Question

Buffers in C programming. Need some help

Asked by: ubuntuguy

Hi I'm having some problems with my code here. Here is what I need to do.
1. Create an array of buffers of size nBuffers
2. Fill up the array with new buffers
3. Initialize each buffer to zero

Note: each buffer needs to hold an integer. (that's all, nothing else).

Problems:
1. Is this how everything should be done?
2. I get a segmentation fault when I try to print the values of the array
3. BUFFER_SIZE is set to 50. Should it just be 1 since I'm only storing an integer in each buffer?






#define BUFFER_LENGTH 50      //size of buffer

int nBuffers;      //the number of buffers to be allocated.
int *bufferArr;            //pointer to future array of buffers

      bufferArr = calloc (nBuffers, sizeof(int *));
      
      int i;
      for (i = 0; i < nBuffers; i++){
            bufferArr[i] = (int) calloc(BUFFER_LENGTH, sizeof(int));      
  }

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Asked On
2009-11-01 at 14:47:23ID24862399
Topics

C Programming Language

,

C++ Programming Language

,

Algorithms

Participating Experts
4
Points
500
Comments
13

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Answers

 

by: pgnatyukPosted on 2009-11-01 at 14:54:48ID: 25716021

You mean you need a buffer for 50 integer values?

int* bufferArr;
bufferArr = (int*)malloc(BUFFER_LENGTH * sizeof(int));
memset(bufferArr, 0, BUFFER_LENGTH * sizeof(int));

it is so.

 

by: ubuntuguyPosted on 2009-11-01 at 14:58:20ID: 25716040

no I need a buffer for 1 integer value only

 

by: pgnatyukPosted on 2009-11-01 at 15:02:22ID: 25716067

You said you need an array?

for one integer:
int* p;
p = (int*)malloc(sizeof(int));
*p = 0;

It does not make a sense.

 

by: Let_Me_BePosted on 2009-11-01 at 15:04:10ID: 25716079

> bufferArr[i] = (int) calloc(BUFFER_LENGTH, sizeof(int));

This line is wrong. You are allocating dynamic memory and casting the returned address to integer value (which will discard part of the address on many architectures).

If you need just 1 integer buffers, then there is absolutely no need to allocate them, just create an array of ints with the length nBuffers. If nBuffers is a variable you will need to use vector<int> or a dynamic array in C++.

 

by: ubuntuguyPosted on 2009-11-01 at 15:14:42ID: 25716129

you are right let_me_be, I should just create an array of integers to store each integer. oh this project says create an array of buffers to store an integer in each

 

by: pgnatyukPosted on 2009-11-01 at 15:20:09ID: 25716144

A TUTORIAL ON POINTERS AND ARRAYS IN C

Pointers

calloc: Allocates a block of memory for an array of num elements, each of them size bytes long, and initializes all its bits to zero.

MSND calloc

/* calloc example */
#include <stdio.h>
#include <stdlib.h>
 
int main ()
{
  int i,n;
  int * pData;
  printf ("Amount of numbers to be entered: ");
  scanf ("%d",&i);
  pData = (int*) calloc (i,sizeof(int));
  if (pData==NULL) exit (1);
  for (n=0;n<i;n++)
  {
    printf ("Enter number #%d: ",n);
    scanf ("%d",&pData[n]);
  }
  printf ("You have entered: ");
  for (n=0;n<i;n++) printf ("%d ",pData[n]);
  free (pData);
  return 0;
}
                                              
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by: trinitrotoluenePosted on 2009-11-01 at 17:16:21ID: 25716508

ubuntuguy:

I think this is what you are trying to do...

#define NOOFBUFFERS 10
#define BUFFER_LENGTH 50 
int* bufferArr[NOOFBUFFERS]; 
for(int bufcnt=0;bufcnt<NOOFBUFFERS;bufcnt++)
 bufferArr[bufcnt] = (int*)malloc(sizeof(int)*BUFFER_LENGTH);

                                              
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by: trinitrotoluenePosted on 2009-11-01 at 17:25:10ID: 25716532

ignore my earlier post. take a look at my snippet below

#define NOOFBUFFERS 10
#define BUFFER_LENGTH 50  
int** bufferArr;
 
   bufferArr= (int**)malloc(sizeof(int*)*NOOFBUFFERS); 
for(int bufcnt=0;bufcnt<NOOFBUFFERS;bufcnt++)
    bufferArr[bufcnt] = (int*)malloc(sizeof(int) * BUFFER_LENGTH);  

                                              
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by: trinitrotoluenePosted on 2009-11-01 at 17:38:41ID: 25716587

here's the complete one with the zero initialization also done

#define BUFFER_LENGTH 50  
int nBuffers;
int** bufferArr; 
//nBuffers should be initialized to a meaningful value before usage 
   
bufferArr= (int**)malloc(sizeof(int*)*nBuffers);
   
for(int bufcnt=0;bufcnt<nbuffers;bufcnt++)
{        
    bufferArr[bufcnt] = (int*)malloc(sizeof(int) * BUFFER_LENGTH);
    memset(bufferArr[bufcnt], 0, sizeof(int)); 
}

                                              
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by: Infinity08Posted on 2009-11-02 at 01:33:19ID: 25718035

>> here's the complete one with the zero initialization also done

If you want to zero-initialize the memory, why not call calloc instead of malloc ?
Note however that zero-initializing memory doesn't guarantee that the all data will have the value 0. so you might be better off with a loop that sets each data value to 0 separately.

 

by: trinitrotoluenePosted on 2009-11-02 at 17:43:58ID: 25725504

right thats why I have put the memset in a loop. missed out initializing all locations. fixed that

#define BUFFER_LENGTH 50  
int nBuffers;
int** bufferArr; 
//nBuffers should be initialized to a meaningful value before usage 
   
bufferArr= (int**)malloc(sizeof(int*)*nBuffers);
   
for(int bufcnt=0;bufcnt<nbuffers;bufcnt++)
{        
    bufferArr[bufcnt] = (int*)malloc(sizeof(int) * BUFFER_LENGTH);
    memset(bufferArr[bufcnt], 0, * BUFFER_LENGTH*sizeof(int)); 
}

                                              
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by: trinitrotoluenePosted on 2009-11-02 at 17:45:01ID: 25725505

oops typo with the *

memset(bufferArr[bufcnt], 0, BUFFER_LENGTH*sizeof(int)); 

                                              
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by: Infinity08Posted on 2009-11-02 at 23:19:38ID: 25726634

That was not my point. memset is meant for zero-initializing a block of memory (and you might as well use calloc, because it already does that internally), but that doesn't guarantee that the data values in that block of memory will also actually have the values 0. For int's it's likely, for other data types less so.

20120131-EE-VQP-002

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