Thread: how to use calloc

  1. #1
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    how to use calloc

    I understood how to use malloc and realloc in dynamic memory allocation but I don't understand how to use calloc ?

    I belive my code allocate dynamic memory for five integer variables.

    Code:
     #include<stdio.h>
    #include<stdlib.h>
    
    
    int main (void)
    {
      // memory is allocated for two integer variables using malloc    
       int * array = (int*) malloc( 2 * sizeof(*array)); 
      
        if ( array != NULL)// Check if the memory has been successfully 
       
            { 
               printf("Memory successfully allocated using malloc.\n"); 
                
                // memory is allocated for three integer variables using realloc    
                array = (int*) realloc ( array, 3 * sizeof(*array)); 
                
                if ( array != NULL)// Check if the memory has been successfully 
                {
                    printf("Memory successfully allocated using realloc.\n"); 
                }
            }
       
       return 0;
    }
    I know calloc also allocate memory, Is there any advice how to know use of calloc ?

  2. #2
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    calloc zeros the memory it allocates.

    From my manpages:
    The calloc() function allocates memory for an array of nmemb elements
    of size bytes each and returns a pointer to the allocated memory. The
    memory is set to zero.

  3. #3
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    Quote Originally Posted by G4143 View Post
    calloc zeros the memory it allocates.

    From my manpages:
    Thank you,

    program proves how calloc is different then malloc

    Code:
     #include<stdio.h>
    #include<stdlib.h>
    
    int main (void)
    {
       // memory is allocated for one integer variables using malloc    
       int *x = malloc(1 * sizeof(int));
       
       // memory is allocated for one integer variables using calloc    
       int *y = calloc(1, sizeof(int));
       
        if ( x != NULL)// Check if the memory has been successfully 
       
            { 
                printf("Memory successfully allocated using malloc.\n"); 
                 
                printf( "x = %d\n", *x);        
            }
            
        if ( y != NULL)// Check if the memory has been successfully 
            {
                printf("Memory successfully allocated using calloc.\n"); 
                
                printf( "y = %d\n", *y);
            }
       
       return 0;
    }
    Memory successfully allocated using malloc.
    x = 134472
    Memory successfully allocated using calloc.
    y = 0
    Last edited by Dadu@; 02-27-2022 at 12:25 AM.

  4. #4
    and the hat of int overfl Salem's Avatar
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    > program proves how calloc is different then malloc
    There's no guarantee that you'll never get x = 0 either.

    In fact, zero filled malloc is quite common because it eliminates one form of information leakage. Imagine if someone could steal your logins from your browser, just by allocating large amounts of memory and looking through it for tell-tale patterns.

    Also, this.
    Question 7.31
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  5. #5
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    Your code does not allocate 5 integers.

    Your code says "hey, allocator, can I have space for 2 integers please?"

    Then later your code says, "hey, allocator, you know this space you gave me for 2 integers? Can you increase it to 3 integers please?"

    realloc doesn't "add" it "grows." So if you want space for 5, you have to request space for 5. And if you're growing something, you probably want a variable to track the current capacity.

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