example segmental error

[grytskiv]

code:

#include <stdio.h>
#include <stdlib.h>

int
main()
{
 int *my_array;
 int key;

 my_array = malloc(sizeof(int) * 10);

 for(key = 0; key < 20; key++)
 {
  my_array[key] = key;
  printf("key %i:\t%i\n",key,my_array[key]);
 }

 return 0;
}

terminal:

grytskiv@ZXDSL831II:~/memory$ gcc -ansi sigmental_memory.c -o sigmental_memory && ./sigmental_memory
key 0:  0
key 1:  1
key 2:  2
key 3:  3
key 4:  4
key 5:  5
key 6:  6
key 7:  7
key 8:  8
key 9:  9
key 10: 10
key 11: 11
key 12: 12
key 13: 13
key 14: 14
key 15: 15
key 16: 16
key 17: 17
key 18: 18
key 19: 19
grytskiv@ZXDSL831II:~/memory$

AND SECOND VARIANT CODE:

#include <stdio.h>
#include <stdlib.h>

int
main()
{
 int my_array[10];
 int key;

 /* my_array = malloc(sizeof(int) * 10); */

 for(key = 0; key < 20; key++)
 {
  my_array[key] = key;
  printf("key %i:\t%i\n",key,my_array[key]);
 }

 return 0;
}

terminal:

grytskiv@ZXDSL831II:~/memory$ gcc -ansi sigmental_memory.c -o sigmental_memory && ./sigmental_memory
key 0:  0
key 1:  1
key 2:  2
key 3:  3
key 4:  4
key 5:  5
key 6:  6
key 7:  7
key 8:  8
key 9:  9
key 10: 10
key 11: 11
key 12: 12
key 13: 13
key 14: 14
key 15: 15
key 16: 16
key 17: 17
key 18: 18
key 19: 19
segmental error
grytskiv@ZXDSL831II:~/memory$

Why in first variant code I don't recive "segmental error"??

And why in second variant my program execute full code? My program assign data for not existing element array (out range defined elements for my_array)

[quzah]

Pure chance.


Quzah.

[TheBigH]

Yeah, what Quzah said. The memory locations things are put in when you declare them are essentially arbitrary, and often there will be big swathes of unused memory between them. And your code will only crash if you attempt to access through your array memory that is allocated to something else. This is why segmentation faults are the most evil, insidious and nasty things ever; you may not get an error message until long after the actual error's happened and end up blaming a perfectly innocent bit of code for the crash.