Threads in C - Textbook answer about multithreading

I have pulled a question from 'Operating Systems - Internals and Design Principles Eighth Edition'

I have tried my answer but really would like to know the proper response to this question.

 Considering the following C declarations and function definition:

 int global_positives = 0;
 typedef struct list { 
    struct list *next;
    double val;
 } *list;

 void count_positives(list l)
 {
   list p;
   for (p = 1; p; p = p -> next) 
      if (p -> val > 0.0)
         ++global_positives;
 }

While thread A does:
count_positives(<list containing only negative numbers>);

and thread B does:
++global_positives;

a) what does the function do? b) The C language only addresses single threaded execution. Does the use of two parallel threads create any problems?

So my answers I would like to know if correct are:

a) The function as of now will do nothing in the case of thread A calling the function with only negative numbers since nothing will be greater than zero as well as... b) the problem of having two threads run in parallel in C because since only one can run at a time global_positives will never increment and the answer will always be zero.

In the ballpark?