The Linux Programming Interface Exercise 5-2

5-2

Write a program that opens an existing file for writing with the O_APPEND flag, and
then seeks to the beginning of the file before writing some data. Where does the
data appear in the file? Why?

It appears at the end. Cause you set O_APPEND, every write will append at the end of the file.

5-2.c

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

off_t lseek(int fd, off_t offset, int whence);

int main(int argc, char const *argv[])
{
    int fd;
    fd = open("test", O_RDWR | O_APPEND);
    if (fd == -1) {
        perror("open");
        return EXIT_FAILURE;
    }
    off_t offset;
    lseek(fd, 0, SEEK_SET);
    write(fd, "9999", 4);
    return 0;
}