C, gtk: ‘window’ undeclared (first use in this function)-CodePudding

I'm very new in C, i've started learning this language about 1 week ago, coming from Python. So,I have difficult understanding the C syntax, and the solution online. I'm trying building a simple graphic interface. In this example project created for this post, if the user types "create_window", the program creates a window, and if after it the user types "create_button" the program should create a button.. but it doesn't work. Below there is the error, but first the code:

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

void main(int argc, char* argv[]){
    char row[50];
    for(int i=0;i<=1;i  ){
        scanf("%s",row);
        if(strstr(row,"create_window")!=NULL){
            GtkWidget* window;
            GtkWidget* button;
            gtk_init(&argc,&argv);
            window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
            gtk_window_set_title(GTK_WINDOW(window), "New window");
            g_signal_connect(window,"destroy",G_CALLBACK(gtk_main_quit),NULL);  
            gtk_window_set_default_size(GTK_WINDOW(window),200,200);
            gtk_widget_show_all(window);
            gtk_main();
        }
        if(strstr(row,"create_button")!=NULL){
            button = gtk_button_new_with_label("Button text");
            gtk_container_add(GTK_CONTAINER(window), button);
            gtk_widget_show_all(window);
            gtk_main();
        }
    }
}

this code create successfully the window, but not the button. After creating the window, i type "create_button" and i receive this error:

error: ‘window’ undeclared (first use in this function)
   80 |             gtk_container_add(GTK_CONTAINER(window), button);

the complete error refers to my """complete""" project, if you need it write in a comment.

CodePudding user response：

Your program doesn't make sense. You cannot create a button till you have a window to place it on (even if you fix the syntax error of making the variable available in the 2nd if statement). Also, gtk_main() runs an event loop and doesn't return. Perhaps you want to optionally create a button?

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

void main(int argc, char* argv[]){
    gtk_init(&argc, &argv);
    GtkWidget* window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), "New window");
    g_signal_connect(window,"destroy", G_CALLBACK(gtk_main_quit),NULL);
    gtk_window_set_default_size(GTK_WINDOW(window),200,200);
    char row[50];
    fgets(row, 50, stdin);
    if(!strcmp(row, "create_button\n")) {
        GtkWidget *button = gtk_button_new_with_label("Button text");
        gtk_container_add(GTK_CONTAINER(window), button);
    }
    gtk_widget_show_all(window);
    gtk_main();
}

I build the program and executed it like this:

gcc 1.c $(pkg-config --cflags atk) $(pkg-config --cflags gdk-3.0) $(pkg-config gtk-3.0) $(pkg-config --libs gtk -3.0) $(pkg-config --libs gdk-3.0) && ./a.out

If you enter create_button\n it will create a window with a button, and if you enter any other line it will just create the window.

Otherwise, you need to tell us what should happen for "create_button"? Do we automatically create a window and then the button? Do we just remember that we need to create the button when user asks to "create_window"? If you want to show the window, then regain control, you need to use gtk_main_iteration_do(FALSE) instead of gtk_main(), and then you can add the button.

CodePudding user response：

Your window variable is scoped exclusively to that first if.

Variables only exist within the scope in which they've been declared.

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

void main(int argc, char* argv[]) {
    char row[50];
    for(int i = 0; i <= 1; i  ) {
        scanf("%s", row);

        GtkWidget* window;
        GtkWidget* button;

        if (strstr(row, "create_window") != NULL) {
            gtk_init(&argc, &argv);
            window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
            gtk_window_set_title(GTK_WINDOW(window), "New window");
            g_signal_connect(window, "destroy", G_CALLBACK(gtk_main_quit), NULL);  
            gtk_window_set_default_size(GTK_WINDOW(window), 200, 200);
            gtk_widget_show_all(window);
            gtk_main();
        }

        if (strstr(row, "create_button") != NULL) {
            button = gtk_button_new_with_label("Button text");
            gtk_container_add(GTK_CONTAINER(window), button);
            gtk_widget_show_all(window);
            gtk_main();
        }
    }
}

Also, I suggest using a width specifier with scanf to avoid overflows.

scanf("Is", row);