gtk_entry_get_text: assertion 'GTK_IS_ENTRY (entry)' failed

I'm creating a small program with C++ and GTK 3 (just learning it), and i have had trouble with this. So, i isolated the problematic part.

It is suposed to get the entry and print it when the button is clicked.

This is the code:

#include <iostream>
#include <gtk/gtk.h>

using namespace std;

GtkWidget *wventana;
GtkWidget *wgrid;

//FUNCS

void ventana(string titulo, int margen)
{
    const char * tituloc = titulo.c_str();

    wventana = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_position (GTK_WINDOW (wventana), GTK_WIN_POS_CENTER);
    gtk_window_set_title (GTK_WINDOW (wventana), tituloc);
    gtk_container_set_border_width(GTK_CONTAINER(wventana), margen);
}

void grid()
{
    wgrid = gtk_grid_new();
    gtk_container_add(GTK_CONTAINER(wventana), wgrid);
    gtk_grid_set_row_spacing (GTK_GRID (wgrid), 10);
    gtk_grid_set_column_spacing (GTK_GRID (wgrid), 25);
}

void boton_clic (GtkEntry *wentrada, gpointer user_data)
{
    const char *nombre;
    nombre = gtk_entry_get_text(wentrada);
    cout << nombre << endl;
}

void entrada(int x, int y, int lx, int ly)
{
    GtkWidget *wentrada;
    wentrada = gtk_entry_new();
    gtk_grid_attach (GTK_GRID (wgrid), wentrada, x, y, lx, ly);
}

void boton(string texto, int x, int y, int lx, int ly)
{
    const char * wtexto = texto.c_str();

    GtkWidget *wboton;
    wboton = gtk_button_new_with_label (wtexto);
    gtk_grid_attach (GTK_GRID (wgrid), wboton, x, y, lx, ly);

    g_signal_connect (GTK_BUTTON (wboton), "clicked", G_CALLBACK (boton_clic), G_OBJECT     (wventana));
}

//MAIN

int main(int argc, char *argv[])
{
    gtk_init (&argc, &argv);

    ventana("ventana",10);
    grid();
    entrada(1, 1, 1, 1);
    boton("Aprietame", 1, 2, 1, 1);

    gtk_widget_show_all (wventana);
    gtk_main ();

    return 0;
}

It doesnt get the entry and shows the following error message:

(boton:6669): Gtk-CRITICAL **: gtk_entry_get_text: assertion 'GTK_IS_ENTRY (entry)' failed

Could anyone please tell what the problem is? and, how can I make it work? Or in any case, what is a good alternative to do this? (it has to be with separated functions)

Thanks in advance.

---

Thanks for your answer @ptomato, I tried replacing with the changes you proposed, been the new code:

#include <iostream>
#include <gtk/gtk.h>

using namespace std;

GtkWidget *wventana;
GtkWidget *wgrid;

//FUNCS

void ventana(string titulo, int margen)
{
    const char * tituloc = titulo.c_str();

    wventana = gtk_window_new (GTK_WINDOW_TOPLEVEL);
    gtk_window_set_position (GTK_WINDOW (wventana), GTK_WIN_POS_CENTER);
    gtk_window_set_title (GTK_WINDOW (wventana), tituloc);
    gtk_container_set_border_width(GTK_CONTAINER(wventana), margen);
}

void grid()
{
    wgrid = gtk_grid_new();
    gtk_container_add(GTK_CONTAINER(wventana), wgrid);
    gtk_grid_set_row_spacing (GTK_GRID (wgrid), 10);
    gtk_grid_set_column_spacing (GTK_GRID (wgrid), 25);
}

void boton_clic (GtkButton *wboton, GtkEntry *wentrada)
{
    const char *nombre;
    nombre = gtk_entry_get_text (wentrada);
    cout << nombre << endl;
}

void entrada(int x, int y, int lx, int ly)
{
    GtkWidget *wentrada;
    wentrada = gtk_entry_new();
    gtk_grid_attach (GTK_GRID (wgrid), wentrada, x, y, lx, ly);
}

void boton(string texto, int x, int y, int lx, int ly)
{
    const char * wtexto = texto.c_str();

    GtkWidget *wboton;
    wboton = gtk_button_new_with_label (wtexto);
    gtk_grid_attach (GTK_GRID (wgrid), wboton, x, y, lx, ly);

    g_signal_connect (wboton, "clicked", G_CALLBACK (boton_clic), wentrada);
}

//MAIN

int main(int argc, char *argv[])
{
    gtk_init (&argc, &argv);

    ventana("ventana",10);
    grid();
    entrada(1, 1, 1, 1);
    boton("Aprietame", 1, 2, 1, 1);

    gtk_widget_show_all (wventana);
    gtk_main ();

    return 0;
}

but when i try to compile it, it shows:

In file included from /usr/include/glib-2.0/gobject/gobject.h:28:0,
                 from /usr/include/glib-2.0/gobject/gbinding.h:29,
                 from /usr/include/glib-2.0/glib-object.h:23,
                 from /usr/include/glib-2.0/gio/gioenums.h:28,
                 from /usr/include/glib-2.0/gio/giotypes.h:28,
                 from /usr/include/glib-2.0/gio/gio.h:26,
                 from /usr/include/gtk-3.0/gdk/gdkapplaunchcontext.h:28,
                 from /usr/include/gtk-3.0/gdk/gdk.h:32,
                 from /usr/include/gtk-3.0/gtk/gtk.h:30,
                 from boton.cpp:2:
boton.cpp: In function ‘void boton(std::string, int, int, int, int)’:
boton.cpp:51:64: error: ‘wentrada’ was not declared in this scope
  g_signal_connect (wboton, "clicked", G_CALLBACK (boton_clic), wentrada);
                                                                ^
/usr/include/glib-2.0/gobject/gsignal.h:472:73: note: in definition of macro     ‘g_signal_connect’
     g_signal_connect_data ((instance), (detailed_signal), (c_handler), (data), NULL,     (GConnectFlags) 0)
                                                                             ^
 Compilation failed.

What is wrong? What should I do???

Could you please explain again about the second argument? (english is not my mother language and this part was a little confusing for me :P )

Thanks again.

The boton_clic signal handler has two arguments: a GtkWidget * and a pointer to some user data that you specify. The first argument is always the object that sent the signal, so that is wboton; but you labeled the argument wentrada and are treating it as a GtkEntry *, hence the error message.

The second argument is the user data that you specify when connecting the signal. In your g_signal_connect() call, you pass wventana here. But if you want to access the entry in your signal handler, you should pass wentrada instead.

So, it should look like this:

void boton_clic (GtkButton *wboton, GtkEntry *wentrada)
{
    const char *nombre;
    nombre = gtk_entry_get_text (wentrada);
    cout << nombre << endl;
}

...

g_signal_connect (wboton, "clicked", G_CALLBACK (boton_clic), wentrada);