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digitalmars.D - Did somebody change the automatic stdout.flush behavior?

reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
In the past, the following program would throw a ConvException *after* 
sending the second message to stdout. Now the second message is not 
displayed at all. Wait! I just checked: The output is actually flushed 
but after the exception output.

import std.stdio;

void main() {
     int a;
     write("Please enter an int: ");
     readf("%s", &a);

     int b;
     write("Please enter another int: ");
     readf("%s", &b);
}

(That is an example to demonstrate the need and usefulness of the space 
character in input format specifiers: Replace "%s" with " %s" and the 
program will work without error.)

I think the current behavior is confusing because the readf error is 
printed *before* stdout is flushed. That's my understanding anyway...

Thank you,
Ali
Feb 16 2022
parent reply Adam Ruppe <destructionator gmail.com> writes:
On Thursday, 17 February 2022 at 00:13:24 UTC, Ali Çehreli wrote:

 In the past, the following program would throw a ConvException 
 *after* sending the second message to stdout. Now the second 
 message is not displayed at all. Wait! I just checked: The 
 output is actually flushed but after the exception output.


the default is to flush on the newline.... (unless it is a pipe, 
which happens with a lot of IDEs, in which case it flushes on a 
buffer size threshold).

since you have no newline here im pretty sure this is the same as 
how it always worked..... i think.
Feb 16 2022
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 2/16/22 4:17 PM, Adam Ruppe wrote:

 On Thursday, 17 February 2022 at 00:13:24 UTC, Ali =C3=87ehreli wrote:

 In the past, the following program would throw a ConvException *after*=




=20

 sending the second message to stdout. Now the second message is not=20
 displayed at all. Wait! I just checked: The output is actually flushed=




=20

 but after the exception output.


=20
 the default is to flush on the newline.... (unless it is a pipe, which =



 happens with a lot of IDEs, in which case it flushes on a buffer size=20
 threshold).
=20
 since you have no newline here im pretty sure this is the same as how i=


t=20

 always worked..... i think.


I know but the other rule is that reading from stdin flushes stdout=20
automatically. At least I've been assuming that there is such a link=20
between those streams. :)

My current assumption is that somebody changed the order of stderr vs.=20
stdout. That's why the output is more confusing to me now. Just a guess..=
=2E

Ali
Feb 16 2022
parent reply =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 2/16/22 16:21, Ali Çehreli wrote:


 the other rule is that reading from stdin flushes stdout
 automatically.


Now I'm in doubt. That's ancient knowledge (belief) for me from C and 
C++ days. I swear there was a feature where you could link (or lock, or 
connect?) an output stream to an input stream and reading from the input 
stream would automatically flush the output stream.

And I've been under the impression that stdin and stdout were such 
streams. But the internet does not agree with me. Was I dreaming? Was 
that a special thing about C++'s cout and cin?

Help! Losing my mind...

Ali

P.S. Regardless, an automatic flush feature would be nice between stdin 
and stdout. But I wouldn't want that unless it "is a tty". Hm... I think 
I see now.
Feb 16 2022
next sibling parent reply Paulo Pinto <pjmlp progtools.org> writes:
On Thursday, 17 February 2022 at 00:36:30 UTC, Ali Çehreli wrote:

 On 2/16/22 16:21, Ali Çehreli wrote:


 the other rule is that reading from stdin flushes stdout
 automatically.


 Now I'm in doubt. That's ancient knowledge (belief) for me from 
 C and C++ days. I swear there was a feature where you could 
 link (or lock, or connect?) an output stream to an input stream 
 and reading from the input stream would automatically flush the 
 output stream.

 And I've been under the impression that stdin and stdout were 
 such streams. But the internet does not agree with me. Was I 
 dreaming? Was that a special thing about C++'s cout and cin?

 Help! Losing my mind...

 Ali

 P.S. Regardless, an automatic flush feature would be nice 
 between stdin and stdout. But I wouldn't want that unless it 
 "is a tty". Hm... I think I see now.


I bet you are mixing it up with connecting FILE streams to 
iostreams.

https://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio
Feb 16 2022
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 2/16/22 23:19, Paulo Pinto wrote:


 I bet you are mixing it up with connecting FILE streams to iostreams.

 https://en.cppreference.com/w/cpp/io/ios_base/sync_with_stdio


Thank you. That must be it.

Regardless whether the automatic flush is standard or not, it is indeed 
the case at least in a terminal. When I run the following program 
without version=input (as commented out below), I see the output in 
waves of 1024 characters. (The first wave appears after about 3 seconds.)

However, when I run it with version=input, I see that the output is 
automatically flushed before the input:

import std.stdio;
import core.thread;

// version = input;
enum inputPeriod = 10;

void main() {
   foreach (i; 1 .. 10_000) {
     Thread.sleep(10.msecs);
     write(' ', i);

     version (input) {
       if (i % inputPeriod == 0) {
         write("Enter a char: ");
         char c;
         readf(" %s", &c);
       }
     }
   }
}

It's perhaps the terminal, which me likes. No big deal...

Ali
Feb 17 2022
prev sibling parent reply Timon Gehr <timon.gehr gmx.ch> writes:
On 17.02.22 01:36, Ali Çehreli wrote:

 Was that a special thing about C++'s cout and cin?
 Help! Losing my mind... 


Afaik it's a more general feature of streams and for cin/cout, the 
following works:

std::cin.tie(nullptr);    // untie cin and cout
std::cin.tie(&std::cout); // tie cin and cout (the default)
Feb 17 2022
parent =?UTF-8?Q?Ali_=c3=87ehreli?= <acehreli yahoo.com> writes:
On 2/17/22 00:02, Timon Gehr wrote:

 On 17.02.22 01:36, Ali Çehreli wrote:

 Was that a special thing about C++'s cout and cin?
 Help! Losing my mind...


 Afaik it's a more general feature of streams and for cin/cout, the
 following works:

 std::cin.tie(nullptr);    // untie cin and cout
 std::cin.tie(&std::cout); // tie cin and cout (the default)


That's the one! I misremembered "tie" as "link".

Ali
Feb 17 2022