man3/strcat.3.html for example. There are also plenty of answers to similar questions on stackoverflow that you would have found helpful:
Segmentation fault- strcat
strcat causing segmentation fault
strcat segmentation fault
However, to save you some anguish, the fault is because to have not allocated any space for the results string p. strcat does not do this in C like it might in C# or other languages.
You would need:
char p[maxletters];
Better still, you should use strncpy with the size limit to prevent memory corruption:
strncpy(p, w1, maxletters);

Answer 3

The root cause leading to your issue is that p does not point anywhere, so trying to copy data to it invokes the infamous Undefined Behaviour.
So allocate some memory to p, either from the heap or from the stack.
To get it from the heap do:
char * p = malloc(some_size);
When then trying to strcat() data to this pointer be aware that strcat() expects its 1st argument to already point to a valid C-"string", that is a 0-teminated char array, to that it can concatenate what the 2nd argument is pointing to.
To take care this, make sure the data p is pointing to is does at least has its first bytes set to 0:
p[0] = '\0';
if not all bytes:
memset(p, 0, some_size);
If going for the latter you might also like to do:
char * p = calloc(some_size, sizeof *p);
As calloc() does the same as malloc() and addtionally initilaises all memory allocated with 0s.
If you feel initalising is to much effort then you cannot start concatenating using strcat(), but nered to start with strcpy() which does not rely on its 1st argument pointing to a valid C-"string", but simply copies what its 2nd argument is pointing to to where its 1st argement is pointing to.
chart * p = malloc(some_size);
strcpy(p, w1);
strcat(p, " ");
...
Finally:
Do not forget to call free() on p if the memory isn't needed anymore, to avoids a memory leak.
Also always check the outcome of memory allocation, that is check the result of malloc/calloc() against NULL, before using it.