Segmentation fault : Reading values from a text file

Question

In my code I am trying to read values from a .txt file so as to build my adjacency matrix but it keeps on returning a segmentation fault. I don't seem to be able to point out where I am going wrong. Please help.

#include <stdio.h>
#include <unistd.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
#include <time.h>
#include <sys/time.h>
#include <math.h>
#include <limits.h>
#include <iostream>


#define MAX_VERTICES 1024

int global_adj_matrix[MAX_VERTICES][MAX_VERTICES];


int **graph_tree;
int **node_data;
int global_weight;
int number_threads;
int max_nodes;
int random_node;
int max_weight;
int finish_flag;

void readAdjMatrix();


int main(int argc, char *argv[]){

    for(int i = 0 ; i < MAX_VERTICES ; i++){
        for(int j = 0 ; j < MAX_VERTICES ; j++){
            global_adj_matrix[i][j] = 0;
        }
    }

    number_threads = atoi(argv[1]);
    max_nodes = 0;

    readAdjMatrix();

}

void readAdjMatrix(){

    int source, destination, edge_weight;

    max_nodes = INT_MIN;
    max_weight = INT_MIN;

    FILE *file_pointer = fopen("graph.txt", "r");

    while(!feof(file_pointer)){
        fscanf(file_pointer, "%d", &source);
        fscanf(file_pointer, "%d", &destination);
        fscanf(file_pointer, "%d", &edge_weight);

        global_adj_matrix[source][destination] = edge_weight;
        global_adj_matrix[destination][source] = edge_weight;

        if(edge_weight > max_weight)
            max_weight = edge_weight;

        if(destination > max_nodes)
            max_nodes = destination;
    }

    printf("%d %d", max_weight, max_nodes);

    for(int i = 0 ; i <= max_nodes ; i++){
        for(int j = 0 ; j <= max_nodes ; j++){
            printf("%d\t", global_adj_matrix[i][j]);
        }
        printf("\n");
    }

    fclose(file_pointer);
}

This is my .txt file

If it is actually C, remove the <iostream> include, because that is C++. You are not using any element of iostream, so you do not need it. — harmic
– harmic, Commented Apr 24, 2017 at 4:29
From core are you able to identify where it fails? Also you need to add check if fopen returns null — Pras
– Pras, Commented Apr 24, 2017 at 4:37
If you don't enter an arg then it should be obvious that atoi(argv[1]); will cause a problem because argv[1] will be NULL. Suggest you learn to use a debugger. — kaylum
– kaylum, Commented Apr 24, 2017 at 4:45

Micrified · Accepted Answer · 2017-04-24 16:56:15Z

Your segmentation fault is because you're trying to read a nonexistent index in the argument vector of main. If you want to avoid that, you should rewrite it to match something like this:

int main (int argc, const char *argv[]) {

    if (argc > 1 && (number_threads = atoi(argv[1]))) {
        max_nodes = 0;
        readAdjMatrix();
    }

    return 0;
}

This ensures that you have an argument to convert to begin with, and also that it is a nonzero number. I believe atoi has undefined behavior if it isn't a valid string though, so you should harden against that. You also do some other unnecessary things. For one, this block here:

   for(int i = 0 ; i < MAX_VERTICES ; i++){
        for(int j = 0 ; j < MAX_VERTICES ; j++){
            global_adj_matrix[i][j] = 0;
        }
    }

is pointless because if you initialize a 2D array as an external/global variable then it is automatically zeroed upon initialization. Only local/automatic variables will be filled with garbage data. Therefore, you can omit it.

Finally, I would also change your while loop to look more or less like this (Credit: Chux for better loop guard).

while(fscanf(file_pointer, "%d %d %d", &source, &destination, &edge_weight) == 3) {

    global_adj_matrix[source][destination] = global_adj_matrix[destination][source] = edge_weight;

    if(edge_weight > max_weight)
        max_weight = edge_weight;

    if(destination > max_nodes)
        max_nodes = destination;
}

This ensures you correctly scanned the amount of variables necessary per line. And the extended assignment just saves a bit of room.

