Can I set a data breakpoint in runtime in System C (or in Plain Vanilla C++)?

Question

I have a class in system-C with some data members as such:

long double x[8];

I'm initializing it in the construction like this:

for (i = 0; i < 8; ++i) {
    x[i] = 0;
}

But the first time I use it in my code I have garbage there.

Because of the way the system is built I can't connect a debugger easily. Are there any methods to set a data breakpoint in the code so that it tells me where in the code the variables were actually changed, but without hooking up a debugger?

Edit: @Prakash: Actually, this is a typo in the question, but not in my code... Thanks!

Answer 1

You could try starting a second thread which spins, looking for changes in the variable:

#include <pthread.h>

void *ThreadProc(void *arg)
{
  volatile long double *x = (volatile long double *)arg;
  while(1)
  {
    for(int i = 0; i < 8; i++)
    {
      if(x[i] != 0)
      {
        __asm__ __volatile__ ("int 3");  // breakpoint (x86)
      }
  }

  return 0;  // Never reached, but placate the compiler
}
...
pthread_t threadID;
pthread_create(&threadID, NULL, ThreadProc, &x[0]);

This will raise a SIGTRAP signal to your application whenever any of the x values is not zero.

Answer 2

Just use printk/syslog.

It's old-fashioned, but super duper easy.

Answer 3

Sure, it will be garbage!

The code should have been as

for (i = 0; i < 8; ++i) {
    x[i] = 0;
}

EDIT: Oops, Sorry for underestimating ;)

Answer 4

@Frank

Actually, that lets me log debug prints to a file. What I'm looking for is something that will let me print something whenever a variable changes, without me explicitly looking for the variable.

Answer 5

How about Conditional breakpoints? You could try for various conditions like first element value is zero or non zero, etc??

Answer 6

That's assuming I can easily connect a debugger. The whole point is that I only have a library, but the executable that linked it in isn't readily available.