Segmentation fault in nested for loop with dynamic memory allocation - c

Question

I was preparing a code for simulating Distance Vector Routing using C, however, I faced Segmentation Fault while running.
The code:-
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
/* Date : 03/06/2018
*
* Algorithm
*
* 1. get number of nodes from user
* 2. dynamic alloc new matrix nxn
* 3. create distance vector matrix, if dist > 1000 consider inf
* _| A B C D E F
* A| 0 5 2 3 i i
* B| 5 0 4 5
* C|
* D|
* E|
* F|
* ---------------
*
* 4. create new routing matrix of nxn
* 5. create new minimizing array for the node
* 6. find minimum of the array, allocate new value
*
* copyleft
*/
#define inf 1000
int min_r(int*, int*, int);
void dvr(int**, int**, char**, int);
void dvtDisp(int**, int);
void dvtDispNew(int **, char**, int);
int main(){
int n; //No of nodes
int i,j; //Counters
printf("> enter the number of nodes in the network... ");
scanf("%d",&n);
int **DisMat = (int **)malloc(n * n * sizeof(int)); //Dynamic allocation of Distance Matrix
for(i=0; i<n; i++){ // x directional loop
printf("> distance vector table for node %c\n",i+65);
for(j=0; j<n; j++){ // y directional loop
printf("> distance from %c... ",j+65);
if(j==i) { DisMat[i][j] = 0; printf("0");}
else scanf("%d",&DisMat[i][j]);
}// y directional loop
}// x directional loop
int **NewDisMat = (int **)malloc(n * n * sizeof(int)); //New Distance Matrix
char **Hop = (char **)malloc(n * n * sizeof(char)); //New Hop Matrix
for(i=0; i<n; i++){
for(j=0; j<n; j++){
Hop[i][j] = '-'; //All Hops Nullified
}
}
dvr(DisMat, NewDisMat, Hop, n); //Distance Vector Routing
return 0;
}//main
void dvr(int *dvt[], int *newdvt[], char *hopper[], int l){ //DVR function
int x=0, y=0, z=0, conCount;
int hopPoint;
int *mini = (int *)malloc((l-1) * sizeof(int));
int *mzer = (int *)malloc((l-1) * sizeof(int));
for(x=0; x<l; x++){ // x directional propagation
mini[0] = x;
z = 1; conCount=0;
do{
if((dvt[x][y] < inf) && (y != x)) {
mini[z] = y;
z++;
conCount++;
}
y++;
}while(y<l);
y = 0; z = 0;
for(y = 0; y<l; y++){
while(z<conCount){
mzer[z] = dvt[mini[z]][y];
z++;
}
newdvt[x][y] = min_r(mzer, &hopPoint, conCount);
hopper[x][y] = hopPoint + 65;
}// y directional propagation
}// x directional propagation
}//dvr
int min_r(int arr[], int *index, int len){
//Sequential minimum search
int min;
int ind = 0;
min = arr[ind];
for(ind = 0; ind<len; ind++){
if(arr[ind] < min){
min = arr[ind];
*index = ind;
}
}
return min;
}//min_r
void dvtDisp(int *dvt[], int size){
int x, y;
printf("_ |");
for(x = 0; x<size; x++){
printf("\t%c",65 + x);
}
printf("\n");
for(y = 0; y<size; y++){
printf("%c |",y + 65);
for(x = 0; x < size; x++)
printf("\t%d",dvt[x][y]);
}
}
void dvtDispNew(int *dvt[], char *hopto[], int size){
int x, y;
printf("_ |");
for(x = 0; x<size; x++){
printf("\t%c\thop",65 + x);
}
printf("\n");
for(y = 0; y<size; y++){
printf("%c |",y + 65);
for(x = 0; x < size; x++)
printf("\t%d\t%c",dvt[x][y],hopto[x][y]);
}
}
I got the following output on the terminal during execution.
anwesh#bionic-Inspiron:~/Documents/NS2/LAB/prog5$ gcc main.c
anwesh#bionic-Inspiron:~/Documents/NS2/LAB/prog5$ ./a.out
> enter the number of nodes in the network... 5
> distance vector table for node A
Segmentation fault (core dumped)
I tried to run it on gdb but could not figure out what the results meant. Here the gdb output:-
Starting program: /home/anwesh/Documents/NS2/LAB/prog5/a.out
> enter the number of nodes in the network... 5
> distance vector table for node A
Program received signal SIGSEGV, Segmentation fault.
0x000055555555487e in main ()
(gdb)
Initially I thought it'd be a problem related to dynamic memory allocation, but I do not know the exact cause. I've checked the code multiple times to see if there are any naive mistakes, but I couldn't.
Please help me out here! Thanks in advance.