Hope this fixed the problem you were having.

Coding Ninja · Accepted Answer · 2017-04-26 13:32:49Z

0

I am posting my answer by neglecting the bigger code which you have not mentioned in your question. I have trimmed down the unnecessary code. The code is given below.

 #include <stdio.h>
 #include <unistd.h>
 #include <ctype.h>
 #include <string.h>
 #include <stdlib.h>
 #include <math.h>
 #include <limits.h>

 #define MAX_VERTICES 1024

 int global_adj_matrix[MAX_VERTICES][MAX_VERTICES];
 int global_weight,max_nodes,random_node,max_weight;

 void readAdjMatrix();


 int main()
 {
  int i,j;

   max_nodes = 0;
   readAdjMatrix();
   return 0;
 }


 void readAdjMatrix()
 {
  int source, destination, edge_weight,i,j;
  max_nodes = INT_MIN;
  max_weight = INT_MIN;

  FILE *file_pointer = fopen("graph.txt", "r");

  while(!feof(file_pointer))
  {
    fscanf(file_pointer, "%d", &source);
    fscanf(file_pointer, "%d", &destination);
    fscanf(file_pointer, "%d", &edge_weight);

    global_adj_matrix[source][destination] = global_adj_matrix[destination][source] =edge_weight;


    if(destination > max_nodes)
        max_nodes = destination;
   }

   printf( "%d\n", max_nodes);

   for( i = 0 ; i <= max_nodes ; i++){
    for( j = 0 ; j <= max_nodes ; j++){
        printf("%d\t", global_adj_matrix[i][j]);
    }
    printf("\n");
   }

   fclose(file_pointer);
  }

PS : Simply execute this code with ./a.out with no command line argument. In case you are using the command line argument (as given in your question), please use the following syntax to execute your code :

./a.out "your desired number which works with the bigger code"

edited Apr 26, 2017 at 13:32

answered Apr 24, 2017 at 8:24

Coding Ninja

11612 bronze badges

8 Comments

chux Over a year ago

while(!feof(file_pointer)) test is too late: code will write outside global_adj_matrix[][]. Better to test the result of fscanf()s.

Coding Ninja Over a year ago

How come ? The two indexes of the global_adj_matrix[][] array are being taken from the file (column 1 --> first index, column 2 --> second index and column 3 --> value to be stored at that location). None of the values in the file are exceeding the number 1024. How will the data be written outside the matrix ?

chux Over a year ago

After the last valid line is read with fscanf(file_pointer, "%d", &edge_weight);, the while(!feof(file_pointer)) does not prevent the next iteration as end-of-file has not happened yet. The 3 fscanf() read fail to read. In this case, likely the values of source, destination remain unchanged. Yet code is brittle. Should int source, destination get moved to inside the while(!feof(file_pointer)) { loop, their value would be undefined. Best to check the return of fscanf() & ditch feof(). See stackoverflow.com/questions/5431941/…

chux Over a year ago

On that last fscanf(file_pointer, "%d", &source);, fscanf() did not return 1 - the scan failed. Try fscanf(file_pointer, "%d", &source); --> printf("retval:%d\n", fscanf(file_pointer, "%d", &source));. Since it do not return 1, the value of source will be 1) unassigned or 2) potentially corrupted. If corrupted or never assigned, global_adj_matrix[source][destination] = ... is undefined behavior - access outside the array may be attempted. Robust code checks the return values of *scanf().

chux Over a year ago

Rather than

while(!feof(file_pointer))   {     fscanf(file_pointer, "%d", &source);     fscanf(file_pointer, "%d", &destination);     fscanf(file_pointer, "%d", &edge_weight);

use while(fscanf(file_pointer, "%d%d%d", &source, &destination, &edge_weight) == 3) {

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