How do I notice when something gets mounted

Question

I want my program always know all of the mountpoints. After a quick google I found that getmntent() and friends can tell me what is mounted. I realize I could do this everytime that I needed to know what was mounted, but is there some way to recognise when something gets mounted/unmounted so I don't have to keep reading the file? Is this what dBus does?

Any hints would be helpful. I'm trying to start programming (I took a course on C in 2002, when I was in college.) and found a feature I want to implement in an open source program.

Answer 1

Based on the mention of getmntent(), I'm going to guess you're dealing with Linux (other Unices also include it, but they're a lot less common nowadays...). If that's the case, man inotify should get you started.

Answer 2

If you're coding for Linux (or are using GNOME libraries nonetheless), GNOME's GIO provides a good API for this. Here's a trivial drive monitor I wrote in C that uses GTK and GIO:

#include <gio/gio.h>
#include <gtk/gtk.h>
#include <string.h>

typedef struct {
    GtkWidget *window;
    GtkWidget *vbox;
    GtkWidget *scrolled_window;
    GtkWidget *view;
    GtkWidget *button;

    GtkListStore *store;
    GVolumeMonitor *monitor;
} Context[1];

enum {
    COL_DRIVE,
    COL_ICON,
    COL_NAME,
    NUM_COLS
};

static int find_drive(GtkTreeModel *model, GDrive *drive, GtkTreeIter *iter)
{
    gboolean valid;

    valid = gtk_tree_model_get_iter_first(model, iter);
    while (valid) {
        GDrive *cur;
        gtk_tree_model_get(model, iter, COL_DRIVE, &cur, -1);

        if (cur == drive)
            return 1;

        valid = gtk_tree_model_iter_next(model, iter);
    }

    return 0;
}

static void set_drive_iter(Context ctx, GDrive *drive, GtkTreeIter *iter)
{
    GIcon *icon = g_drive_get_icon(drive);
    gchar *name = g_drive_get_name(drive);

    gtk_list_store_set(ctx->store, iter,
        COL_DRIVE, drive,
        COL_ICON, icon,
        COL_NAME, name,
        -1);

    g_free(name);
}

static void add_drive(Context ctx, GDrive *drive)
{
    GtkTreeIter iter;
    gtk_list_store_append(ctx->store, &iter);
    set_drive_iter(ctx, drive, &iter);
}

static void refresh_drive_list(Context ctx)
{
    GList *drives, *l;
    drives = g_volume_monitor_get_connected_drives(ctx->monitor);

    gtk_list_store_clear(ctx->store);

    for (l = drives; l; l = l->next)
        add_drive(ctx, l->data);
}

static void update_drive(Context ctx, GDrive *drive)
{
    GtkTreeModel *model = GTK_TREE_MODEL(ctx->store);
    GtkTreeIter iter;

    if (find_drive(model, drive, &iter))
        set_drive_iter(ctx, drive, &iter);
    else
        refresh_drive_list(ctx); //Shouldn't happen
}

static void remove_drive(Context ctx, GDrive *drive)
{
    GtkTreeModel *model = GTK_TREE_MODEL(ctx->store);
    GtkTreeIter iter;

    if (find_drive(model, drive, &iter))
        gtk_list_store_remove(ctx->store, &iter);
    else
        refresh_drive_list(ctx); //Shouldn't happen
}

static void init_drive_list(Context ctx)
{
    ctx->store = gtk_list_store_new(3, G_TYPE_POINTER, G_TYPE_ICON, G_TYPE_STRING);
    refresh_drive_list(ctx);
}

static void init_drive_view(Context ctx)
{
    GtkTreeViewColumn *column;
    GtkCellRenderer *renderer;

    ctx->view = gtk_tree_view_new_with_model(GTK_TREE_MODEL(ctx->store));
    g_object_set(ctx->view,
        "headers-visible", FALSE,
        NULL);

    renderer = gtk_cell_renderer_pixbuf_new();
    g_object_set(renderer, "stock-size", GTK_ICON_SIZE_DIALOG, NULL);
    column = gtk_tree_view_column_new_with_attributes("Icon", renderer, "gicon", COL_ICON, NULL);
    gtk_tree_view_column_set_resizable(column, TRUE);
    gtk_tree_view_append_column(GTK_TREE_VIEW(ctx->view), column);

