C language fcntl function

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>


int main(int argc, char *argv[])
{
    
    int fd = open("a.txt", O_RDWR);
    char buf[12138] = {
    0};
    //  Writing again at this time is additional writing 
    read(fd, buf, 12138);
    printf(" Original content ：%s\n", buf);
    //  Get the original flag And plus O_APPEND flag
    int flag = fcntl(fd, F_GETFL) | O_APPEND;
    //  Set up flag
    fcntl(fd, F_SETFL, flag);
    char* str = "\tafter append";
    //  Writing again at this time is additional writing 
    write(fd, str, strlen(str));
    close(fd); 
    fd = open("a.txt", O_RDWR);
    read(fd, buf, 12138);
    printf(" Additional content ：%s\n", buf);
    return 0;
}