Question related to NULL assignment in pointers to structs

**C_me_run** · 12-07-2023

Let's say we have this code under the pe-processor directives, as if we were going to create a linked list:

Code:

struct node
{
  int value;
  struct node *next;
};

struct node *first = NULL; 
struct node *current = NULL; 
struct node *previous = NULL;

The assignment here doesn't appear to be necessary, as I've tested pointers to struct without the NULL assignment, and when you make the linked list...the next pointer is NULL by default. This appears to be true for both gcc and clang.

However, in the interest of preventing un-defined behavior...does assigning NULL like that to those 3 pointers mean the next pointer inside of them is also NULL?

**laserlight** · 12-07-2023

Originally Posted by C_me_run

The assignment here doesn't appear to be necessary, as I've tested pointers to struct without the NULL assignment

That's initialisation, not assignment. The key here is that you have declared these variables outside of a function, so they have static storage duration (this is true even though the static keyword was not used; if you did use it, they would still have static storage duration, but would have internal linkage instead of the external linkage that they have now), and are initialised at startup. If you don't specify any constant for initialisation, they are zero initialised, which for pointers means that they are initialised to be null pointers.

Originally Posted by C_me_run

and when you make the linked list...the next pointer is NULL by default.

How are you making the linked list?

Originally Posted by C_me_run

However, in the interest of preventing un-defined behavior...does assigning NULL like that to those 3 pointers mean the next pointer inside of them is also NULL?

It doesn't, because there is no next pointer inside of them. They are pointers to struct node objects, but not struct node objects themselves, thus they don't have any members, hence they don't have a next pointer member.

**entrocraft** · 12-08-2023

Here are step-by-step ANSI C citations to confirm the explicit NULL assignments aren’t required.

3.1.2,1 Scopes of Identifiers
“If the declarator or type specifier that declares the identifier appears outside of any block or list of parameters, the identifier has file scope, which terminates at the end of the translation unit. If the declarator or type specifier that declares the identifier appears inside a block or within the list of parameter declarations in a function definition, the identifier has block scope, which terminates at the } that closes the associated block.”

These struct declarations aren't within a block, so they have file scope.

3.1.2.2 Linkages of Identifiers
“If the declaration of an identifier for an object has file scope and no storage-class specifier, its linkage is external.”

These file-scoped struct definitions have external linkage.

3.1.2.4 Storage Durations of Objects
“An object whose identifier is declared with external or internal linkage, or with the storage class specifier static has static storage duration.”

These file-scoped, externally-linked struct definitions have static storage duration.

3.5.7 Initialization -> Semantics
“If an object that has static storage duration is not initialized explicitly, it is initialized implicitly as if every member that has arithmetic type were assigned 0 and every member that has pointer type were assigned a null pointer constant.”

These file-scoped, externally-linked struct definitions with non-explicit static storage duration are initialized with NULL.

**rstanley** · 12-08-2023

Originally Posted by entrocraft

Here are step-by-step ANSI C citations to confirm the explicit NULL assignments aren’t required.
...

What document are you quoting? Even in the C90 Standard RFC, the sections are:

6.1.2.1 Scopes of identifiers
6.1.2.2 Linkages of identifiers
...

You should be quoting the latest C18 Standard,

**john.c** · 12-08-2023

Yeah, he's referring to the initial ANSI C standard (aka C89 or C90).
https://web.archive.org/web/20161223....uk/ansi.c.txt

Isn't the current standard called C17 (not C18)?
C (programming language) - Wikipedia

What you've posted is referred to as "C2x working draft" here:
WG 14 Document log

This seems to be the closest to the final draft of C17:
https://www.open-std.org/jtc1/sc22/w...docs/n2176.pdf

Working draft of C23 (coming in 2024?) here:
https://www.open-std.org/jtc1/sc22/w...docs/n3096.pdf

**rstanley** · 12-08-2023

Yes, he was probably using a copy of C89 (ANSI) not C90 (ISO)

At some point, I did see C17 listed on an ISO page as C18, and really should be called C18 as it was adopted in 2018:
"C17 ISO/IEC 9899:2018"

Which breaks the naming convention of C90, C99, and C11.

This file is password protected:

This seems to be the closest to the final draft of C17:
https://www.open-std.org/jtc1/sc22/w...docs/n2176.pdf

**C_me_run** · 12-08-2023

Originally Posted by laserlight

That's initialisation, not assignment.

It's initialization and assignment, unless "=" is not an assignment operator (and yes it is)

If you don't specify any constant for initialisation, they are zero initialised, which for pointers means that they are initialised to be null pointers.

