What´s the explenation for this in this piece of code, (related to strings in C)?-CodePudding

Im new to coding and C, and cant found an explenation to this can someone explain me please: Piece of my code Why does those two last random characters pop up if I enter 8, but if I input 6 it doesn´t. It is related to null characters or something? Please I need an explenation.

I tried almost everything, still figuring out, can´t found an explenation.

CodePudding user response：

In C, strings need to end with a special character called the NUL termination character. The value of that character is 0.

If you pass a string to printf (%s), it expects the string to end with 0.

Since you don't set the NUL termination, the printf function does not know when to stop reading characters from the string.

This is called a buffer over read: you're reading past your allocated buffer (aster in that case).

Make sure to terminate aster and it'll work.

One way of achieving this would be to change char aster[n]; to char aster[n 1]; (allocate one character more for the NUL termination character) and then setting aster[n] = 0;.

Consider n = 3 memory will look like this: (OUT OF BOUNDS means the memory afterwards doesn't belong to aster and is something else, we don't know what it is).

aster[0] = ?
aster[1] = ?
aster[2] = ?
aster[3] = ? <OUT OF BOUNDS>
aster[4] = ? <OUT OF BOUNDS>
aster[5] = ? <OUT OF BOUNDS>
aster[6] = ... and so on

After your code memory will look like this:

aster[0] = *
aster[1] = *
aster[2] = *
aster[3] = ? <OUT OF BOUNDS>
aster[4] = ? <OUT OF BOUNDS>
aster[5] = ? <OUT OF BOUNDS>
aster[6] = ... and so on

printf will look at each character. It should stop at aster[3] but it doesn't know that because there isn't a well-defined character at that place. This is why it might work sometimes, but other times it will not work. It's undefined behaviour.

Changing the code to always set the NUL character it will look like this:

aster[0] = *
aster[1] = *
aster[2] = 0
aster[3] = ? <OUT OF BOUNDS>
aster[4] = ? <OUT OF BOUNDS>
aster[5] = ? <OUT OF BOUNDS>
aster[6] = ... and so on

In that case printf will NEVER read past aster[2].

In any case, it's always a no-no to read or write past your memory buffers ;)

Also you need to intialize your loop variable i to 0.