segmentation fault while writing to file

Question

i am getting segmentation fault while writing data into file. here is code

 int arr[N]; // where N = 1508065

f = fopen( "datafile.txt", "w" );

if (f == NULL)

{

    printf("Error opening file!\n");
    exit(1);
}

for( i=1;i<=N;i++)

    {
       arr[i]=rand()%100;
       fprintf(f, "%d  %d  %d\n",i,1,arr[i] );
    } 

fclose(f);

I have tried with double arr[N] even then is a segmentation fault. It works fine when N is small.

You are likely blowing up you stack --> *STACK OVERFLOW* with an integer array of `1,508,065` integers or `6,032,260` bytes of data. Be careful. It may be wise to dynamically allocate that storage. — David C. Rankin, Feb 27 '18 at 08:24

score 2 · Answer 1 · answered Feb 27 '18 at 08:20

2

for( i=1;i<=N;i++) will give undefined behavior.

C arrays start at 0 and go to N-1. Accessing element N is a no-no.

Should be for( i=0;i<N;i++)

answered Feb 27 '18 at 08:20

John3136

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user2736738 · Answer 2 · 2018-02-27T08:30:40.363

1

Accessing the N-th index of a N-size array is undefined behavior. You are accessing array out of bound. It will be for( i = 1; i < N; i++).

From standard: J.2 Under Undefined Behavior

An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the lvalue expression a1[7] given the declaration int a[4][5])

As you were using - arrays are 0 indexed in c.

edited Feb 27 '18 at 08:30

answered Feb 27 '18 at 08:20

user2736738

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score 1 · Accepted Answer · answered Feb 27 '18 at 08:24

1

It works fine when N is small.

I suppose you create arr as local variable, for N = 1508065 it is more than 5 MB memory allocated onto stack - you got stack overflow. You should allocate arr using malloc function or create this array as global variable.

Problem with reading out of range was pointed out in previous answers.

answered Feb 27 '18 at 08:24

rafix07

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Thanks for quick answer. declaring it with malloc works fine. No segmentation fault even with for( i=1;i<=N;i++) – Mani Feb 27 '18 at 08:51
Change `i<=N` to `i < N`, you cannot read out of range - it is undefined behaviour. – rafix07 Feb 27 '18 at 08:52
Thanks I have changed it to i < N – Mani Feb 27 '18 at 09:40

segmentation fault while writing to file

3 Answers3