Bad file descriptor when running cat command in a shell

Question

I'm trying to implement I/O redirection in a shell that I'm writing for a Unix-like OS (xV6). In the manual that I'm reading for the OS, I found the following code that would run in the shell for the execution of the cat command:

char *argv[2];
argv[0] = "cat";
argv[1] = 0;
if(fork() == 0) {
    close(0);
    open("input.txt", O_RDONLY);
    exec("cat", argv);
}

I modified the code to run in my shell which has the argv array located in another function, but it maintains the functionality. For some reason, when I run the cat < input.txt the shell outputs:

cat: -: Bad file descriptor
cat: closing standard input: Bad file descriptor

I'm still new to OS programming so I'm not exactly clear on all of the functionality of I/O redirection, but I think the code I have should work. What could be causing the problem. I have the code for the I/O redirection below:

case '<':
    ecmd = (struct execcmd*)cmd;
    rcmd = (struct redircmd*)cmd;

    if(fork() == 0){
      close(0);
      open("input", O_RDONLY);
      execvp(ecmd->argv[0], ecmd->argv );
    }
    runcmd(rcmd->cmd);
    break;

EDIT

I did strace -e open ls and got:

open("/etc/ld.so.cache",    O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libselinux.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libacl.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libpcre.so.3", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libattr.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/proc/filesystems", O_RDONLY)     = 3
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
a.out  sh.c  test
+++ exited with 0 +++

EDIT 2

For some reason, this code for the case works, but I'm not sure why:

case '<':
    rcmd = (struct redircmd*)cmd;
    close(rcmd->fd);
    if(open(rcmd->file, rcmd->mode) < 0){
      printf(2, "Cannot open file %s\n", rcmd->file);
      perror(rcmd->file);
      exit(1);
    }
    runcmd(rcmd->cmd);
    break;

Answer 1

You can't just close stdin and then open another file. It doesn't automatically become your new stdin. What you want is to use the syscall dup2 which can "redirect" a file descriptor to another.

int fd = open("input.txt", O_RDONLY);
dup2(fd, 0); // stdin now points to fd
close(fd);

For more info see man 2 dup2. Note that open and dup2 can both fail, so you should check their return values if this is a concern.

EDIT: This will actually work sometimes because POSIX guarantees the kernel will always allocate the lowest free file-descriptor. But it is not thread-safe and is just bad style in general. I recommend to always do the dup2 even though in some cases you could get away without it.