Does C guarantee the data layout of the memory allocated by malloc function?

[lovecreatesbeauty]

Hello experts,


1. Does C guarantee the data layout of the memory allocated by malloc
function on the heap. I mean, for example, if I allocate a array of 100
elements of structure, can I always reference a correct/valid structure
member upon that allocated memory?

If I allocate memory, for example like this way:

lp_Person = (person_t *)malloc(CNT * sizeof(person_t));

I guess the malloc/compiler doesn't know the detail of the data type
person_t. It only know the size of the object of the data type. So, how
can I use the members of the object of person_t (suppose it had
members) without compiler knowing it.

2. Is the base type of a pointer very important? I ever read one book,
it says "A pointer is not just a pointer, but a pointer with some
particular data type" (perhaps like this). Is it right?

3. If I allocate memory inside a function named f1(), is it better for
me to delay the de-allocation of that memory in some other functions
later?


Could you please also give me some more suggestion on my following
code, thank you very much.


lovecreatesbeauty


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


#define NMLEN 30

typedef enum {
male = 1,
female = 2
} gender_t;

typedef struct {
char name[NMLEN];
gender_t gender;
int age;
} person_t;

typedef enum {
false = 0,
true = 1
} bool_t;


int NameListInitialization(void){
const int CNT = 100;
int ln_LoopCnt = 0;
person_t * lp_Person = NULL;
char la_PersonName[NMLEN] = {'\0'};
bool_t lb_IsAllocated = false;


lp_Person = (person_t *)malloc(CNT * sizeof(person_t));
if (lp_Person != NULL){
lb_IsAllocated = true;

for (ln_LoopCnt = 0; ln_LoopCnt < CNT ; ++ln_LoopCnt){
memset(la_PersonName, 0x00, sizeof(la_PersonName));
memset(lp_Person[ln_LoopCnt].name,
0x00,
sizeof(lp_Person[ln_LoopCnt].name));

itoa(ln_LoopCnt, la_PersonName, 10);
strcat(la_PersonName, "-Anonymous");
strcpy(lp_Person[ln_LoopCnt].name, la_PersonName);
lp_Person[ln_LoopCnt].gender = (gender_t)(
ln_LoopCnt % 2 + 1);
lp_Person[ln_LoopCnt].age = ln_LoopCnt;
}
}

/* do something here ... */

if (lb_IsAllocated == true){
free(lp_Person); /* Can I call this free in other place? */
}

return 0;
}

 

[Gordon Burditt]

1. Does C guarantee the data layout of the memory allocated by malloc

function on the heap. I mean, for example, if I allocate a array of 100
elements of structure, can I always reference a correct/valid structure
member upon that allocated memory?

What does reference ... upon mean? Is that some kind of kinky byte sex?

The memory allocated by malloc() is guaranteed to be contiguous
within itself, otherwise there would be no way to find it with just
the pointer returned by malloc(). The memory allocated in one call
to malloc() is not guaranteed to be contiguous with the memory in
any other call to malloc() (and in one implementation I'm familiar
with, no two chunks of memory allocated by malloc() will EVER be
contiguous to each other).

If I allocate memory, for example like this way:

lp_Person = (person_t *)malloc(CNT * sizeof(person_t));

Don't cast the return value of malloc().

I guess the malloc/compiler doesn't know the detail of the data type
person_t. It only know the size of the object of the data type. So, how
can I use the members of the object of person_t (suppose it had
members) without compiler knowing it.

Assuming a lp_Person is declared as a person_t *, the first
element is lp_Person[0], and the next one is lp_Person[1], ...
Then you can refer to things like lp_Person[0].hat_size .

2. Is the base type of a pointer very important? I ever read one book,
it says "A pointer is not just a pointer, but a pointer with some
particular data type" (perhaps like this). Is it right?

Yes. The difference in address between lp_Person[0] and lp_Person[1]
is the size of a person_t. The difference between ((char *)
lp_Person)[0] and ((char *) lp_Person)[1] is one byte.

3. If I allocate memory inside a function named f1(), is it better for
me to delay the de-allocation of that memory in some other functions
later?

Don't free the memory before you finish needing it.
Free the memory after you do finish needing it.

Could you please also give me some more suggestion on my following
code, thank you very much.


lovecreatesbeauty


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


#define NMLEN 30

typedef enum {
male = 1,
female = 2
} gender_t;

typedef struct {
char name[NMLEN];

30 characters isn't nearly enough for real human names.

gender_t gender;
int age;
} person_t;

typedef enum {
false = 0,
true = 1
} bool_t;


int NameListInitialization(void){
const int CNT = 100;
int ln_LoopCnt = 0;
person_t * lp_Person = NULL;
char la_PersonName[NMLEN] = {'\0'};
bool_t lb_IsAllocated = false;


lp_Person = (person_t *)malloc(CNT * sizeof(person_t));
if (lp_Person != NULL){
lb_IsAllocated = true;

for (ln_LoopCnt = 0; ln_LoopCnt < CNT ; ++ln_LoopCnt){
memset(la_PersonName, 0x00, sizeof(la_PersonName));
memset(lp_Person[ln_LoopCnt].name,
0x00,
sizeof(lp_Person[ln_LoopCnt].name));

itoa(ln_LoopCnt, la_PersonName, 10);

There is no function itoa() in standard C.

strcat(la_PersonName, "-Anonymous");
strcpy(lp_Person[ln_LoopCnt].name, la_PersonName);
lp_Person[ln_LoopCnt].gender = (gender_t)(
ln_LoopCnt % 2 + 1);
lp_Person[ln_LoopCnt].age = ln_LoopCnt;
}
}

/* do something here ... */

if (lb_IsAllocated == true){
free(lp_Person); /* Can I call this free in other place? */

As written, lp_Person is about to be lost, so you CAN'T use its value
in another place. If, on the other hand, you returned lp_Person to
the function, it would be possible to free it in another place.

