why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Mark Galeck]

Hello,

a dash script like: 

date > foobar.txt 
date 

is (as an SSCCE) handled like this: 


int fd; 
int saved; 

fd = open64("foobar.txt", O_WRONLY|O_CREAT); 
saved = fcntl(1, F_DUPFD, 10); 
dup2(fd, 1); 
if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 

dup2(saved, 1); 
if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 



This is strange. Why save, dup and dup again to restore, descriptors in the parent, when it would be much simpler to just dup in the child, and not have to save and restore. This is simpler and I checked it works the same: 


int fd; 

if (!fork()) { 
    fd = open64("foobar.txt", O_WRONLY|O_CREAT); 
    dup2(fd, 1); 
    execl("/bin/date", "date", (char *)NULL); 
} 

if (!fork()) { 
    execl("/bin/date", "date", (char *)NULL); 
} 



I am sure there must be a good reason and I am not understanding something deeper. What is it?


Thank you,

Mark

Re: why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Harald van Dijk]

On 31/01/2017 21:00, Mark Galeck wrote:
> Why save, dup and dup again to restore, descriptors in the parent, when it would be much simpler to just dup in the child, and not have to save and restore.

This assumes there's a child in the first place. Depending on the 
command, that might not be the case, think of built-in commands such as 
echo for instance. Code is needed to handle that case, and I suspect 
that if that code is there anyway, then having one place to set up 
redirections is simpler than having two places to set up redirections.

Cheers,
Harald van Dijk

Re: why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Parke]

On Tue, Jan 31, 2017 at 12:00 PM, Mark Galeck <mark_galeck@pacbell.net> wrote:
> This is strange. Why save, dup and dup again to restore, descriptors in the parent, when it would be much simpler to just dup in the child, and not have to save and restore. This is simpler and I checked it works the same:

> I am sure there must be a good reason and I am not understanding something deeper. What is it?

I am not a dash developer, but one reason to make system calls in the
parent is that it is much simpler to handle errors in the parent.

In your example:

> if (!fork()) {
>     fd = open64("foobar.txt", O_WRONLY|O_CREAT);
>     dup2(fd, 1);
>     execl("/bin/date", "date", (char *)NULL);
> }

What happens if open64 fails?  How should the child inform the parent
of this specific error?

-Parke

Re: why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Jilles Tjoelker]

On Tue, Jan 31, 2017 at 06:27:00PM -0800, Parke wrote:
> On Tue, Jan 31, 2017 at 12:00 PM, Mark Galeck <mark_galeck@pacbell.net> wrote:
> > This is strange. Why save, dup and dup again to restore, descriptors
> > in the parent, when it would be much simpler to just dup in the
> > child, and not have to save and restore. This is simpler and I
> > checked it works the same:

> > I am sure there must be a good reason and I am not understanding
> > something deeper. What is it?

> I am not a dash developer, but one reason to make system calls in the
> parent is that it is much simpler to handle errors in the parent.

> In your example:

> > if (!fork()) {
> >     fd = open64("foobar.txt", O_WRONLY|O_CREAT);
> >     dup2(fd, 1);
> >     execl("/bin/date", "date", (char *)NULL);
> > }

> What happens if open64 fails?  How should the child inform the parent
> of this specific error?

In general, you are right, but in the shell's case there is no
difficulty. The error is reported via an error message to stderr and a
non-zero exit status of the command, and the child process can easily do
those things.

-- 
Jilles Tjoelker

Re: why does dash save, dup, and restore redirected descriptor in the parent, rather than redirect in the child?

[Mark Galeck]

Thank you Harald and Parke for your very informative comments, I appreciate!


Mark