Does exec preserve file descriptors

Question

This is actually a two-step question:

1. What exactly is a file descriptor? I thought it was the most fundamental way to represent an open file. But since dup2 can make two different file descriptors point to the same file, then what is it that represents a unique file?

2. If I do dup2 before exec, the whole program is then wiped out, does it still have the same file descriptor table? Do the redirected file descriptors still are redirected?

Answer 1

Yes. Open file descriptors are preserved across a call to exec. From the execve man page (all exec* calls are just a wrapper around this system call):

By default, file descriptors remain open across an execve(). File descriptors that are marked close-on-exec are closed; see the description of FD_CLOEXEC in fcntl(2).

1. Yes, a file descriptor is the way that userspace refers to open files when making system calls. (Or socket, or pipe, etc.) The fact that they can be duplicated doesn't change that. They are simply multiple references to the same open file. See also:

   • Can I check if two FILE* or file descriptor numbers refer to the same file?
   • two file descriptors to same file

2. Yes, as mentioned in the man page quote.

In fact, many programs (including your shell) rely upon this feature. If it wasn't so, your program would start up without the standard in/out/error file descriptors open! When the shell runs a program, it forks, dup2's the open tty file descriptors to 0,1,2, and then execve's your new program.