Problem with short program

Dev Shed Forums Sponsor:

December 6th, 2012, 12:38 PM

hs101

Registered User

Join Date: Dec 2012

Posts: 1

Time spent in forums: 36 m 26 sec
Reputation Power: 0

Problem with short program

This is my first time writing in C and I'm trying to compile and run a program in C that is supposedly standard code for dense matrix multiplication. However i'm getting this segmentation fault (core dumped) error and i have no idea what the problem is. Here is my code:

int main(){

int N = 1024;
int x[N][N];
int y[N][N];
int z[N][N];
int i;
int r;
int k;
int j;

for (i = 0; i < N; i = i+1) {
for (j = 0; j < N; j = j+1)
{r = 0;
for (k = 0; k < N; k = k+1){
r = r + y[i][k]*z[k][j];};
x[i][j] = r;
};
};

return 0;
}

December 6th, 2012, 01:41 PM

salem

Contributed User

Join Date: Jun 2005

Posts: 3,840

Folding Points: 153 Folding Title: Novice Folder

Time spent in forums: 2 Months 3 Weeks 2 Days 19 h 21 m 53 sec
Reputation Power: 1774

Yet another cross-poster

__________________
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
If at first you don't succeed, try writing your phone number on the exam paper

December 6th, 2012, 07:19 PM

Scorpions4ever

Banned ;)

Dev Shed God 9th Plane (9000 - 9499 posts)

Join Date: Nov 2001

Location: Woodland Hills, Los Angeles County, California, USA

Posts: 9,389

Time spent in forums: 1 Month 4 Weeks 1 Day 22 h 17 m 29 sec
Reputation Power: 4080

Since you said you got a Segfault and a core file, I'll assume you're running on a Linux system. It looks like you've got a stack-smashing issue (basically, you're allocating too much data on the stack). For instance,

Code:

int N = 1024;
int x[N][N];

Assuming an int is 4 bytes, we have 1024 * 1024 * 4 = 4 MB just for this one array. Now you have 3 such arrays declared x, y and z. Therefore total stack needed for just these three arrays is 4 * 3 = 12 MB. And we also need stack space for the remaining variables as well (i, j, k, r) as well as for function calls. So we'll need stack somewhat more than 12 MB to run this program.

Now if you go to your linux system and type:

Code:

ulimit -s

This shows the max. amount of stack assigned to your programs by default. On my system, it says:

Code:

mb@motorhead:~$ ulimit -s
8192

i.e. 8MB max. stack is allocated to my program when it starts up. This is less than the min. of 12 MB+ stack that the program needs.

Now if I change the max. amount to something else (such as 16384 (i.e.) 16 MB), the code runs without segfault.

Code:

mb@motorhead:~/cpp$ ./matrix 
Segmentation fault
mb@motorhead:~/cpp$ ulimit -s 16384
mb@motorhead:~/cpp$ ./matrix
.... runs all the way without segfault ....

Of course the result will be nonsense, since you haven't set y or z, but I'll assume you knew that already.
Hope this helps.

__________________
Up the Irons
What Would Jimi Do? Smash amps. Burn guitar. Take the groupies home.
"Death Before Dishonour, my Friends!!" - Bruce D ickinson, Iron Maiden Aug 20, 2005 @ OzzFest
Down with Sharon Osbourne

Problem with short program

Developer Shed Advertisers and Affiliates