segmentation fault when calling a function

Question

Segmentation fault when calling the Update_Multiplier and gdb debugger shows these:

Program received signal SIGSEGV, Segmentation fault. 0x080b74e8 in Update_Multiplier() ()

double upperbound = 116325;
double objective = 1.1707e+07;
int main()
{
    Update_Multiplier();
}
void Update_Multiplier()
{
    cout << "function 0" << endl;
    // Determine subgradient vectors
    double gra[1000][1000];
    double grb[1000][1000];
    double dumX = 0;
    double stepsize[1000][1000];
    double tuning=2;
    double LRADum[1000][1000];
    double LRBDum[1000][1000];

    cout << "function 1" << endl;
    // update subgradient vectors
    for (int i=1; i<=noOfNodes; i++)
    {
        for (int j=1; j<=noOfNodes; j++)
        {
            if (C[i][j] != 0)
            {
                dumX=0;
                for (int p=1; p<=noOfCommodity; p++)
                {
                    dumX += X[i][j][p];
                }
                gra[i][j]=dumX-U[i][j]*Y[i][j]-Q[i][j];
                grb[i][j]=Q[i][j]-B[i][j]*Y[i][j];
            }
        }
    }

    // update stepsize
    cout << "function 2" << endl;
    for (int i=1; i<=noOfNodes; i++)
    {
        for (int j=1; j<=noOfNodes; j++)
        {
            if (C[i][j] != 0)
            {
                stepsize[i][j]=(tuning*(UpperBound-Objective))/sqrt((gra[i][j]*gra[i][j])*(grb[i][j]*grb[i][j]));
                LRADum[i][j]=LRA[i][j]+stepsize[i][j]*gra[i][j];
                LRA[i][j]=LRADum[i][j];
                LRBDum[i][j]=LRB[i][j]+stepsize[i][j]*grb[i][j];
                LRB[i][j]=LRBDum[i][j];

            }
        }
    }

}

Hmmm, possibly 40MB of stack space required. What's the platform, and do you know how much memory is allocated for the stack? — Lara Bailey
– Lara Bailey, Commented Jan 22, 2010 at 14:53
I think he's programming one of those computers with 256-bit processors, ~200TB RAM that nobody else knows. — isekaijin
– isekaijin, Commented Jan 22, 2010 at 15:00
To be honest, it's pretty surprising that it even gets to segfault. I can't see declarations of cout, endl, noOfNodes, C, noOfCommodity, X, U, Y, Q, B, UpperBound, Objective, sqrt, LRA, LRB. In addition there's no declaration of Update_Multiplier in scope where it's called from main. — Lara Bailey
– Lara Bailey, Commented Jan 22, 2010 at 15:04

Khaled Alshaya · Accepted Answer · 2010-01-22 14:56:03Z

7

I see two suspicious things in your code.

First, you are taking too much stack space (about ~40 MB).
Second, you are starting the index of the array at 1, where it should be 0:

for (int i=1; i<=noOfNodes; i++)

Change it to:

for (int i=0; i<noOfNodes; i++)

answered Jan 22, 2010 at 14:56

Khaled Alshaya

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5 Comments

Randolpho Over a year ago

heh.. beat me to it. Although the large stack is a possibility, I think this is the most likely scenario.

Willis Blackburn Over a year ago

The comments about the loops starting at 1 are also valid, but we don't know the value of noOfNodes. If it's less than 1,000 then that part at least will be fine. But what if it's greater than 1,000? It looks like the function is just allocating 1,000 x 1,000 arrays with the assumption that noOfNodes will never be that large. It should use the actual value of noOfNodes and allocate them dynamically from the heap.

Randolpho Over a year ago

@Willis Blackburn: Even if noOfNodes is set to the exact number of items in the array, starting at 1 and using i<=noOfNodes will segfault.

Willis Blackburn Over a year ago

For that matter we don't know what B, C, Q, U, X, Y, or noOfCommodity are either.

Willis Blackburn Over a year ago

Randolpho: That's why I said "If it's less than 1,000 ..." :-)

anon · Accepted Answer · 2010-01-22 14:53:16Z

4

At a guess, you have a stack overflow! You cannot reliably create gigantic arrays on the stack. You need to create them dynamically or statically.

answered Jan 22, 2010 at 14:53

anon

Comments

dirkgently · Accepted Answer · 2010-01-22 14:55:13Z

3

Where did you define noOfNodes? What is the initial value of this? Or, do you read this in from the console? If this is uninitialized, it probably has junk data -- which may or may not explain the crash.

answered Jan 22, 2010 at 14:55

dirkgently

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Comments

Willis Blackburn · Accepted Answer · 2010-01-22 14:57:51Z

2

You need a stack of at least 40 megabytes to run this function because you're allocating five arrays of one million eight-byte doubles each.

Change the function to allocate the double arrays from the heap using new.

answered Jan 22, 2010 at 14:57

Willis Blackburn

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1 Comment

Khaled Alshaya Over a year ago

+1 Ya it is ~40 MB. I first mentioned it is ~5 MB in my post :)

liwp · Accepted Answer · 2010-01-22 14:58:23Z

1

You should really give us the whole code, e.g. noOfNodes is not defined anywhere.

Just a stab in the dark: are you possibly overflowing C since your indices (i and j) go from 1 to noOfNodes?

answered Jan 22, 2010 at 14:58

liwp

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Comments

isekaijin · Accepted Answer · 2010-01-22 14:59:23Z

0

First, what Neil said is true.

Second, C and C++ arrays start from index zero. If you declare

int a[100]; // 100 elements, from zeroth to ninety-ninth.

Then its elements are a[0], a[1] ... a[99].

answered Jan 22, 2010 at 14:59

isekaijin

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Comments

user253984 · Accepted Answer · 2010-01-22 15:00:29Z

0

I can not see anything wrong with this code as given, BUT: You might have an off-by-one error if noOfNodes is 1000.

Remember that Arrays are 0-indexed so you have to access indexes 0 - 999 instead of 1 - 1000 as you are doing

answered Jan 22, 2010 at 15:00

user253984

Comments

user11783276 · Accepted Answer · 2019-07-14 17:13:39Z

-1

me too i have this problem and my function returned std::string now i just do reference an dreturn type void like that:

void readOnDicoFile(std::ifstream file/*If you have "using namespace std;" instruction, put ifstream, not std::ifstream (you can put both)*/)

and before:

std::string readOnDicoFile(std::string fileName/*If you have "using namespace std;" instruction, put ifstream, not std::ifstream (you can put both)*/)

answered Jul 14, 2019 at 17:13

user11783276

1

1 Comment

user1781290 Over a year ago

How does this relate to the question asked?

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