473,379 Members | 1,377 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,379 software developers and data experts.

Setting pointers to null after freeing them

Hi,

My question is related to setting freed pointers to NULL.

After freeing a pointer:
1) Should the freeing routine also be responsible for setting the
pointer to null?
2) Or, should the client/user code be responsible for doing it?

On what basis should a decision be made favouring either case ?

Rgds.
Amogh
May 2 '06 #1
5 3123
Amogh wrote:
Hi,

My question is related to setting freed pointers to NULL.

After freeing a pointer:
1) Should the freeing routine also be responsible for setting the
pointer to null?
2) Or, should the client/user code be responsible for doing it?

On what basis should a decision be made favouring either case ?


Most of the time, a freed pointer does not get set to null. Typically,
when the object is no longer necessary, it is freed, and then the
pointer itself is deallocated. This may occur with an explicit free
statement, or it may happen when the function returns and its automatic
variables are lost.

The following example code contains two "freeing routines". Neither the
routine itself, nor the client/user code actually sets any pointers to null.

Note that pointers are passed by value in C. If the freeing routine
wanted to set the caller's pointer to NULL, it would have to take a
pointer to that pointer. That seems like poor style to me.

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

typedef struct {
int year;
int month;
int day;
} Date;

Date *new_date(int year, int month, int day)
{
Date *d = malloc(sizeof *d);
if(!d) return NULL;

d->year = year;
d->month = month;
d->day = day;

return d;
}

void destroy_date(Date *d)
{
free(d);

/* There is no point setting 'd = 0' here, as
the value of d is lost when the function
returns anyway.

One would have to write the function as:
void destroy_date(Date **pd)
{
free(*pd);
*pd = NULL;
}
and the caller would call it as
destroy_date(&date);
that would actually set the caller's 'date' variable to null.
I don't recommend doing this.
*/
}

typedef struct {
char *name;
Date *birthdate;
} Person;

Person *new_person(const char *name, int year, int month, int day)
{
Person *p = malloc(sizeof *p);
if(!p) return NULL;

p->name = malloc(strlen(name) + 1);
if(!p->name)
{
free(p);
return NULL;
}
strcpy(p->name, name);

p->birthdate = new_date(year, month, day);
if(!p->birthdate)
{
free(p->name);
free(p);
return NULL;
}

return p;
}

void destroy_person(Person *p)
{
free(p->name);
destroy_date(p->birthdate);
free(p);
}

const char *ordinal(int n)
{
if(n % 100 == 11 || n % 100 == 12 || n % 100 == 13) return "th";
if(n % 10 == 1) return "st";
if(n % 10 == 2) return "nd";
if(n % 10 == 3) return "rd";
return "th";
}

const char *monthname(int n)
{
const char *names[] = {
"January", "February", "March", "April",
"May", "June", "July", "August",
"September", "October", "November", "December"};

if(n >= 1 && n <= 12)
return names[n - 1];
else
return NULL;
}

void show_person(const Person *p)
{
printf("%s was born on the %d%s of %s, %d\n",
p->name,
p->birthdate->day,
ordinal(p->birthdate->day),
monthname(p->birthdate->month),
p->birthdate->year);
}

int main(void)
{
size_t i;
Person *family[5];
family[0] = new_person("Simon", 1982, 9, 6);
family[1] = new_person("Andrew", 1986, 2, 25);
family[2] = new_person("Thomas", 1987, 6, 5);
family[3] = new_person("Sue", 1957, 5, 27);
family[4] = new_person("Michael", 1953, 2, 6);

for(i = 0; i < sizeof family / sizeof *family; i++)
{
if(family[i])
{
show_person(family[i]);
destroy_person(family[i]);
}
}
return 0;
}

--
Simon.
May 2 '06 #2
Amogh opined:
Hi,

My question is related to setting freed pointers to NULL.

After freeing a pointer:
1) Should the freeing routine also be responsible for setting the
pointer to null?
2) Or, should the client/user code be responsible for doing it?

On what basis should a decision be made favouring either case ?


Standard C function `free()` that de-allocates memory pointed to by its
argument does not modify the pointer in question.

Whether you need this pointer reset to NULL after memory's been freed
is really a design choice for your particular application. I guess the
most important thing is, once you make a choice of who resets the
pointers (and whether they're reset at all) you stick to it throughout
the application.

My personal preference would likely be for the freeing routine to reset
the pointer to NULL, but as I said it really depends on the job at
hand. I'm sure arguments can be found for both.

--
The light at the end of the tunnel is the headlight of an approaching
train.

<http://clc-wiki.net/wiki/Introduction_to_comp.lang.c>

May 2 '06 #3
On 2006-05-02, Amogh <am*****@nospam.com> wrote:
Hi,

My question is related to setting freed pointers to NULL.

After freeing a pointer:
1) Should the freeing routine also be responsible for setting the
pointer to null?
2) Or, should the client/user code be responsible for doing it?

On what basis should a decision be made favouring either case ?


It's a matter of taste really. I prefer (2) to (1).

The problem with (1) is you have to use pointers-to-pointers, and
there's no way the called routine can know about aliases anyway.
Example:

#include <stdlib.h>

void destroy(int **p)
{
free(*p);
*p = NULL;
}

int main(void)
{
int *p = malloc(100);
int *q = p;

free(&p);
/* OK p is NULL, but I can still use q... */

return 0;
}

The other benefit of (2) is that it mirrors the way free works, so is
clearer to most people. Otherwise the reader of the code has to ask,
this is pointer-to-pointer, why?

