How to avoid a set of values /collected in .so callback calls/ from pointing to same address?

jollyroger <>
Mon, 22 Nov 2010 16:52:47 -0800 (PST)
Hi all.
It seems that here's a problem with a pointer constantly bound to some
fixed address.

// compile with: g++ -o test -Wall `pkg-config --cflags --libs
glib-2.0` -lgthread-2.0
// or g++ -o testq -lglib-2.0 -I/usr/include/glib-2.0 -I/usr/
lib64/glib-2.0/include/ -lgthread-2.0c

#include <glib.h>
#include <iostream>
#include <string.h>
int counter;
GAsyncQueue *q;

typedef struct MyStruct {
int foo;
char *data;
} *MyStructPtr;

gpointer looper(gpointer data) {
        while (1) {
                while (g_async_queue_length(q) > 0) {
                        MyStructPtr xtransport;
                        xtransport = new struct MyStruct;
                        xtransport = (MyStructPtr)g_async_queue_try_pop(q);

 // The actual code is acting as weird, as if we have here, xtransport-

data = last_text_value_ever_received;
std::cout << "str # " << xtransport->foo << ":" << xtransport-
data << ":str\n";
void adder(char *data) {
        char *processedData;
        // someExternalAPICallToprocessData(data, &processeddata);
        processedData = (char *)data;
        MyStructPtr dtransport;
        dtransport = new struct MyStruct;

        dtransport->data = processedData;//processeddata;
        dtransport->foo = ++counter;

        g_async_queue_push(q, (gpointer*) dtransport);
int main() {
        GThread *l;
        q = g_async_queue_new();
        l = g_thread_create(looper, NULL, FALSE, NULL);
        char *t;
        t = strdupa("1111");
        t = strdupa("222");
        sleep (5);

This is a working lab mockup of some larger project. On the project, a
strange behavior is noticed;

instead of main(), we're sitting in a callback function, that gets
some data from a system component. Trying to print (e.g. cout) that
data before putting to queues, can get something like:

N1 (in Callback): aaaabbbbcccc
N2 (in Callback): bbbbaaaacccc.

Same, if the access to (newly set pointer ) dtransport->data is made
in a same code where it was created (mainloop/callback function)t;
that's fine.

But can't get the correct value of the first elements in the queue!
Data fields is always the same - accessing the collected data via
queue, can get only the value that was set in last callback; evil
pointer hell supposed!

str #1: bbbbaaaacccc.
str #2: bbbbaaaacccc.

The question is, **would that
 someExternalAPICallTOprocessData() fill &processeddata with a pointer
to a fixed address storing only the data of last call**?
How to get a copy of char* being returned and safely put it into
MyStruct ?Note, that a next function uses offsets...

    void process(char *data) {
     my *ptr = NULL;
     ptr = data + SOME_SIZE_CONST

And somehow, memcpy'ng the content of processedData seems to segfault
the .so library which contains someExternalAPICallTOprocessData(). In
the mockup context, it is the end of adder(); in the real context it
is end of callback function to kernel networking stuff; so funny.

gcc 4.4.5.


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