c++ memset cause segment fault of int** pointer

Question

    int **dpTable = new int* [iMatrixHeight + 1];
    for (int i = 0; i < iMatrixHeight + 1; i++)
    {
        dpTable[i] = new int [iMatrixWidth + 1];
    }

    memset(dpTable, 0, (sizeof(int)) * (iMatrixHeight + 1)*(iMatrixWidth + 1));

I'm using operator new to allocate a two-dimensional array, but if I use memset to initialize the array, I got a segmentfault when I access the array later. Without the memset, it's ok.

Am I doing anything wrong? THX!

you're trying to fill an array of pointers with an array of integers, and you're overshooting by a factor of (iMatrixWidth + 1) (+/- the difference between pointer size and int). — BeyelerStudios
– BeyelerStudios, Commented Jun 5, 2015 at 14:57
You're using memset to obliterate the entire sequence of pointers your for-loop just worked so hard to acquire. (and breaching the sequence in the process just to add salt to the wound). — WhozCraig
– WhozCraig, Commented Jun 5, 2015 at 14:58
Another question I'd ask is what's the intended purpose for the +1. Why not just set iMetrixHeight accordingly? — Keith
– Keith, Commented Jun 5, 2015 at 15:00
possible duplicate of Initialize a double pointer with zeros in C++ — Toby Speight
– Toby Speight, Commented Jun 5, 2015 at 15:12

marom · Accepted Answer · 2015-06-05 14:59:14Z

5

The arrays dpTable[i] do not point to contiguous memory. You have to initialize then one by one

for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1];
    memset(dpTable[i], 0, (iMatrixWidth + 1) * sizeof(int)) ;
}

answered Jun 5, 2015 at 14:59

marom

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2 Comments

WhozCraig Over a year ago

Simply dpTable[i] = new int [iMatrixWidth + 1](); will work as well, if the OP has anything closely resembling a modern C++ compiler. No memset required in that case.

Jayce Over a year ago

Oh yes, it suddenly struck me

Cheers and hth. - Alf · Accepted Answer · 2015-06-05 15:21:16Z

Instead of this code:

int **dpTable = new int* [iMatrixHeight + 1];
for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1];
}

memset(dpTable, 0, (sizeof(int)) * (iMatrixHeight + 1)*(iMatrixWidth + 1));

… you can do this:

int **dpTable = new int* [iMatrixHeight + 1];
for (int i = 0; i < iMatrixHeight + 1; i++)
{
    dpTable[i] = new int [iMatrixWidth + 1]();
}

Look ma, no memset – instead asking for zeroing of the memory.

Still that's very ugly in C++.

So, do this:

vector<vector<int>> table( iMatrixHeight + 1, vector<int>( iMatrixWidth + 1 ) );

where vector is std::vector.

Or, consider defining a matrix class with just a single vector as backing store.

Or just use an existing matrix class, e.g. from Boost.

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