Memory allocation for converting a large 2d array to a 1d array in c++

Question

A is a 2d array and data is 1d array

double **A, *data;

fullsize is 9222, memory allocation:

A = (double **)malloc((fullsize)*sizeof(double *));         //allocate memory dynamically for matrix A
for (i = 0; i < fullsize; i++)
    A[i] = (double *)malloc((2 * fullsize)*sizeof(double));

data = (double *)malloc((fullsize*fullsize)*sizeof(double *));

conversion logic:

{
    for (int n = 0; n<fullsize; n++)
    for (int m = 0; m<fullsize; m++)
    {
        data[n*fullsize + m] = A[n][m];
    }
}

error log:

The thread 0xad80 has exited with code 0 (0x0).
Unhandled exception at 0x00F5E6E9 in ConsoleApplication1.exe: 0xC0000005: Access violation writing location 0x69E65000.

The program '[50964] ConsoleApplication1.exe' has exited with code 0 (0x0).

data = (double *)malloc((fullsize*fullsize)*sizeof(double *)); You should use sizeof( double ) here. — olgierdh, Aug 02 '17 at 11:35
Large data doesn't mean `malloc`. Use `std::vector>` by default to get your code correct first. If you come from programming C, put everything you know on hold and pick up a [good C++ book](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list) instead. — StoryTeller - Unslander Monica, Aug 02 '17 at 11:37
see the comment from @olgierdh. Also, is any of the malloc returning 0 (failing)? Anyway I would not use malloc to allocate memory in C++ — Paolo Brandoli, Aug 02 '17 at 12:01

score 1 · Answer 1 · answered Aug 02 '17 at 11:36

1

You are allocating data using size of double pointer:

data = (double *)malloc((fullsize*fullsize)*sizeof(double *));
//                                                        ^

It should be allocated using sizeof(double) instead.

You can avoid using malloc in C++ by using vectors. If you want exact sizes on allocations, use new double[...] instead. In either case you will end up with more readable code:

double **A = new double*[fullsize];
for (int i = 0 ; i != fullsize ; i++) {
    A[i] = new double[fullsize];
}
double *data = new double[fullsize*fullsize];

answered Aug 02 '17 at 11:36

Sergey Kalinichenko

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I tried `sizeof(double)` its still giving the same error, I need arrays for conversion into Eigen matrix; – Sanju Aug 02 '17 at 11:39
@Sanju Did you try `new`? Also, `2` in `malloc((2 * fullsize)*sizeof(double))` looks suspicious. Is your matrix not square? – Sergey Kalinichenko Aug 02 '17 at 12:06
1

Is any of the malloc failing (returning 0)? – Paolo Brandoli Aug 02 '17 at 12:06
It seems that your malloc returns nullptr somewhere. Also @dasblinkenlight is right you don't need the 2*fullsize. Try compiling your application using clang with: `-fsanitize=address -fno-omit-frame-pointer -O1 -g` Plus you can do this trick if you want to: `double* data = ( double* ) malloc( fullsize * fullsize * sizeof( double ) ); double** A = static_cast< double** >( &data );` This way you will have both views onto one continuous memory block. – olgierdh Aug 02 '17 at 15:24

Memory allocation for converting a large 2d array to a 1d array in c++

1 Answers1