Malloc,calloc..Sscanf.

Hello ,

I will have two questions . First I havent got a enough information about the funcitons : malloc,calloc,realloc. Can anyone show me a direction to a source or can anyone directly explain their jobs detailed?
Secondly , does the vary the sscanf function returns depends on the how many variable it used? By variable I mean the variables that we save the varies that we took from the string we put into the first argument of sscanf function.

I hope I made my questions clear. I will be glad if you help. Thank you..

(for example sscanf(s,"%d",&a)==1 ??? )

Yes for sscanf(), RTFM.

Consider using a manual, eg http://www.opengroup.org/onlinepubs/...ns/malloc.html

Real brief (and from memory, so check what I'm saying yourself):

• malloc()
  
  Allocates contiguous memory in terms of bytes. Accepts a size_t as an argument and returns a void *. If the memory could not be allocated, the return value is NULL.
  
• calloc()
  
  Similar to malloc(), with a few small changes. It accepts two args, one being the size of one element, and the other being the amount of elements. Another difference is that calloc() initializes all bytes of the memory it allocates to 0. As with malloc(), if the memory could not be allocated, NULL is returned.
  
• realloc()
  
  This function is used to change the size of a block of memory allocated by either malloc(), calloc(), or realloc(). You pass it the pointer to the original block of memory, and specify the new size. If it is able to perform the memory resize, it returns a pointer to the new block of memory. If it couldn't resize the old block, it'll try to make a new block and copy all of the data from the old block to the new block. It then frees the old block. If the pointer you give it to resize is NULL, it behaves similar to malloc(). If it couldn't manage to resize the block of memory, or create a new block upon receiving a NULL pointer, it returns NULL.
  

As far as sscanf() and the scanf() family of functions, yes, the return value is the number of variables that received valid values. If this return value is not equal to the number of variables you gave it, something went wrong with the assignment of at least one variable (or something major went wrong).

Try http://www.hmug.org/man/3/malloc.php

Yes, sscanf(s, "%d", &i) would return 0 if s doesn't contain a number, 1 if there is a number in s. Note also that i will only be filled in if it returns one.

sscanf(s, "%d %d %d", &i, &j, &k) could return any number between 0 and 3, depending on what's in s.

--
Mats

I think your hopes didn't come out true.
The question is kinda not clear to me.

for the 1st question refer the man pages, if you dont know what they are then search Google may be the search query "malloc man page" and see the links, you'll get all you need.

Yea zombie I did that before , then I got the teorical base , I just wanted to know if there are more about them...Thank you all , you clarified all my questions..

So , I wonder if I do this :

>char *array=malloc(10*sizeof(char));
>printf("%d",sizeof(array));

output is 10?? Or I didnt get anything right?

Depending on your computer, the output would most probably be 4.

You're asking for the sizeof the pointer, not what it points to.

yes it should give you 4 as array is the pointer and the sizeof operator when applied on a pointer shall return you 4 on the machines we normally use today.

I am sorry It should have been
>printf("%d",sizeof(*array));

That will print 1 because a char is one byte.

But I did array=malloc(10*sizeof(char)) before I printed the sizeof(*array) .. Dint I allocate a 10 char memory to array ? So shouldnt it print out 10??

But Is it because When I do that I set a 10 lenght string and the array points to the first component of the string and that is 1 byte?

Also I wonder :

>char *array=malloc(10*sizeof(char));
>gets(array);

I deliberately use gets to lead program to an error. But I enter an input more than 10 character , it does not give error. What is the problem. I think I couldnt get what malloc does..

gets does not check the buffer size...
it starts from the first byte pointed by its argument and writes till the user input end...

that's why you should not use it - use fgets as FAQ suggests

Vart you are not serious are you? Dont you read people's questions? Or are you ??????!!!!! Please read my lines carefully..! What did I say "I DELIBERATELY used gets to lead program to an error" ... So what are you talking about ? Dont walk aroun with the crave of teaching people ...

you make an error - you write to the memory you do not own.
You do not get crash - you're lucky...

Next time - pay attention - to what is talled, and be more polite in your talking

as vart pointed out, you're lucky. You're stepping past memory you've allocated (or possibly memory you have allocated) and into 'the unknown'. And I would say hat you're unlucky you didn't get a crash.

Read the FAQ, and listen to what is said.

> it does not give error. What is the problem.
That's just dumb luck on your part.

Sooner or later, the next malloc() or free() call for instance, it would all go pear shaped in spectacular fashion, and your question would have morphed into "why does my foo() call fail here?".

Because C has little or no run-time checks, there is no failure at the point the mistake is made, only at the point where it is first noticed. This "cause and effect" really confuses a lot of newbies into looking in the wrong place.

I am praying on you SALEM , thank you for your understanding...

