Segmentation fault initialising an array

Question

The following program produces a segmentation fault, and I'm not sure why. malloc succeeds, so it doesn't seem to be an initialisation error, but for some reason, it segfaults when I access the 253900th element. The array is only 4 * 1e6 bytes, or a about a Megabyte.

This does produce a lot of output

#include <stdlib.h> #include <stdio.h> int *long_array(size_t N) { int *arr = (int *) malloc(N); if (arr == NULL) { printf("could not malloc"); exit(1); } for (size_t i = 0; i < N; i++) { printf(".. %ld ", i); arr[i] = 10; } printf("done with loop\n"); return arr; } int main(void) { int *arr = long_array(1000000); printf("%d", arr[5050]); return 0; }

I compile this with gcc -std=c99 and run the output to see the final few numbers printed before the segfault:

253899 .. 253900 .. 2 segmentation fault (core dumped) ./a.out

I don't understand why accessing a particular index is causing the segmentation fault. I can guess that I must be accessing a memory location outside of my processes address space, but this seems like a bug if I successfully allocated the memory from within my address space.

Your array is 1Mb roughly, but you're trying to access elements beyond that. When allocating, you're not taking into account the size of int. Use int *arr = malloc(N*sizeof(*arr)); — AntonH
– AntonH, Commented Feb 2, 2014 at 16:14

Moshe Gottlieb · Accepted Answer · 2014-02-02 16:16:47Z

6

malloc(sizeof(int)*N) instead of malloc(N). Otherwise you'd get an array of N bytes, not N integers.

answered Feb 2, 2014 at 16:16

Moshe Gottlieb

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1 Comment

Charles Over a year ago

Oh jeez, I just read about that. Sorry, brain farts. Thanks for pointing it out!

Tim Child · Accepted Answer · 2014-02-02 16:17:15Z

0

use

 int *arr = (int *) malloc(N * sizeof(int));

answered Feb 2, 2014 at 16:17

Tim Child

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Comments

Prashant Anuragi · Accepted Answer · 2014-02-02 16:57:35Z

Your code is correct but the problem here is that your did not consider the size of an integer while allocating memory. You just allocated :

int *arr=(int *)malloc(N);

here you just allocated size of N but you have to consider size of integer in your machine. If you allocate just 10000000 size of memory and sizeof(int) in you machine is 4 then it can just access (10000000/4) memory ......... So you have to allocate memory like:

int *arr=(int *)malloc(N*sizeof(int))

or

int *arr=(int *)malloc(N*sizeof(*arr));

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