    renderer = gtk_cell_renderer_text_new();
    column = gtk_tree_view_column_new_with_attributes("Name", renderer, "text", COL_NAME, NULL);
    gtk_tree_view_column_set_resizable(column, TRUE);
    gtk_tree_view_append_column(GTK_TREE_VIEW(ctx->view), column);
}

static void print_drive_info(GDrive *drive)
{
    GIcon *icon;
    gchar *name, *icon_string;

    name = g_drive_get_name(drive);
    icon = g_drive_get_icon(drive);
    icon_string = g_icon_to_string(icon);
    g_object_unref(icon);

    g_print("\tname: %s\n\ticon: %s\n",
        name ? name : "(null)",
        icon_string ? icon_string : "(null)");

    g_free(name);
    g_free(icon_string);
}

static void on_drive_changed(GVolumeMonitor *volume_monitor, GDrive *drive, Context ctx)
{
    g_print("Drive changed:\n");
    print_drive_info(drive);
    update_drive(ctx, drive);
}

static void on_drive_connected(GVolumeMonitor *volume_monitor, GDrive *drive, Context ctx)
{
    g_print("Drive connected:\n");
    print_drive_info(drive);
    add_drive(ctx, drive);
}

static void on_drive_disconnected(GVolumeMonitor *volume_monitor, GDrive *drive, Context ctx)
{
    g_print("Drive disconnected:\n");
    print_drive_info(drive);
    remove_drive(ctx, drive);
}

static void on_refresh_clicked(GtkButton *button, Context ctx)
{
    refresh_drive_list(ctx);
}

int main(int argc, char *argv[])
{
    Context ctx;
    memset(ctx, 0, sizeof(ctx));
    gtk_init(&argc, &argv);

    ctx->window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    g_signal_connect(ctx->window, "destroy", G_CALLBACK(gtk_main_quit), NULL);

    ctx->vbox = gtk_vbox_new(FALSE, 0);
    gtk_widget_show(ctx->vbox);
    gtk_container_add(GTK_CONTAINER(ctx->window), ctx->vbox);

    ctx->scrolled_window = gtk_scrolled_window_new(NULL, NULL);
    gtk_scrolled_window_set_policy(
        GTK_SCROLLED_WINDOW(ctx->scrolled_window),
        GTK_POLICY_AUTOMATIC,
        GTK_POLICY_AUTOMATIC);
    gtk_box_pack_start(GTK_BOX(ctx->vbox), ctx->scrolled_window, TRUE, TRUE, 0);
    gtk_widget_show(ctx->scrolled_window);

    ctx->monitor = g_volume_monitor_get();
    g_signal_connect(ctx->monitor, "drive-changed", G_CALLBACK(on_drive_changed), ctx);
    g_signal_connect(ctx->monitor, "drive-connected", G_CALLBACK(on_drive_connected), ctx);
    g_signal_connect(ctx->monitor, "drive-disconnected", G_CALLBACK(on_drive_disconnected), ctx);

    init_drive_list(ctx);
    init_drive_view(ctx);
    gtk_widget_show(ctx->view);
    gtk_container_add(GTK_CONTAINER(ctx->scrolled_window), ctx->view);

    ctx->button = gtk_button_new_from_stock(GTK_STOCK_REFRESH);
    g_signal_connect(ctx->button, "clicked", G_CALLBACK(on_refresh_clicked), ctx);
    gtk_widget_show(ctx->button);
    gtk_box_pack_start(GTK_BOX(ctx->vbox), ctx->button, FALSE, FALSE, 0);

    gtk_window_set_default_size(GTK_WINDOW(ctx->window), 500, 500);
    gtk_widget_show(ctx->window);

    gtk_main();
    return 0;
}

Compile with gcc $(pkg-config --cflags --libs gtk+-2.0) volumes.c -o volumes . Test it by running it, then plugging in a pen drive. It should update on the fly. It even displays appropriate icons.

Note that this monitors drives, not mounts. GVolumeMonitor can monitor mounts as well (see "mount-added" and "mount-removed" in devhelp). You'll have to learn a little bit about the GObject system to use this. Good luck!