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    <p>The following code does what I asked. It uses pthreads. I don't know if it's the most elegant but it seems to work. The trick was using two flags: one for the idle state and one for a cancel request, which avoids needing to cancel the thread using the "pthread_cancel" function, which I find to be unusual in real code. </p> <pre><code>#include &lt;gtk/gtk.h&gt; #include &lt;pthread.h&gt; #include &lt;errno.h&gt; #include &lt;stdlib.h&gt; #define UNUSED(x) (void)(x) #define handle_error_en(en, msg) do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0) typedef struct _Data { GtkWidget *window1, *button1; } Data; static Data *data; static pthread_mutex_t calcmutex = PTHREAD_MUTEX_INITIALIZER; static pthread_t calcthread=0; static gboolean idle=TRUE,cancel_request=FALSE; void *calcfunc(void *arg) { int i,s; UNUSED(arg); g_print("\tstarting thread\n"); s = pthread_setcancelstate(PTHREAD_CANCEL_ASYNCHRONOUS, NULL); if (s != 0) { handle_error_en(s, "pthread_setcancelstate"); } gdk_threads_enter(); gtk_button_set_label(GTK_BUTTON(data-&gt;button1),"Stop"); gdk_threads_leave(); g_print("\tstarting work...\n"); for (i=0; i&lt;100000000 ;++i) { /* check for cancelation */ pthread_mutex_lock(&amp;calcmutex); if ( cancel_request ) { g_print("\t[cancel request noted].\n"); pthread_mutex_unlock(&amp;calcmutex); break; } pthread_mutex_unlock(&amp;calcmutex); /* do "calculation" */ i=i*1*-1*1*-1; } g_print("\tdone work.\n"); gdk_threads_enter(); gtk_button_set_label(GTK_BUTTON(data-&gt;button1),"Start"); gdk_threads_leave(); pthread_mutex_lock(&amp;calcmutex); cancel_request=FALSE; idle=TRUE; pthread_mutex_unlock(&amp;calcmutex); g_print("\tdone thread.\n"); pthread_exit(NULL); } void on_button1_clicked(GtkWidget *widget, gpointer *ldata) { int s; UNUSED(widget); UNUSED(ldata); g_print("entered on_button1_clicked\n"); pthread_mutex_lock(&amp;calcmutex); if ( idle ) { g_print("idle, starting thread\n"); s = pthread_create(&amp;calcthread, NULL, calcfunc, NULL); if (s != 0) { handle_error_en(s, "pthread_create"); } idle=FALSE; } else { g_print("not idle and not first time, making canceling request.\n"); cancel_request=TRUE; } pthread_mutex_unlock(&amp;calcmutex); g_print("finished on_button1_clicked\n"); } /* just defines our setup */ int main (int argc, char *argv[]) { g_thread_init(NULL); gdk_threads_init(); gdk_threads_enter(); gtk_init(&amp;argc, &amp;argv); data=g_slice_new0(Data); printf("initial idle is %d\n",idle); /* add widgets and objects to our structure */ data-&gt;window1=gtk_window_new(GTK_WINDOW_TOPLEVEL); gtk_window_set_default_size(GTK_WINDOW(data-&gt;window1),250,250); data-&gt;button1=gtk_button_new_with_label("Start"); gtk_container_add(GTK_CONTAINER(data-&gt;window1),GTK_WIDGET(data-&gt;button1)); gtk_signal_connect(GTK_OBJECT(data-&gt;window1), "delete-event", gtk_main_quit, NULL); gtk_signal_connect(GTK_OBJECT(data-&gt;button1), "clicked", G_CALLBACK(on_button1_clicked), NULL); gtk_widget_show_all(GTK_WIDGET(data-&gt;window1)); gtk_main(); /* free the memory and stuff */ g_slice_free(Data, data); pthread_mutex_destroy(&amp;calcmutex); gdk_threads_leave(); return 0; } </code></pre> <p>This compiles warningless with</p> <pre><code>gcc -Wall -Wextra -Wconversion -pedantic `pkg-config --cflags --libs gtk+-2.0` start_stop.c -o start_stop </code></pre>
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    1. POUsing GTK and C, how can I start/stop a long calculation (in a seperate thread) using a button?
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