question about variables

[jackhasf]

hi

if we have this code

char x[10];

just this code nothing else, can we know what there might be inside the elements of the array?

if you print an element for example printf("%c",x[5]) it will give something to the screen

i ve heard it's independent but how can C just print this stuff? where does C get this stuff from?

thanks in advance

[MK27]

i ve heard it's independent but how can C just print this stuff? where does C get this stuff from?

Because x is assigned 10 bytes of real physical memory. There cannot be "nothing" there, but you have not done anything with it, so it contains whatever it was used for last by some other program -- possibly -- or it is just in a random state.

[jackhasf]

thanks MK27

i have one more question

what's the probability of this variable to contain a normal string of 3 characters?

i mean if we would make printf("%s",x) what's the probability of this to get us a normal string that has 3 characters+1('\0') to the screen for example "abc"?

is this even possible? just curious

[quzah]

printable/256 + printable/256 + printable/256 + 1/256

Where printable is the total number of printable character values for your current character set.


Quzah.

[cpjust]

If it's a global variable (outside of any functions), then all the elements will be initialized to '\0', so it will be an empty string.
If it's a local variable, it could have any value.

[C_ntua]

printable/256 + printable/256 + printable/256 + 1/256

Where printable is the total number of printable character values for your current character set.


Quzah.

You mean

printable/256 * printable/256 * printable/256 * 1/256

where printable would usually be 255 (not including 0). For "abc" it would be 1/(256^4)

[C_ntua]

Because x is assigned 10 bytes of real physical memory. There cannot be "nothing" there, but you have not done anything with it, so it contains whatever it was used for last by some other program -- possibly -- or it is just in a random state.

Wouldn't be reading what was used for last by some other program kind of a security issue?

[quzah]

where printable would usually be 255 (not including 0).

There's a reason they have a function called isprint. For example: \b\b\b\0

I always get probability wrong (probably because it's boring).


Quzah.

[cpjust]

Wouldn't be reading what was used for last by some other program kind of a security issue?

That's why good programs don't store things like plain text passwords in memory, they store them encrypted.

[C_ntua]

That's why good programs don't store things like plain text passwords in memory, they store them encrypted.

Wouldn't it be simpler if the OS erased the stack after the program finished or crashed? As a safety measure lets say. Or is it kind of "not necessary" safety measurement. You could do it yourself if the program normally returns.

[cpjust]

Wouldn't it be simpler if the OS erased the stack after the program finished or crashed? As a safety measure lets say. Or is it kind of "not necessary" safety measurement. You could do it yourself if the program normally returns.

People with root/Administrator access can force a memory dump whenever they want and then they'd be able to see anything that's not encrypted in the dump file. But yeah, I guess that might make things a tiny bit more secure.