how to access pointer member of nested structure in c code from another file

Question

Below is complete code

brief summary of my problem is below: I have one global structure as mentioned below in file1.c and its getting initialized with some value in file1.c by function"assign_value()" and now I am printing this value by function "print_value()" from file2.c.

Problem is that: its not printing proper value from file2.c on the other hand if I call "print_value()" from file1.c inside function "assign_value()" as mentioned below then it shows proper value.

Please suggest what I am missing, why I am not able to print proper value by calling print_value() function in file2.c

  file1.c

My_struct_one is nested structure containing another structure My_struct_two and my_struct_obj_global is global variable in file1.c

    //file1.c

    #include <stdio.h>

    #include "file1.h"

    typedef unsigned long int List;

    typedef struct
    {

     List* my_list;
    }My_struct_two;


     typedef struct
    {

     My_struct_two struct_two;
    }My_struct_one;


     My_struct_one struct_global;



      void assign_value()
     {
     List value=9;

     struct_global.struct_two.my_list = &value;
     print_value();
     }

    void print_value()
    {
    printf("inside print");
    printf("value=%u\n",*(struct_global.struct_two.my_list));
    }

file1.h //file.h

  #ifndef _file1_c

  #define _file1_c

  void print_value();
  void assign_value();

  #endif

file2.c #include

  #include "file1.h"

  int main()

  {

  assign_value();

  print_value();

  return 0;

   }

OUTPUT: inside printvalue=9 inside printvalue=4195506

Mine doubt is why I cant access value from file2.c,

Start by putting common definitions and declarations in a header file that both source files could include. — Some programmer dude
– Some programmer dude, Commented Jan 15, 2019 at 13:13
And don't forget that all symbols you use must be declared or defined before you use them. — Some programmer dude
– Some programmer dude, Commented Jan 15, 2019 at 13:14
Lastly, listen to your compiler and what it tells you. List* value=9; is wrong. So is my_struct_obj_global.my_struct_obj.my_list = &value. It seems you either learning C by guessing (which will not work much longer) or you skipped a few chapters in whatever book your reading (or possibly skipped a few classes). My recommendation is that you get a couple of books and start over. Good books will also describe how to split programs into multiple source and header files. — Some programmer dude
– Some programmer dude, Commented Jan 15, 2019 at 13:16
Do you link object files (file1.o and file2.o) together or do you have an include statement in file2.c (do you include a header file or the c file)? Because, in one case it might work, in others it will not. — kmartin
– kmartin, Commented Jan 15, 2019 at 13:38
Please read the help pages, especially "What topics can I ask about here?" and "What types of questions should I avoid asking?". Also take the tour and read about how to ask good questions and this question checklist. Lastly please learn how to create a minimal reproducible example. — Some programmer dude
– Some programmer dude, Commented Jan 15, 2019 at 14:02

AlexP · Accepted Answer · 2019-01-15 14:03:02Z

1

value is a local variable in function assign_value(). It is created when the function is entered, and it is destroyed when the function returns.
In function print_value(), the value of struct_global.struct_two.my_list is a dangling pointer: its value is the address of a variable which does not exist any more.
Dereferencing the value of a dangling pointer is undefined behavior.

answered Jan 15, 2019 at 14:03

AlexP

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