No, actually this is not true at all. Let me prove it:

Code:

#include<stdio.h>
int main()
{
  int *value;


  if(!value)
  {
    printf("it's a NULL pointer!\n");
  }
  else
  {
    puts("it's not a NULL pointer\n");
  }


  return 0;
}

Here's the output:

it's not a NULL pointer

And that's the reason I posted this anyway: it seems that the pointer inside of the struct is NULL when I allocate memory for it.

**rstanley** · 12-08-2023

Global variables, defined outside of any function, not explicitly initialized, are automatically initialized to zero or NULL in the case of pointers.

Local variables, defined inside of a function, are NOT initialized, and pickup whatever garbage value is in memory when the function is called.

You should initialize ALL local variables!!! Explicitly initializing ALL variables is always a god idea, especially pointers!

Example:

Code:

#include <stdio.h>

char *ptr1;  // Global automatically initialized to NULL

int main(void)
{
  char *ptr2;  // Local NOT initialized!
  
  printf("ptr1 == %p\n", (void *)ptr1);
  printf("ptr2 == %p\n", (void *)ptr2);
  
  return 0;
}

Code:

Output:

ptr1 == (nil)
ptr2 == 0x7fdbdf952ab0

**C_me_run** · 12-08-2023

Originally Posted by rstanley

Global variables, defined outside of any function, not explicitly initialized, are automatically initialized to zero or NULL in the case of pointers.

Local variables, defined inside of a function, are NOT initialized, and pickup whatever garbage value is in memory when the function is called.

You should initialize ALL local variables!!! Explicitly initializing ALL variables is always a god idea, especially pointers!

Example:

Code:

#include <stdio.h>

char *ptr1;  // Global automatically initialized to NULL

int main(void)
{
  char *ptr2;  // Local NOT initialized!
  
  printf("ptr1 == %p\n", (void *)ptr1);
  printf("ptr2 == %p\n", (void *)ptr2);
  
  return 0;
}

Code:

Output:

ptr1 == (nil)
ptr2 == 0x7fdbdf952ab0

Thanks for clarifying that, i believe that's what laserlight was saying but i was distracted by the other terminology.

**john.c** · 12-09-2023

Non-password protected version of C17 draft:
https://web.archive.org/web/20181230...posed_fdis.pdf
I don't know where you got the 404 from (or how you knew it was password protected if you got a 404).

**rstanley** · 12-09-2023

Originally Posted by john.c

Non-password protected version of C17 draft:
https://web.archive.org/web/20181230...posed_fdis.pdf
I don't know where you got the 404 from (or how you knew it was password protected if you got a 404).

I just clicked on the link and it asked for a password to open the PDF file. No 404.

**C_me_run** · 12-09-2023

Originally Posted by rstanley

Yes, he was probably using a copy of C89 (ANSI) not C90 (ISO)

At some point, I did see C17 listed on an ISO page as C18, and really should be called C18 as it was adopted in 2018:
"C17 ISO/IEC 9899:2018"

Which breaks the naming convention of C90, C99, and C11.

This file is password protected:

I didn't get that error, oh well, the internet is strange.

**laserlight** · 12-11-2023

Originally Posted by C_me_run

It's initialization and assignment, unless "=" is not an assignment operator (and yes it is)

No, in this context "=" is not the assignment operator. While initialisation certainly involves assignment in the computer science sense, C distinguishes between initialisation and assignment, that's why I noted that "that's initialisation, not assignment". I did not mean to imply that there was no assignment of value, because of course initialisation involves the initial assignment of a value.

Originally Posted by C_me_run

No, actually this is not true at all. Let me prove it:

Good to see that rstanley cleared it up, though you must have missed my preceding sentence about static storage duration because "you have declared these variables outside of a function" and how that means that those variables are "are initialised at startup".

Originally Posted by rstanley

Explicitly initializing ALL variables is always a god idea, especially pointers!

Ascribing divinity might be taking it a step too far, but I suppose it works!

**rstanley** · 12-11-2023

Originally Posted by rstanley
Explicitly initializing ALL variables is always a god idea, especially pointers!

Ascribing divinity might be taking it a step too far, but I suppose it works!

Typo, obviously! "Good" not "god"! ;^)

Although, some programmers do think C is a religion! ;^)

**stahta01** · 12-13-2023

Originally Posted by rstanley

Typo, obviously! "Good" not "god"! ;^)

Although, some programmers do think C is a religion! ;^)

No; But, I think "spaces" versus "tabs" is a religious split.

Tim S.

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