Answer 1

The line
int **DisMat = (int **)malloc(n * n * sizeof(int)); //Dynamic allocation of Distance Matrix
is not valid. DisMat is an array of pointers to an array of ints. So we need to allocate n pointers to ints first:
int **DisMat = malloc(n * sizeof(int*));
Then we need to n-times allocate array of n ints:
for(size_t i = 0; i < n; ++i) {
DisMat[i] = malloc(n * sizeof(int));
}
The same goes for Hop and NewDisMat.
Remember, that malloc does not check for multiplication overflow.
The following code runs fine:
#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <assert.h>
/* Date : 03/06/2018
*
* Algorithm
*
* 1. get number of nodes from user
* 2. dynamic alloc new matrix nxn
* 3. create distance vector matrix, if dist > 1000 consider inf
* _| A B C D E F
* A| 0 5 2 3 i i
* B| 5 0 4 5
* C|
* D|
* E|
* F|
* ---------------
*
* 4. create new routing matrix of nxn
* 5. create new minimizing array for the node
* 6. find minimum of the array, allocate new value
*
* copyleft
*/
#define inf 1000
int min_r(int*, int*, int);
void dvr(int**, int**, char**, int);
void dvtDisp(int**, int);
void dvtDispNew(int **, char**, int);
int main(){
int n; //No of nodes
int i,j; //Counters
printf("> enter the number of nodes in the network... ");
scanf("%d",&n);
int **DisMat = malloc(n * sizeof(*DisMat)); //Dynamic allocation of Distance Matrix
assert(DisMat != NULL);
for(size_t i = 0; i < n; ++i) {
DisMat[i] = malloc(n * sizeof(*DisMat[i]));
assert(DisMat[i] != NULL);
}
for(i=0; i<n; i++){ // x directional loop
printf("> distance vector table for node %c\n",i+65);
for(j=0; j<n; j++){ // y directional loop
printf("> distance from %c... ",j+65);
if(j==i) { DisMat[i][j] = 0; printf("0");}
else scanf("%d",&DisMat[i][j]);
printf("\n");
}// y directional loop
}// x directional loop
int **NewDisMat = malloc(n * sizeof(*NewDisMat)); //New Distance Matrix
assert(NewDisMat != NULL);
for(size_t i = 0; i < n; ++i) {
NewDisMat[i] = malloc(n * sizeof(*NewDisMat[i]));
assert(NewDisMat[i] != NULL);
}
char **Hop = malloc(n * sizeof(*Hop)); //New Hop Matrix
assert(Hop);
for(size_t i = 0; i < n; ++i) {
Hop[i] = malloc(n * sizeof(*Hop[i]));
assert(Hop[i] != NULL);
}
for(i=0; i<n; i++){
for(j=0; j<n; j++){
Hop[i][j] = '-'; //All Hops Nullified
}
}
dvr(DisMat, NewDisMat, Hop, n); //Distance Vector Routing
for(size_t i = 0; i < n; ++i) {
free(DisMat[i]);
}
free(DisMat);
for(size_t i = 0; i < n; ++i) {
free(NewDisMat[i]);
}
free(NewDisMat);
for(size_t i = 0; i < n; ++i) {
free(Hop[i]);
}
free(Hop);
return 0;
}//main
void dvr(int *dvt[], int *newdvt[], char *hopper[], int l){ //DVR function
int x=0, y=0, z=0, conCount;
int hopPoint;
int *mini = (int *)malloc((l-1) * sizeof(int));
int *mzer = (int *)malloc((l-1) * sizeof(int));
for(x=0; x<l; x++){ // x directional propagation
mini[0] = x;
z = 1; conCount=0;
do{
if((dvt[x][y] < inf) && (y != x)) {
mini[z] = y;
z++;
conCount++;
}
y++;
}while(y<l);
y = 0; z = 0;
for(y = 0; y<l; y++){
while(z<conCount){
mzer[z] = dvt[mini[z]][y];
z++;
}
newdvt[x][y] = min_r(mzer, &hopPoint, conCount);
hopper[x][y] = hopPoint + 65;
}// y directional propagation
}// x directional propagation
}//dvr
int min_r(int arr[], int *index, int len){
//Sequential minimum search
int min;
int ind = 0;
min = arr[ind];
for(ind = 0; ind<len; ind++){
if(arr[ind] < min){
min = arr[ind];
*index = ind;
}
}
return min;
}//min_r
void dvtDisp(int *dvt[], int size){
int x, y;
printf("_ |");
for(x = 0; x<size; x++){
printf("\t%c",65 + x);
}
printf("\n");
for(y = 0; y<size; y++){
printf("%c |",y + 65);
for(x = 0; x < size; x++)
printf("\t%d",dvt[x][y]);
}
}
void dvtDispNew(int *dvt[], char *hopto[], int size){
int x, y;
printf("_ |");
for(x = 0; x<size; x++){
printf("\t%c\thop",65 + x);
}
printf("\n");
for(y = 0; y<size; y++){
printf("%c |",y + 65);
for(x = 0; x < size; x++)
printf("\t%d\t%c",dvt[x][y],hopto[x][y]);
}
}
Side note: Remember that sizeof(int*) == sizeof(*DisMat), so I prefer:
int **DisMat = malloc(n * sizeof(*DisMat));
By using that expression type *variable = malloc(n * sizeof(*variable)) I can remember, that I am allocating the correct type, an array of pointers to ints in case of DisMat, cause typeof(*DisMat) == int*, and make less errors.