}

return 0;
}

Gordon L. Burditt

 

[KJ]

Friend when u allocate memory using malloc the bytes allocaed is
always contiguous. It returns the starting byte offset. Now questions
comes how do we manipulate the data stored in bytes. For example
int *p = (int*)malloc(10*sizeof(int));

p will get the starting byte offset. where will p+1 refer to. Here
comes the type of pointer. Since the pointer p is of int type (assuming
sizeof(int) is 4) p+1 or p[1]refers to starting offset + 4bytes. so now
it points to the second int value of the allocation.

if u uset structure then increasing pointer of structure by one means
pointing to next structure values.

U can free the allocation anywhere but u must have the pointer pointing
to the starting offset of the allocation.

 

[=?ISO-8859-1?Q?=22Nils_O=2E_Sel=E5sdal=22?=]

Hello experts,


1. Does C guarantee the data layout of the memory allocated by malloc
function on the heap. I mean, for example, if I allocate a array of 100
elements of structure, can I always reference a correct/valid structure
member upon that allocated memory?

If I allocate memory, for example like this way:

lp_Person = (person_t *)malloc(CNT * sizeof(person_t));

Don't cast the return value of malloc !

I guess the malloc/compiler doesn't know the detail of the data type
person_t. It only know the size of the object of the data type. So, how
can I use the members of the object of person_t (suppose it had
members) without compiler knowing it.

malloc doesn't know, but it's required to give you memory aligned suitable
for any data type.

You can't access the members of a structure without the compiler knowing
about them.

2. Is the base type of a pointer very important? I ever read one book,
it says "A pointer is not just a pointer, but a pointer with some
particular data type" (perhaps like this). Is it right? Yes.

3. If I allocate memory inside a function named f1(), is it better for
me to delay the de-allocation of that memory in some other functions
later?

Deallocate when done with it, wherever that is.

 

[lovecreatesbeauty]

What does reference ... upon mean? Is that some kind of kinky byte sex?

Hi Gordon, thanks for your kindly help. Sorry for my poor English. I
mean that Whether I refer to the members of the object of the
particular data type in the allocated member?

When allocate with malloc(), it only knows the size but don't know the
layout of the object of that particular data type. I'm a little
confused on it.

Don't cast the return value of malloc().

Does it comply to the latest Language Standard?

Gordon L. Burditt

Best Regards

lovecreatesbeauty

 

[Jason Curl]

Hi Gordon, thanks for your kindly help. Sorry for my poor English. I
mean that Whether I refer to the members of the object of the
particular data type in the allocated member?

When allocate with malloc(), it only knows the size but don't know the
layout of the object of that particular data type. I'm a little
confused on it.

Sure, as Gordon mentions, your only guarantee then is that the memory
block for the size you requested is contiguous if malloc() was successful.

malloc() doesn't need to know about the layout of your memory. You
define that by the datatype pointer you assigned with malloc(). e.g.

struct mystruct {
int element1;
char element2;
} *pmystruct;

pmystruct = malloc(sizeof(mystruct));

So, the type 'struct mystruct' defines the layout. You just need to make
sure that you allocate enough memory, and this is what the 'sizeof'
operator is used for.

 

[SM Ryan]

# Hello experts,
#
#
# 1. Does C guarantee the data layout of the memory allocated by malloc
# function on the heap. I mean, for example, if I allocate a array of 100
# elements of structure, can I always reference a correct/valid structure
# member upon that allocated memory?
#
# If I allocate memory, for example like this way:
#
# lp_Person = (person_t *)malloc(CNT * sizeof(person_t));

If you allocate N*sizeof(T) chars, you will have enough space for
at least N elements. If you save the pointer,
T *p=malloc(N*sizeof(T)), you can access successive elements as
p[0], p[1], p[2], ... , p[N-1]. Any alignment issues and filler space
are taken care for you.

# 2. Is the base type of a pointer very important? I ever read one book,
# it says "A pointer is not just a pointer, but a pointer with some
# particular data type" (perhaps like this). Is it right?

You can cast a function pointer to a function pointer and a data
pointer to a data pointer. All struct pointers are the same; for
other data pointers, the compiler is allowed to use different
representations.

malloc returns a (void*). If you assign or cast to (T*), the
compiler will insert any magic if it needs to change the pointer
value.

On some systems, a function pointer can be a code address and
a data address. That's why data and function pointers are different
beasts.

# 3. If I allocate memory inside a function named f1(), is it better for
# me to delay the de-allocation of that memory in some other functions
# later?

There is no 'better' or 'worse'. It is a matter of interface design.