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

Question

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.

Answer 1

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("%49s", row);

Answer 2

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.