5.c

#include <stdio.h>

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

#include <unistd.h>

#define FILENAME "./test"

int main(void)
{
    int fd;
    int ret;
    int flag;

    fd = open(FILENAME, O_WRONLY);
    printf("fd = %d\n");

    ret = write(fd, "1", 1);
    if (ret == -1)
    {
        printf("ret = -1\n");
        perror("write");
    }

    flag = fcntl(fd, F_GETFL);
    flag |= O_APPEND;
    ret = fcntl(fd, F_SETFL, flag);

    printf("fcntl ret = %d\n");

    ret = write(fd, "2", 1);
    printf("ret = %d\n", ret);

    flag = fcntl(fd, F_GETFL);
    flag &= ~(O_APPEND);
    fcntl(fd, F_SETFL, flag);

    return 0;
}