/* Exercise 5-4 */

#include <unistd.h>

#include <fcntl.h>

#include <assert.h>

#include <limits.h>

#include "tlpi_hdr.h"

/* guess */

#define OPEN_MAX 256

int my_dup (int);

int my_dup2 (int, int);

off_t get_offset (int);

int my_dup (int oldfd) {

return fcntl(oldfd, F_DUPFD, 0);

}

int my_dup2 (int oldfd, int newfd) {

// based on the information from `man -s2 dup2`

if (newfd < 0 || newfd >= OPEN_MAX) {

errno = EBADF;

return -1;

}

if (fcntl(oldfd, F_GETFL) == -1) {

return -1;

}

if (oldfd == newfd) {

return oldfd;

}

close(newfd);

return fcntl(oldfd, F_DUPFD, newfd);

}

off_t get_offset (int fd) {

off_t o = lseek(fd, 0, SEEK_CUR);

if (o == -1) { errExit("getting offset"); }

return o;

}

int main (int argc, char *argv[]) {

if (argc != 2) {

usageErr("%s testfile\n", argv[0]);

}

int fd = open(argv[1], O_RDWR | O_CREAT | O_TRUNC, S_IWUSR | S_IRUSR);

if (fd == -1) { errExit("open testfile"); }

assert(my_dup(-1) == -1);

assert(my_dup(42) == -1);

int dup = my_dup(fd);

assert(dup != -1);

assert(dup != fd);

assert(get_offset(fd) == 0);

assert(get_offset(dup) == 0);

char text[] = "abc";

size_t text_len = strlen(text);

if (write(dup, text, text_len) != (ssize_t) text_len) { errExit("writing 'abc'"); }

assert(get_offset(fd) == (off_t) text_len);

assert(get_offset(dup) == (off_t) text_len);

assert(my_dup2(42, 42) == -1);

assert(my_dup2(STDOUT_FILENO, STDOUT_FILENO) == STDOUT_FILENO);

assert(my_dup2(fd, -1) == -1);

assert(my_dup2(fd, OPEN_MAX) == -1);

assert(my_dup2(42, STDOUT_FILENO) == -1);

dup = my_dup2(fd, STDOUT_FILENO);

assert(dup == STDOUT_FILENO);

char greeting[] = "Hello World!";

size_t combined_len = text_len + strlen(greeting);

fprintf(stderr, "this should be the only output of this program\n");

printf(greeting);

fflush(stdout);

assert(get_offset(dup) == combined_len);

assert(get_offset(fd) == combined_len);

exit(EXIT_SUCCESS);

}