implements dup and dup2 using fcntl.

dup2.c

// implements dup and dup2 using fcntl.

#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <assert.h>
#include <errno.h>

int mydup(int oldfd);
int mydup2(int oldfd, int newfd);

int mydup(int oldfd) {
    int newfd = fcntl(oldfd, F_DUPFD, oldfd + 1);
    return newfd;
}

int mydup2(int oldfd, int newfd) {
    int fd = fcntl(oldfd, F_GETFL);
    if (fd == -1) {
        errno = EBADF;
        return -1;
    }
    else {
        if (oldfd == newfd) {
            return oldfd;
        }
        else {
            close(newfd);
            return fcntl(oldfd, F_DUPFD, newfd);
        }
    }

}

int main(int argc, char const *argv[]) {
    int newfd1 = mydup(0);
    close(newfd1);
    printf("%d\n", newfd1);
    int newfd2 = dup(0);
    printf("%d\n", newfd2);
    close(newfd2);

    int fd = open("test.txt", O_RDONLY);
    int newfd3 = mydup2(fd, fd);
    printf("%d\n", newfd3);
    int newfd4 = dup2(fd, fd);
    printf("%d\n", newfd4);
    close(newfd3);
    close(newfd4);

    return 0;
}