Question

       int num_arrays;
       char *p[20];
       char tempc;       
       int i=0;
       do
       { p[i]=malloc(sizeof(int));
        scanf("%s",p[i]);
        tempc=*p[i];
        ++i;
        }while(tempc=='x');
        num_arrays=atoi(p[0]);

When i write num_arrays=atoi(..),gcc give me segmentation fault or memory
stack is exceeded, I don’t understand why it behaves like that
can anyone explain, why?

Answer 1

You haven’t allocated any memory in p, so its elements point to random locations. You can allocate memory for strings with a maximum length of 100 like this:

int i;
for (i = 0; i < 20; i++)
  p[i] = malloc(101);

Or else you want an array of characters, not of strings, in which case you should declare it like

char p[20];

In this case, you must not try to read strings into each element with scanf though.

Update after the missing code part has been added:

You are allocating sizeof(int) bytes of memory for your strings, which is most likely 4, i.e. your input strings read by scanf must not be longer than 3 characters. Otherwise you have a buffer overrun error, which may lead to the segmentation fault you are struggling with.

Moreover, tempc=p[i] assigns a pointer value to a character variable! This value, converted to a character, will almost surely never be equal to 'x'.

I guess you are trying to get the first character of p[i], which would be p[i][0] or *p[i].

I also suspect that your loop condition is the opposite of what you had in mind: right now the loop repeats as long as tempc is equal to 'x' – you probably wanted to repeat until tempc becomes 'x'. And you should also check for your loop not running more than 20 times:

  ...
}while(tempc != 'x' && i < 20);