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Views: 2164 | Replies: 4 | Solved
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Join Date: Aug 2005
Location: Bosnia and Herzegovina
Posts: 147
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Junior Poster
Hello guys,
It's been a while since I last time posted in this forum.
I have strange problem regarding matrix allocation. I was asked to write code that includes 2D array dynamic allocation. I write two version of matrix allocation.
Here is in my opinio relevant part of the code:
I have tested both versions with Dev-Cpp on Windows platform and sent to my friend who discovered that if he uses version which is commented, everything is OK, but if he the use version as in the above code he gets segmentation fault when entering elements. He tested it in Linux. I don't have linux installed and i ask you to test this code and check if it will fail on linux machine. I don't see a reason for such behaviour and simply can't figure out what is wrong. He sad he got segmentation fault with rows = 3 and cols = 2.
Can you please check it?
Thanks
It's been a while since I last time posted in this forum.
I have strange problem regarding matrix allocation. I was asked to write code that includes 2D array dynamic allocation. I write two version of matrix allocation.
Here is in my opinio relevant part of the code:
#include <stdio.h>
#include <stdlib.h>
int main( void )
{
int rows, cols;
int i, j;
/*int * mat1Blok;*/
int ** mat1;
printf("Enter number of rows: ");
scanf("%d", &rows);
printf("Enter number of columns: ");
scanf("%d", &cols);
/*memory allocation*/
/* mat1Blok = malloc(rows * cols * sizeof(int));
mat1 = malloc(rows * sizeof(int*));
for (i = 0; i < rows; ++i)
{
mat1[i] = &mat1Blok[i * cols];
}
*/
mat1 = malloc(cols * sizeof(int*));
for(i = 0; i < cols; i++)
{
mat1[i] = malloc(rows * sizeof(int));
}
printf("\nEnter elements row by row:\n");
for (i = 0; i < rows; i++)
for(j = 0; j < cols; j++)
scanf("%d",&mat1[i][j]);
/*
free (mat1);
free (mat1Blok);
*/
for (i = 0; i < cols; i++)
{
free (mat1[i]);
}
free (mat1);
system("PAUSE");
return 0;
}
I have tested both versions with Dev-Cpp on Windows platform and sent to my friend who discovered that if he uses version which is commented, everything is OK, but if he the use version as in the above code he gets segmentation fault when entering elements. He tested it in Linux. I don't have linux installed and i ask you to test this code and check if it will fail on linux machine. I don't see a reason for such behaviour and simply can't figure out what is wrong. He sad he got segmentation fault with rows = 3 and cols = 2.
Can you please check it?
Thanks
Last edited by Micko : Jun 6th, 2007 at 1:28 am.
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Join Date: Dec 2005
Posts: 3,257
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> mat1 = malloc(cols * sizeof(int*));
You've got the rows and cols mixed up in a couple of places.
You've got the rows and cols mixed up in a couple of places.
If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
Do not PM me for help; You'll be ignored, or told to learn to read.
Do not ask me if I'm muslim - I'm not. Nor do I care about yours or anyone else's mysticism. Religion is a matrix, take the RED PILL.
Do not PM me for help; You'll be ignored, or told to learn to read.
Do not ask me if I'm muslim - I'm not. Nor do I care about yours or anyone else's mysticism. Religion is a matrix, take the RED PILL.
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Join Date: Aug 2005
Location: Bosnia and Herzegovina
Posts: 147
Reputation:
Rep Power: 4
Solved Threads: 1
Junior Poster
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Originally Posted by Salem 
> mat1 = malloc(cols * sizeof(int*));
You've got the rows and cols mixed up in a couple of places.
Hmm, this is important part:
mat1 = malloc(cols * sizeof(int*));
for(i = 0; i < cols; i++)
{
mat1[i] = malloc(rows * sizeof(int));
}Still don't understand, why this code works on windows machine and fails on linux.
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Join Date: Sep 2004
Posts: 6,017
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Solved Threads: 414
You're mixing up the meaning of a row and a column. The following allocates using column major order (a[cols][rows]):
And this prints using row major order (a[rows][cols]):
Unless rows and cols have the same value, the two aren't interchangeable and you're accessing memory outside the bounds of the array you just allocated. Linux is giving you a segmentation fault.
mat1 = malloc(cols * sizeof(int*));
for(i = 0; i < cols; i++)
{
mat1[i] = malloc(rows * sizeof(int));
}for (i = 0; i < rows; i++)
for(j = 0; j < cols; j++)
scanf("%d",&mat1[i][j]); Member of: Beautiful Code Club.
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