strange behavior of random numbers with gtkmm

[jody <jody xha gmail com>]

Hi
I've found a strange effect in gtkmm programs involving random numbers:
for a given seed not always produce the same sequence is produced.

I wrote a test program which shows this. There are two buttons in the window,
'reset' to set the random seed to a constant value, and 'step' to get a random
number and print it.
The reset function is called in the constructor.

This is the output i get when i run the program and do the following:
- press the step-button 3 times
- press the reset-button
- press the step-button 3 times
- press the reset-button
- press the step-button 3 times

[jody localhost rtest]$ ./gtkrand
Reset
One step: 1851975387
One step: 1789185830
One step: 545709798
Reset
One step: 1889402444
One step: 1202690506
One step: 785052894
Reset
One step: 1889402444
One step: 1202690506
One step: 785052894

So the first time, the sequence is different, all following ones are
always the same.

If i do not call srand in the constructor but instead start the
sequence with the reset-button,
the first sequence of random numbers is the same as the following.

So my question is: does gtkmm call random numbers somewhere in its code?
If yes, at what points?
If no, how can this strange behaviour be explained?

Thank You
  Jody


Here is the code

///// gtkrandwin.h ------------------------------------------------
#ifndef __GTKRANDWIN_H__
#define __GTKRANDWIN_H__

#include <gtkmm/window.h>
#include <gtkmm/button.h>
#include <gtkmm/box.h>

class gtkrandwin : public Gtk::Window {
public:
    gtkrandwin();

protected:

    Gtk::HBox    m_HBox0;
    Gtk::Button  m_butReset;
    Gtk::Button  m_butStep;
    virtual void reset_action();
    virtual void step_action();

};

#endif


///// gtkrandwin.cpp ------------------------------------------------
#include <gtkmm/window.h>
#include <gtkmm/button.h>
#include <gtkmm/box.h>

#include "gtkrandwin.h"

#define CAPTION_RESET  "Reset"
#define CAPTION_STEP   "Step"

//-------------------------------
// constructor
//
gtkrandwin::gtkrandwin()
    : m_HBox0(false, 5),
      m_butReset(CAPTION_RESET),
      m_butStep(CAPTION_STEP) {


    m_butStep.signal_clicked().connect(sigc::mem_fun(*this,
&gtkrandwin::step_action) );
    m_butReset.signal_clicked().connect(sigc::mem_fun(*this,
&gtkrandwin::reset_action) );

    m_HBox0.pack_start(m_butReset,   Gtk::PACK_EXPAND_WIDGET,5);
    m_HBox0.pack_start(m_butStep,   Gtk::PACK_EXPAND_WIDGET,5);
    add(m_HBox0);

    reset_action();
    show_all_children();
}


//-----------------------------------------------------------------------------
// step_action
//   callback for reset action
//
void gtkrandwin::reset_action() {
    // one step
    srand(12343271);
    printf("Reset\n");
}

//-----------------------------------------------------------------------------
// step_action
//   callback for reset action
//
void gtkrandwin::step_action() {
    // one step
    printf("One step: %d\n", rand());
}


///// gtkrand.cpp ------------------------------------------------
#include <glibmm.h>
#include <gtkmm/main.h>

#include "gtkrandwin.h"


int main(int iArgC, char *apArgV[]) {
    Gtk::Main kit(iArgC, apArgV);

    gtkrandwin m_randwin;

    Gtk::Main::run(m_randwin);

    return 0;
}