Simulate backspace button press gtk in C

Question

I am writing an app on raspberry in gtk+3/C which requires a virtual numpad (e.g 0-9, backspace and enter). I will use it with a touch screen, so no keyboard or mouse available (just like a kiosk).

I am adopting this code which allows me to use button press to emulate the number and display to an entry, however I am stuck at giving it the backspace function (go back and delete one previous char) and enter function (e.g when I'm done with input, press that button will help me to get back to the main screen).

/*
   gcc -Wall keyboard1.c -o keyboard1 `pkg-config --cflags --libs gtk+-3.0`
   Tested with GTK3.22 and GTK3.22 on Ubuntu18.04   
*/
#include<gtk/gtk.h>

struct key{
    gint id;
    GtkWidget *button;
  };

static const gchar letters[18]="QWERTYASDFGHZXCVBN";
//Need single chars as strings.
static gchar single_char[2]={'A', '\0'};

static void button_clicked(GtkWidget *button, gpointer *user_data)
  {
    gpointer *button_index=g_hash_table_lookup((GHashTable*)user_data[0], button);
    g_print("Button index %i\n", (gint)(*button_index));
    gint index=(gint)(*button_index);
    single_char[0]=letters[index];
    gchar *string=g_strdup_printf("%s%s", gtk_entry_get_text(GTK_ENTRY(user_data[1])), single_char);
    gtk_entry_set_text(GTK_ENTRY(user_data[1]), string);
    g_free(string);
  }
int main(int argc, char *argv[])
  {
    gtk_init (&argc, &argv);
    gint i=0;
    gint j=0;
    
    GtkWidget *window=gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), "Keyboard");
    gtk_window_set_default_size(GTK_WINDOW(window), 400, 200);
    gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER);
    g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);

    GtkWidget *entry=gtk_entry_new();
    gtk_widget_set_hexpand(entry, TRUE);

    //Save buttons in an array.
    struct key k1;
    GArray *keyboard=g_array_new(FALSE, FALSE, sizeof(struct key));    
    for(i=0;i<18;i++)
      {
        single_char[0]=letters[i];
        k1.id=i;
        k1.button=gtk_button_new_with_label(single_char);
        g_array_append_val(keyboard, k1);
      }   
 
    //A hash table to look up array index values.
    struct key *p1=NULL;
    GHashTable *hash_table=g_hash_table_new(NULL, NULL);
    for(i=0;i<18;i++)
      {
        p1=&g_array_index(keyboard, struct key, i);
        g_hash_table_insert(hash_table, p1->button, &(p1->id));
      }

    gpointer user_data[2]={hash_table, entry};
    GtkWidget *grid1=gtk_grid_new();
    for(i=0;i<3;i++)
      {
        for(j=0;j<6;j++)
          {
            p1=&g_array_index(keyboard, struct key, i*6+j);
            gtk_grid_attach(GTK_GRID(grid1), p1->button, j, i, 1, 1);
            g_signal_connect(p1->button, "clicked", G_CALLBACK(button_clicked), user_data);
          }
      } 

    GtkWidget *scroll=gtk_scrolled_window_new(NULL, NULL);
    gtk_widget_set_vexpand(scroll, TRUE);
    gtk_widget_set_hexpand(scroll, TRUE);
    gtk_container_add(GTK_CONTAINER(scroll), grid1);

    GtkWidget *expander=gtk_expander_new("Keyboard");
    gtk_widget_set_vexpand(expander, TRUE);
    gtk_widget_set_hexpand(expander, TRUE);
    gtk_container_add(GTK_CONTAINER(expander), scroll);

    GtkWidget *grid2=gtk_grid_new();
    gtk_grid_attach(GTK_GRID(grid2), expander, 0, 0, 1, 1);
    gtk_grid_attach(GTK_GRID(grid2), entry, 0, 1, 1, 1);

    gtk_container_add(GTK_CONTAINER(window), grid2);

    gtk_widget_show_all(window);

    gtk_main();

    g_hash_table_destroy(hash_table);
    g_array_free(keyboard, TRUE);

    return 0;
  }  

Here is the link to the post.

I have tried to replace a char in the gchar letters[18]="QWERTYASDFGHZXCVBN"; with \b, \n and \t as to replace backspace, newline and tab, but only \t works. \b gave me a strange symbol:

Could you give me suggestions on how to do it. Thank you very much!

Answer 1

You seem to confuse two things: gtk_entry_set_text sets the caption of a GTK element to a string of characters. It does not interpret it's characters.

That's why gtk_entry_set_text works well with non-control characters but stuck at \b. You had to send a GtkEvent using gtk_propagate_event to handle this properly (see here and here). \n is not a control character is this manner: It's possible that the GTK element your writing to just doesn't support newlines and Return would be interpreted as confirmation.