Help me for an memory access violation error in C

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I get memory access violation error, please help me quickly!!!!

I wrote a code like this, kindly note my commented line of error

C++

float  fValue=0;

stpData ->dbpMG1G1_data = ( float  ** ) malloc (sizeof ( float * ) * nRows * nColumns);

while ( NULL != fgets( cpReadString, 6144, fStream ) && nRow < nRows )

{

while ( NULL != cpSubString && nColumn < nColumns )

{
  fValue =  atof(cpSubString) ; 

  stpData ->dbpMG1G1_data[nRow][nColumn] = fValue ; // here I am getting error	
			
  ++nColumn ;
  
}

++nRow ; 
 
}

I delibratly wrote the memory allocation code line outside the loops so as to
optimze the execution speed. Is it indexing problem or something else?

Complete debugg error notification:

Unhandled exception at 0x0042b9e2 in On_The_Fly-A350.exe: 0xC0000005: Access violation writing location 0xcdcdcdcd.

Thanks in advance!!!!!!!

Posted 12-Dec-12 0:23am

Ajain A K

Updated 12-Dec-12 0:24am

Jochen Arndt

v2

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2 solutions

Solution 1

You need a matrix of floats (you don't need (nRows * nColumns) of float pointers).
I think an easy approach would be (using data instead of stpData ->dbpMG1G1_data to simplify notation):

C

data = (float *) malloc( sizeof(float) * nRows * nColumns); // linear array of floats
//..
{
  fValue = atof(cpSubString) ; 
  int i = nRow * Columns + nColumn; // bidimensional to linear: [Row][Column] => [i]
  data[i] = fValue;
}

Posted 12-Dec-12 0:33am

CPallini

Comments

nv3 12-Dec-12 8:05am

Good advice. +5.

Ajain A K 12-Dec-12 9:31am

Dear CPallini,

Thanks a lot for very quick response and very useful answer.

CPallini 13-Dec-12 3:33am

You are welcome.

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Jochen Arndt · Accepted Answer · 2012-12-12T00:40:00

You are allocating an 1-dimensional array with nRows * nColumns elements of pointers to float and casting this to a float**. This will not work (the total array size would match for floats in this case because sizeof(float) == sizeof(float *) for 32-bit builds, but accessing 2-dim members will cause access violations).

To fix the problem, you have two options:

1. Use a 1-dimensional array and calculate the index when accessing elements:

stpData->dbpMG1G1_data = (float *)malloc(sizeof(float) * nRows * nColumns);
// ...
stpData ->dbpMG1G1_data[nRow * nColumns + nColumn] = fValue;

1. Use a 2-dimensional array which must be allocated using a loop:

stpData->dbpMG1G1_data = (float **)malloc(sizeof(float *) * nRows);
int i;
for (i = 0; i < nRows; i++)
    stpData ->dbpMG1G1_data[i] = (float *)malloc(sizeof(float) * nColumns);

The first version is more performat and does not require freeing the memory inside another loop.