Strcopy understanding

[fusi0n]

Hi,
my code is as follows:

#include <iostream>
#include <string.h>

int main()
{

    char bla[2][4];
    strcpy(bla[0],"haha");
    strcpy(bla[1],"hihi");
    strcpy(bla[2],"huhu");
    printf("%s",bla[0]);
    return 0;
}

My question is, that why does it print out hahahihihuhu / all the strings I added to different indexes?

[vlad from moscow]

Let consuder the program step by step.

Here

strcpy(bla[0],"haha");

5 characters (do not forget the terminating zero of the string literal) is copied to bla[0] that has type char[4]. So the fifth character that is '\0' will be copyed in bla[1][0].

Here

strcpy(bla[1],"hihi");

is the same problem. Only the bla[1][0] that contains '\0' will be overwritten by the first character of the string literal. The terminating zero will be written beyond the array.

Here

strcpy(bla[2],"huhu");

you are overwriting the memory that does not belong to the array because the array has acceptable indexes in the range [0, 1].. Again you are overwriting the last terminating zero of the previous operastion.

So you get one long string which occupies the memory beyond the array. The type of the expression b[0] is char[]. So then you use

printf("%s",bla[0]);

the whole string will be outputed.

[Chervil]

The program is invalid, the array size is too small.

String "haha" is five characters long.
It is equivalent to { 'h', 'a', 'h', 'a', '\0' }

Thus the terminating null character '\0' falls in the first position of the next string.
Each strcpy will over-write the null byte of the previous string.

The null of strcpy(bla[1],"hihi"); will fall outside boundaries of the array.

In addition, bla[2] is the third entry in an array of just two elements, so it as writing to an area outside the array.

If you want predictable and valid results, change line 7 to read:
char bla[3][5];

[S G H]

Vlad is right, let me show how your memory will look like with all the strcpy's, assuming you correct your bla's size and my written 0 is binary 0:

Beginning Data into bla (4x3 + 1):
0000000000000
First strcpy:
haha000000000
Second strcpy:
hahahihi00000
Third strcpy:
hahahihihuhu0

When printf calculates the length of the string, it goes looking for that 0, but only finds one at the end of all the variables, so prints them all.

[fusi0n]

Thank you guys very much. I totally forgot about the '\0' char.