Why does the code in c not end and forces me to press crl+z to end the execution?

I´ve made a simple code to print in reverse the numbers written between letters.
For example: if I write on the terminal:
1 2 3 hello 4 5 6 end
or enter that in the function through a txt its returning the expected values:
3 2 1\n
6 5 4\n
but it doesn´t end, Im forced to press crl+z to be able to use the trminal again.

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

#define MAX_NUM 64

int main() {
    int num[MAX_NUM];
    int size = 0, i;
    char word[20];
    while (scanf("%d", &num[size]) != 0) {
		size++;
		while(scanf("%d", &num[size]) != 0){
			size++;
		}
        if (scanf("%s",word) != 0 || size == MAX_NUM) {
			for (i = size - 1; i >= 0; i--) {
				if(i == 0)
					printf("%d", num[i]);
				else
					printf("%d ", num[i]);
			}
			printf("\n");
			size = 0;
        }
    }
    return 0;
}

> Why does the code in c not end and forces me to press crl+z to end the execution?

Because scanf cannot read your mind. For instance, when you call scanf("%d", &num[size]) and there is not presently any more data available to read, scanf doesn't know that you don't intend ever to present any more. Before it returns, it needs to see an end-of-input or error signal or a character that it cannot match to the format. In Windows, Ctrl-Z signals end-of-input on the standard input of a program running in the terminal.

> Im forced to press crl+z to be able to use the trminal again.

Then you can't be running on Windows. On Windows ctrl-z in a terminal means EOF (End Of File). If your program receives an EOF freom a scanf call it will enter an endless loop.

But you do write:

> Im forced to press crl+z to be able to use the terminal again.

In that case you are on a *nix system (or a similar system). On *nix ctrl-z means "suspend". It will give you the terminal prompt back but it will not end the program. The program is still there in the background. Run pidof <program-name> to see that.

Some comments about your program:

while(scanf(...) != 0)

is nearly always bad (perhaps always bad). The reason is that you can't catch an EOF with such code.

The better way is

while(scanf(...) == N)

where N is the number of items you expect to scan (it's 1 in your case). By doing that EOF will work.

Code like

scanf("%s",word)

is always bad - very bad. The user may overflow the buffer. Always limit the maximum number of characters to read. In your case like:

scanf("%19s",word)

However, fixing the above won't make the program end when you give input like

1 2 3 hello 4 5 6 end

The program will call the outer (first) scanf("%d", &num[size]) and wait forever because it doesn't receive further input.

You can end your program by giving it two consecutive non-integer inputs. Example

1 2 3 hello 4 5 6 end abc
                      ^
                      This will cause outer scanf to return zero
                      and the program will stop

If you want "end" to mean "end the program", you can add code like:

if (strcmp(word, "end") == 0) return 0;