Also Zac7 we disscussed this before. If you wont point another thing , just dont write. I want you to have a look at your last post. What did you do?Nothing. ALL you said is "Vart is true , see FAQ" .Maybe all you always say is see FAQ. I respect all of your knowledge . But sometimes you dont respect newbies. You treat them like a seperated low level learner , and avoid helping them , you dont even read what people say. I dont care what you do or how you treat me , I just appeal , DO NOT waste my time , you do not look bigger when you post empty posts..

Quote:

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Originally Posted by ozumsafa

But I did array=malloc(10*sizeof(char)) before I printed the sizeof(*array) .. Dint I allocate a 10 char memory to array ? So shouldnt it print out 10??

But Is it because When I do that I set a 10 lenght string and the array points to the first component of the string and that is 1 byte?

Also I wonder :

>char *array=malloc(10*sizeof(char));
>gets(array);

I deliberately use gets to lead program to an error. But I enter an input more than 10 character , it does not give error. What is the problem. I think I couldnt get what malloc does..

---

Well, it will crash EVENTUALLY if you input a long enough string - unless of course the compiler generated some "memory protection code" or some such.

The case here is that you don't really know what you're overwriting, and how far that is from anything "important" that will make the code crash. Imagine that malloc internally works like this:

Code:

---

void *malloc(size_t size) {
    if (list_of_blocks == NULL) {
        list_of_blocks = allocate_block(MEGABYTES * 4);

    block = split_block(list_of_blocks, size);
    return block;
}

---

Imagine also that your call to malloc is the first one (so list_of_blocks is NULL), we grab a 4MB block of memory, and split it into smaller blocks. So the next "bad memory address" is 4MB from where you got your memory...

Under other circumstances, it may be much closer to the end.

If you're using the keyboard to type into the gets, it may take quite some time to type megabytes of data... [keyboard repeat rate is something like 10 keys per second if you hold a key down. 4M / 10 = 400000s = 100+ hours...]

Now of course, I just made that malloc function up - but I do know that malloc at least SOMETIMES allocates large chunks of memory and then splits it into smaller pieces, because that's one way to implement a malloc function - it is generally "expensive" to ask the operating system for memory, so you don't want to do that every time some user wants ten or a hundred bytes - particularly since most OS's don't support allocating small blocks of memory, only in multiples of 4K or some such. So to spread the cost of asking for memory from the OS, malloc "guesses" what the app needs, and then splits it to smaller chunks as needed.

Each combination of OS and C-library will implement malloc slightly differently, but the general principle is similar in most cases.

--
Mats

All I wanted was this , matsp , thank you a lot . Bless you =)

Quote:

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Also Zac7 we disscussed this before. If you wont point another thing , just dont write.

---

Evidently you don't read posts, I pointed out what exactly you were doing. Please don't be so rude, it hurts my feelings ;)

Heyyy, I didnt mean to be rude . It is cool.

Quote:

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Originally Posted by ozumsafa

But I did array=malloc(10*sizeof(char)) before I printed the sizeof(*array) .. Dint I allocate a 10 char memory to array ? So shouldnt it print out 10??

---

Code:

---

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

unsigned long foo( int (*p_stack)[5] )
{
  return sizeof p_stack;
}

int main( void )
{
  int p_auto[] = { 0, 1, 2, 3, 4, };
  int *p_heap = malloc( sizeof *p_heap * 5 );
  if ( p_heap )
  {
    printf( "sizeof p_auto = %lu\n", sizeof p_auto );
    printf( "sizeof p_auto[0] = %lu\n", sizeof p_auto[0] );
    printf( "sizeof p_auto / sizeof p_auto[0] = %lu ;)\n", sizeof p_auto / sizeof p_auto[0] );
    printf( "sizeof p_heap = %lu\n", sizeof p_heap );
    printf( "sizeof *p_heap = %lu\n", sizeof *p_heap );
    printf( "sizeof p_stack = %lu\n", foo( &p_auto ) );

    free( p_heap );
    p_heap = NULL;
  }
  return 0;
}

/**
* sizeof p_auto = 20
* sizeof p_auto[0] = 4
* sizeof p_auto / sizeof p_auto[0] = 5 ;)
* sizeof p_heap = 4
* sizeof *p_heap = 4
* sizeof p_stack = 4
*/

---

Building on what the others had said, the size of some object really depends how it was allocated, and where in the code you decide to ask. sizeof is a special function provided by the compiler that computes the size of an object on a function's stack.

And you will notice that pointers often occupy the stack in a function; ones like p_heap might be 4 or longer on other systems. The size of a pointer is the byte-width RAM requires to store a memory address. The size of an array on the other hand is much different. The array uses all the memory it needs from the function stack. Therefore we can conclude that the size of p_auto is accurate.

So, what to learn from all of this: keep a variable handy that knows the length of a pointed to array, because it is hard to tell otherwise. If you need to know the size of a pointed to object (like int), dereference the pointer first.