SIGSEGV in C error - c

Question

I always get an SIGSEGV error whenever i am dynamically initializing arrays in C. Please tell me what am I doing wrong all the time?
The code works fine on TurboC but it gives SIGSEGV on an online judge which uses GCC.
Programming Problem
My code:
#include<stdio.h>
#include<stdlib.h>
int main(void)
{
long n,h,i,crane=0,box=0,temp=0;
long *comm;
scanf("%ld %ld",&n,&h);
long *a = (long*)malloc(n*sizeof(long));
for(i=0;i<n;i++)
scanf("%ld",&a[i]);
scanf("%ld",&comm[0]);
i=0;
while(comm[i]!=0)
{
i++;
scanf("%ld",&comm[i]);
}
for(i=0;comm[i]!=0;i++)
{
if(comm[i]==3)
box=1;
if(comm[i]==4 && box==1)
{
a[crane]=(a[crane]+1);
temp=0;
}
if(box==1 && (comm[i]==1 || comm[i]==2) && temp==0)
{
a[crane]=(a[crane]-1);
temp=1;
}
if(crane!=0 && comm[i]==1)
crane--;
if(comm[i]==2)
crane++;
if(comm[i]==0)
break;
}
for(i=0;i<n;i++)
printf("%ld ",a[i]);
free(a);
free(comm);
return 0;
}

Answer 1

For a start, nowhere in that code are you actually allocating memory for comm to point to. You apparently know this is required since you've done something similar for a and you free both a and comm at the end.
You need to malloc the memory for comm to point to, before using it. For example, if you wanted that to depend on the second value input (h, probable since it's not used anywhere else), you would need to add in:
comm = malloc(h*sizeof(long));
after the first scanf, noting that I don't cast the return value - you shouldn't do that in C.
If you don't know how big comm should be before using it, the usual way to handle that is to allocate a certain number of elements (the capacity) and keep track of how many you've used (the size). Each time when your size is about to exceed your capacity, use realloc to get more space.
The following (C-like) pseudo-code shows how to do this, starting with an empty array and expanding it by thirty elements each time more space is needed:
comm = NULL
capacity = 0
size = 0
for each value in input():
if size == capacity:
capacity += 30
comm = realloc (comm, capacity), exit if error
comm[size++] = value
Note that, on loop exit, size is the indicator of how many elements are in the array, despite the fact there may be more capacity.
And, as an aside, you should always assume that calls subject to failure (such as scanf and malloc) will fail at some point. In other words, check the return values.