Finding unexpected storage overwrite errors in C

During program run time, some storage might unexpectedly change its value and you want to find out when and where this happens. Consider this example where function set_i changes more than the caller expects it to change.

struct s { int i; int j;};
struct s a = { 0, 0 };

⁄* function sets only field i *⁄
void set_i(struct s * p, int k)
{
  p–>i = k;
  p–>j = k;   ⁄* error, it unexpectedly sets field j also *⁄
}
main() {
  set_i(&a,123);
}

Find the address of a with the command

LIST &(a.j) ;

Suppose the result is 0x7042A04. To set a breakpoint that watches for a change in storage values starting at that address for the next 4 bytes, issue the command:

AT CHANGE %STORAGE(0x7042A04,4)

When the program is run, z/OS® Debugger will halt if the value in this storage changes.