C - problem with const - error: initializer element is not constant

Question

I try add global variable in my buttons.c file, but have a error - initializer element is not constant. Example headers.h file

struct MainStruct {
  GtkEntryBuffer *buffer;
  GtkWidget *entry;
  GtkWidget *label;
};

extern struct MainStruct *p;
extern const char *text_entry;
void lower_button_clicked(GtkWidget *lowerbutton);

and when file main.c calls file buttons.c, I cannot define the variable text_entry. What am I doing wrong?

buttons.c

#include <gtk/gtk.h>
#include "headers.h"

const char *text_entry = gtk_entry_buffer_get_text(p -> buffer); // is not constant, why?
void lower_button_clicked(GtkWidget *lowerbutton)
{
  printf("%s\n", text_entry);
}

I saw a lot of similar questions that talk about static, but

static const char *text_entry = gtk_entry_buffer_get_text(p -> buffer);

not working. How do I define this variable to be global? to avoid duplication in similar functions

Answer 1

From the C Standard (6.7.9 Initialization)

4 All the expressions in an initializer for an object that has static or thread storage duration shall be constant expressions or string literals.

And in this declaration

const char *text_entry = gtk_entry_buffer_get_text(p -> buffer);

the initializer is not a constant expression. So the compiler issues an error.

Also pay attention to that these declarations

extern const char *text_entry;

and that follows it

static const char *text_entry = /*...*/;

contradict each other. The first declaration declares the variable text_entry as having external linkage while the second declaration declares the variable as having internal linkage.