Some people use macros:

#define DESTROY(p) do { FREE(p); p = NULL; } while 0

or something, to simulate "pass-by-reference"; I'd really advise
against this though.
May 2 '06 #4
Simon Biber wrote:
Amogh wrote: -snip-

int main(void)
{
size_t i;
Person *family[5];
family[0] = new_person("Simon", 1982, 9, 6);
family[1] = new_person("Andrew", 1986, 2, 25);
family[2] = new_person("Thomas", 1987, 6, 5);
family[3] = new_person("Sue", 1957, 5, 27);
family[4] = new_person("Michael", 1953, 2, 6);

for(i = 0; i < sizeof family / sizeof *family; i++)
{
if(family[i])
{
show_person(family[i]);
destroy_person(family[i]);
}
}
return 0;
}


I know it isn't, but it looks very much like C++.... The functions are
like from inside C++ classes... Did you take it from some C++ code? :-)

Again: I know it's C code but perhaps a bit C++'ish?... Nothing wrong
with that however... Actually I like the example...
Best regards / Med venlig hilsen
Martin Jørgensen

--
---------------------------------------------------------------------------
Home of Martin Jørgensen - http://www.martinjoergensen.dk
May 2 '06 #5
Martin Jørgensen wrote:
Simon Biber wrote:

int main(void)
{
size_t i;
Person *family[5];
family[0] = new_person("Simon", 1982, 9, 6);
family[1] = new_person("Andrew", 1986, 2, 25);
family[2] = new_person("Thomas", 1987, 6, 5);
family[3] = new_person("Sue", 1957, 5, 27);
family[4] = new_person("Michael", 1953, 2, 6);

for(i = 0; i < sizeof family / sizeof *family; i++)
{
if(family[i])
{
show_person(family[i]);
destroy_person(family[i]);
}
}
return 0;
}

I know it isn't, but it looks very much like C++.... The functions are
like from inside C++ classes... Did you take it from some C++ code? :-)


No, I wrote it from scratch. I have been doing more object-oriented
programming lately so I suppose it has rubbed off on my C code too.
Again: I know it's C code but perhaps a bit C++'ish?... Nothing wrong
with that however... Actually I like the example...


Thanks.

--
Simon.
May 2 '06 #6

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

2
by: mosfets | last post by:
Hi, I'm having a little trouble figuring out the difference in terms of memory allocation between: class person_info; class A { private:
8
by: John Hanley | last post by:
I working in C. I haven't paid much attention to void pointers in the past, but I am wondering if I can use them for my various linked lists to save work. I have two different linked lists that...
7
by: Rano | last post by:
/* Hello, I've got some troubles with a stupid program... In fact, I just start with the C language and sometime I don't understand how I really have to use malloc. I've readden the FAQ...
7
by: Akhil | last post by:
How much is Pointers different in C# compared to C++? Do we have destructors in C#? if yes, for what do we use it? -- Cheers, Akhil
31
by: Yevgen Muntyan | last post by:
6.2.4 of standard says: "The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime." Do I understand it right that value of pointer may or may...
20
by: sam_cit | last post by:
Hi Everyone, I just heard from a friend of mine that there are few c compilers that give an error when pointers are not initialised to NULL. Is it correct? and if so, is there any standard for...
11
by: Piotrek | last post by:
Hi, I have no idea why my reallocation function (myadd) is breaking up my array. I'm trying to solve it and it took me few hours already. Maybe you can look at my code and explain me what is...
27
by: rocco.rossi | last post by:
I've been trying to write a function capable of checking if a pointer value is set to NULL, and if it isn't, of deallocating and setting it's value to NULL regardless of the pointer's type. I...
3
by: JoxC | last post by:
Hey guys, I was wondering if there is an *efficient* way in setting every pointer in an array of pointers to NULL? memset cannot work for compatibility reasons, but I was playing a bit with...
1
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: Faith0G | last post by:
I am starting a new it consulting business and it's been a while since I setup a new website. Is wordpress still the best web based software for hosting a 5 page website? The webpages will be...
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...
0
by: ryjfgjl | last post by:
In our work, we often need to import Excel data into databases (such as MySQL, SQL Server, Oracle) for data analysis and processing. Usually, we use database tools like Navicat or the Excel import...
0
by: taylorcarr | last post by:
A Canon printer is a smart device known for being advanced, efficient, and reliable. It is designed for home, office, and hybrid workspace use and can also be used for a variety of purposes. However,...
0
by: Charles Arthur | last post by:
How do i turn on java script on a villaon, callus and itel keypad mobile phone
0
by: aa123db | last post by:
Variable and constants Use var or let for variables and const fror constants. Var foo ='bar'; Let foo ='bar';const baz ='bar'; Functions function $name$ ($parameters$) { } ...
0
by: ryjfgjl | last post by:
If we have dozens or hundreds of excel to import into the database, if we use the excel import function provided by database editors such as navicat, it will be extremely tedious and time-consuming...
0
by: emmanuelkatto | last post by:
Hi All, I am Emmanuel katto from Uganda. I want to ask what challenges you've faced while migrating a website to cloud. Please let me know. Thanks! Emmanuel

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.