[PATCH v2] Ensure standard file descriptors are open on start

[Joseph Myers]

On Wed, 19 Aug 2020, Arsen Arsenović via Libc-alpha wrote:

> ISO C requires that standard input, output and error are always open on
> program startup.

ISO C doesn't talk about file descriptors at all.  The objects stdin, 
stdout and stderr need to be initialized, but it's fine for all I/O on 
them to fail.

> +  /* Ensure the standard streams are opened, as required by POSIX and C. For
> +     dynamic programs this is already handled in the dynamic loader.  */

Please give specific references, not just "as required by POSIX and C".  
What exactly do you think requires these descriptors to be open?  The 
POSIX specification for execve explicitly says "If file descriptor 0, 1, 
or 2 would otherwise be closed after a successful call to one of the exec 
family of functions, implementations may open an unspecified file for the 
file descriptor in the new process image. If a standard utility or a 
conforming application is executed with file descriptor 0 not open for 
reading or with file descriptor 1 or 2 not open for writing, the 
environment in which the utility or application is executed shall be 
deemed non-conforming, and consequently the utility or application might 
not behave as described in this standard.".  So at least that description 
of the process of creating a new process image only permits opening those 
file descriptors but does not require it and indeed explicitly does not 
impose requirements on how code behaves if started with those file 
descriptors not open.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/execve.html

-- 
Joseph S. Myers
joseph@codesourcery.com