C - program stops when printf('\033c') used

printf('\033c');

printf() expects a char const * as its first parameter, not a character constant.

Try

printf("\033c");

instead.

On Windows use SetConsoleMode() to enable virtual terminal sequences:

#include <windows.h>
#include <stdio.h>

int main() {

    HANDLE con_out = GetStdHandle(STD_OUTPUT_HANDLE);
    DWORD mode;
    GetConsoleMode(con_out, &mode);
    SetConsoleMode(con_out, mode | ENABLE_VIRTUAL_TERMINAL_PROCESSING);

    printf("Hello!\n");
    getchar();
    printf("\033[H\033[2J");
    getchar();
}

There are multiple problems in your code:

• The array pixelsArr pixels[world_y_size] is not dereferenced incorrectly in create_world and update_thread, you should index first by y, then by x.

• '\033c' is a character constant, you should pass a string to printf:

  printf("\033c");
  

  Note however that this escape sequence might not have the desired effect on your system.

• the array output is not null terminated: passing it to printf has undefined behavior. It is defined with the correct size, but local objects with automatic storage are uninitialized, so the last element in the array is uninitialized. You should set it to '\0' before printing the string.

• you should not pass a computed string directly to printf as printf(output);. This has undefined behavior if the string contains one or more % characters, which could happen depending on how you update the pixels. Use printf("%s", output); or fputs(output, stdout);.

Here is an improved version:

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

#define world_x_size 53
#define world_y_size 50

struct Pixel {
    char what_here;
};

typedef struct Pixel pixelsArr[world_x_size];

void create_world(pixelsArr *pixels) {
    int x = 0;
    int y = 0;
    while (y < world_y_size) {
        x = 0;
        while (x < world_x_size) {
            pixels[y][x].what_here = 'O';
            x++;
        }
        y++;
    }
}

void update_thread(pixelsArr *pixels) {
    while (1) {
        int x = 0;
        int y = 0;
        while (y < world_y_size) {
            char output[world_x_size + 1];
            x = 0;
            while (x < world_x_size) {
                output[x] = pixels[y][x].what_here;
                x++;
            }
            output[x] = '\0';
            printf("%s\n", output);
            y++;
        }
        printf("\033c");
    }
    return NULL;
}

int main() {
    pixelsArr pixels[world_y_size];

    create_world(pixels);

    pthread_t thread;
    pthread_create(&thread, NULL, update_thread, pixels);
    pthread_join(thread, NULL);

    return 0;
}

You are accessing pixels items incorrectly: Change it

            output[x] = pixels[x][y].what_here;

to

            output[x] = pixels[y][x].what_here;

look at this definition:

pixelsArr pixels[world_y_size];

pixels can be treated as Pixel [world_y_size][world_x_size].

And pixels[x][y].what_here = 'O'; to pixels[y][x].what_here = 'O';.