inexplicable change in variable value

Question

I'm being faced with an unexplicable change in a variable value. Now I'm not very experienced in C, most of the code I wrote with one hand on the keyboard and the other tracking a page in K&R, so please be gentle.

I have a C project, in Visual Studio 2010, a Lua binding for the pupnp library. Here's some relevant code;

file: luaUPnPdefinitions.h (excerpt)

#ifndef LuaUPnPdefinitions_h
#define LuaUPnPdefinitions_h
...
// tracker for library being started or not
volatile static int UPnPStarted;
...
#endif  /* LuaUPnPdefinitions_h */

file: LuaUPnP.h (complete file)

#ifndef LuaUPnP_h
#define LuaUPnP_h

#include "upnp.h"
#include "upnptools.h"
#include "uuid.h"
#include <lua.h>
#include <lauxlib.h>
#include "luaIXML.h"
#include "darksidesync_aux.h"
#include "luaUPnPdefinitions.h"
#include "luaUPnPsupport.h"
#include "luaUPnPcallback.h"

#endif  /* LuaUPnP_h */

file: LuaUPnP.c (excerpt)

#include "luaUPnP.h"  // only include in this file
...
static int L_UpnpSendAdvertisement(lua_State *L)
{
    int result = UpnpSendAdvertisement(checkdevice(L, 1), luaL_checkint(L,2));
    if (result != UPNP_E_SUCCESS) return pushUPnPerror(L, result, NULL);
    lua_pushinteger(L, 1);
    return 1;
}
...

file: LuaUPnPsupport.h (excerpt)

#ifndef LuaUPnPsupport_h
#define LuaUPnPsupport_h

//#include <ixml.h>
#include <lua.h>
#include <lauxlib.h>
#include "luaIXML.h"
#include "upnptools.h"
#include "luaUPnPdefinitions.h"
...
UpnpDevice_Handle checkdevice(lua_State *L, int idx);
...
#endif  /* LuaUPnPsupport_h */

file: LuaUPnPsupport.c (excerpt)

#include "luaUPnPsupport.h"  // only include in this file
...
UpnpDevice_Handle checkdevice(lua_State *L, int idx)
{
    pLuaDevice dev;
    luaL_checkudata(L, idx, LPNP_DEVICE_MT);
    if (! UPnPStarted) luaL_error(L, UpnpGetErrorMessage(UPNP_E_FINISH));
    dev = (pLuaDevice)lua_touserdata(L, idx);
    return dev->device;
}

Now for the problem; the UPnPstarted static variable basically tracks whether the pupnp library background processes have been started. Now when debugging at the L_UpnpSendAdvertisement function at some point (reproducable), then UPnPstarted == 1, but when I hit this line;

int result = UpnpSendAdvertisement(checkdevice(L, 1), luaL_checkint(L,2));

and step into it, the debugger jumps to the checkdevice function (in file LuaUPnPsupport.c), and the value of UPnPstarted immediately changes to UPnPstarted == 0.

I'm lost. It's a static variable, so should be shared, why does it change value, just by stepping into that other function? In the debugger, the watch window, the values are lit up red, indicating that they just changed. Initially I thought that the files where included in the wrong order, and the UPnPstarted variable was duplicated (or had 2 instances), but when adding a watch to the watch window; &UPnPstarted to track the memory location of the variable, I don't see a change when I step into the function, so it seems to me it refers to the same memory location.

I just don't get it. Any ideas on what is happening?

Answer 1

Marking the variable as static does not mean there will be a single instance shared across translation units (essentially a .c source file plus all the headers it pulls in). Each translation until will have its own definition of UPnPStarted as each .c file includes the header file where UPnPStarted is defined.

This means when the function UpnpSendAdvertisement() is invoked for the first time it is accessing its own version of UPnPStarted, which has not been modified and will have its initial, unchanged, value of 0.

To share the same variable across translation units use extern at declaration and provide exactly one definition:

/* In the header file. */
extern volatile int UPnPStarted;

and then in one, and only one, .c file:

volatile int UPnPStarted;