CODEWITHSUNDEEP
clinuxsegmentation-fault
Ahmed Suffian Javed
Ahmed Suffian Javed

Reputation: 19

C: rand () function occasionally causes Segmentation fault

Given Below is the code which stores random integers in a binary file. The size of the binary file should be 65536 bytes, so I have to generate total 16634 integers and then save them 16 integers at a time. This is has been implemented in populateBackingStore() function. The Problem here is that a segmentation fault occurs "sometimes" at line: 

buffer [i%16] = rand ()  % MAX_INT;.

Note: For debugging purpose I have printed the iteration number and I have found that the segmentation fault only occurs at 16358th iteration.

#include <stdio.h>

#define     BACKING_STORE   65536
#define     MAX_INT     42949678295

int populateBackingStore () {
    FILE * backingStore = fopen ("BACKING_STORE.bin", "wb");
    int num, i;
    int * buffer = (int* ) malloc (16);     // 64 * (4 bytes) = 256 bytes 

    if (backingStore == NULL) {
        printf ("Error while creating file BACKING_STORE.bin\n");
        return NULL;
    }

    srand (time (NULL));
    for (i = 0; i < BACKING_STORE/sizeof (int) ;i ++) {
        buffer [i%16] = rand ()  % MAX_INT;
        if ( i % 16 == 0 ) {
            fwrite (buffer, sizeof (int), 16, backingStore);
        }           
        printf ("%d ", i);
    }
    fclose (backingStore);  
}

int main () {
    populateBackingStore ();

    return 0;
}

Upvotes: 1

Views: 2951

Answers (1)

Sourav Ghosh
Sourav Ghosh

Reputation: 134276

int * buffer = (int* ) malloc (16);

You're not allocating enough space for 16 ints. You're allocating 16 bytes.

Change to

int * buffer = malloc (16 * sizeof (*buffer));

Related reading :

  1. malloc() man page.
  2. Do not cast the return value of malloc().

Upvotes: 7

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