Get the selected text from GTK3 ComboBox in C

Question

I'm only starting to learn GTK3, but I'm having a real struggle. All of the documentation that i could find is for GTK2 and the gnome dev web site is simply not good enough. Now I am trying to get the selected text/item from a GtkComboBox, here is some of the code which I have tried.

  gint array =gtk_combo_box_get_active(cbox_quaity);
  //or 
  gchar *array =gtk_combo_box_get_active(cbox_quaity);

If i use the gint, the printout will be "0" (zero) no matter what itom of the list I select. If I use gchar, I get back "(null)". In addition, I get these warnings while compiling.

gcc -c -g -O0 -Wall -pthread -pipe src/main.c `pkg-config --cflags --libs gtk+-3.0` -o main.o
src/main.c: In function ‘btn_start_click’:
src/main.c:41:17: warning: initialization makes pointer from integer without a cast [-Wint-conversion]
   gchar *array =gtk_combo_box_get_active(cbox_quaity);
                 ^~~~~~~~~~~~~~~~~~~~~~~~
gcc -o tstreamer-gtk main.o -pthread `pkg-config --cflags --libs gtk+-3.0` -export-dynamic

The user interface was created in glade and the combo box used a GtkListStore. Also, here is the source code of the C code:

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

GtkWidget *btn_start;
GtkComboBox *cbox_quaity;
#define MAX_WORD_SIZE 32

void btn_start_click(){

  gint active_item = gtk_combo_box_get_active(cbox_quaity);
  printf("%d\n",active_item);
}

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

  GtkBuilder *builder;
  GtkWidget *window;

  gtk_init(&argc, &argv);

  builder = gtk_builder_new();
  gtk_builder_add_from_file (builder, "glade/window_main.glade", NULL);

  window = GTK_WIDGET(gtk_builder_get_object(builder, "window_main"));
  gtk_builder_connect_signals(builder, NULL);

  btn_start = GTK_WIDGET(gtk_builder_get_object(builder, "btn_start"));

  g_object_unref(builder);
  gtk_widget_show(window);
  gtk_main();
  return 0;
}

// called when window is closed
void on_window_main_destroy(){
  gtk_main_quit();
}

Answer 1

You are doing a few things wrong.

First, GtkBuilder does not automatically bind controls to variables. You explicitly have to call gtk_builder_get_object() on all the controls you need to get to, including cbox_quaity.

Now, you can't program on examples alone; you'll need the documentation to find out what you can do and how. If you check the docs, you'll see gtk_combo_box_get_active() returns a gint with the index of the selected item, so the gint array step was right. The reason you kept getting 0 from that function was because, as above, you forgot to initialize cbox_quaity, so it was doing gtk_combo_box_get_active(NULL), which of course makes no sense.

(GtkBuilder can connect signals automatically by name because of how dynamic linking works, but that is a different discussion.)

Your signal method signatures are also wrong. You need to have the same signatures as the signal you are connecting, which you can get... from the documentation. In your case, btn_start_click() needs to match the signal for GtkButton::clicked and on_window_main_destroy() needs to match the signal for GtkWidget::destroy. Each signal function begins with a parameter that is the widget in question, and each signal function ends with a parameter that allows you to pass data of your choice around, as a more futureproof alternative to using global variables. In the case of GtkBuilder signals, this is the second argument to gtk_builder_connect_signals(). Mind your return values too; if a signal returns a value, that return value is important!

All GTK+ types have macros like GTK_WIDGET() (which you did use at one point) and GTK_COMBO_BOX() to allow run-time-type-checked conversions between GTK+ widget types; this allows GTK+ widgets to be polymorphic in C. Therefore, most GTK+ code returns and stores GtkWidget *s directly.

And finally, the documentation is NOT devoid of examples. Not only is there a full introductory section in the documentation proper, which includes a detailed tutorial as well as smaller examples, there is also a program included with GTK+ called gtk3-demo that provides a lot of sample applications AND source code for those applications. (Some Linux distributions may split this program and a few related ones into a separate package named something like gtk3-examples.)

Good luck!

Answer 2

I resolved this way:

GtkTreeIter   iter;
GtkTreeModel* model;
gchar*        value;

1.- I get model of combobox
    model = gtk_combo_box_get_model(GTK_COMBO_BOX(combobox));

2.- I get the iter selected in the model
    gtk_tree_model_get(model, &iter, 1, &value,  -1);

3.- in value is written the active text

Greetings !
Note:
combobox is gtk_combo_box

Answer 3

This is the "right way to do it" ™:

gint active_item = gtk_combo_box_get_active(cbox_quaity);

Yes, you must use an int, not a pointer to int, as explained in the documentation of gtk_combo_box_get_active.

If that doesn't work, this is because you're not calling it at the right moment or place, and hence we need more context to determine the problem.

Answer 4

Here is what I would

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

GtkWidget *btn_start;
GtkComboBox *cbox_quaity;
#define MAX_WORD_SIZE 32

void btn_start_click(GtkWidget *widget, gpointer builder){
  cbox_quaity=GTK_COMBO_BOX(gtk_builder_get_object(builder, "ID of combobox"));
  gint active_item = gtk_combo_box_get_active(cbox_quaity);
  printf("%d\n",active_item);
}

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

  GtkBuilder *builder;
  GtkWidget *window;

  gtk_init(&argc, &argv);

  builder = gtk_builder_new();
  gtk_builder_add_from_file (builder, "glade/window_main.glade", NULL);

  window = GTK_WIDGET(gtk_builder_get_object(builder, "window_main"));
  gtk_builder_connect_signals(builder, NULL);

  /* Add a signal to the button to call your function and pass in the builder */
  btn_start = gtk_builder_get_object(builder, "btn_start");
  g_signal_connect(btn_start,"clicked",G_CALLBACK (btn_start_click),builder);

  g_object_unref(builder);
  gtk_widget_show(window);
  gtk_main();
  return 0;
}

// called when window is closed
void on_window_main_destroy(){
  gtk_main_quit();
}