UNIX for Dummies Questions & Answers

I've been referring bash info for processes and came across a structure for a process which is defined like
typedef struct process
{
struct process *next;
char ** argv
.
.
.
}process;

What I don't understand is that in the program there's a for loop which goes like this
job *j;//pointer to a structure that holds data about the jobs running
process *p;//pointer to a structure that holds process information
for(p=j->first_process;p;p=p->next) {...}
I don't understand how the p highlighted in red can be used as a condition in a for loop. What purpose does it serve? How does the for loop evaluate p as a terminating condition for the loop?

NULL is defined to be<datatype> *)0 - this means regardless of the underlying architecture
if p == 0 and p is a pointer it is guaranteed to be unassigned. Therefore

Code:

if (p)
if(p==NULL)

evaluate the same. NULL is fairly new. Old C code uses code like (char**)0 all the time. Example - this is in the strtoul definition on the manpage for systems that have C89 compilers.

Thanks I had a feeling it could be a null thingy....but didn't expect C to actually support that