function system() from stdlib.h

**Zack** · Feb 15 '07, 10:05 PM

Re: function system() from stdlib.h

On Feb 15, 4:48 pm, "britzkrieg " <eap.blitzkr... @gmail.comwrote :

pause the screen until I push a button.

**santosh** · Feb 15 '07, 10:25 PM

Re: function system() from stdlib.h

britzkrieg wrote:

Hi,
>
I had write a program for Windows OS and put the following line:
>
system("pause")
>
that use the "pause" command from DOS to pause the screen until I push
a button. How to get the same result in Linux's bash ?

Why not use getchar() and discard the return value? It's more portable
than executing OS specific commands.

**Chris Dollin** · Feb 16 '07, 08:55 AM

Re: function system() from stdlib.h

britzkrieg wrote:

Hi,
>
I had write a program for Windows OS and put the following line:
>
system("pause")
>
that use the "pause" command from DOS to pause the screen until I push
a button. How to get the same result in Linux's bash ?

Well, what /I'd/ do is do

program | more

and not bother doing anything in the code. But if you really want
to wait for a character, why not call:

void waitForRETURN(v oid)
{
fprintf( stderr, "press RETURN to continue:" );
fflush( stderr );
while (getchar() != '\n') {}
}

Pickier people might want to handle EOF. Prettier people might
want to parametrise with a reason message. Perfectionist people
will probably see five things wrong I won't notice until after
I post, or after my coffee.

--
Chris "problemati c hedgehog" Dollin
"The path to the web becomes deeper and wider" - October Project

**santosh** · Feb 16 '07, 10:45 AM

Re: function system() from stdlib.h

Chris Dollin wrote:

britzkrieg wrote:
>

Hi,

I had write a program for Windows OS and put the following line:

system("pause")

that use the "pause" command from DOS to pause the screen until I push
a button. How to get the same result in Linux's bash ?

>
Well, what /I'd/ do is do
>
program | more
>
and not bother doing anything in the code. But if you really want
to wait for a character, why not call:
>
void waitForRETURN(v oid)
{
fprintf( stderr, "press RETURN to continue:" );
fflush( stderr );
while (getchar() != '\n') {}
}

Is there a specific reason for doing while( ... ) {} instead of
while( ... ); other than readability? I consider the latter form to be
more readable, but I may be in the minority regarding that.

Pickier people might want to handle EOF. Prettier people might
want to parametrise with a reason message. Perfectionist people
will probably see five things wrong I won't notice until after
I post, or after my coffee.

What if stdin is already at end-of-file, when getchar() is invoked?

**Chris Dollin** · Feb 16 '07, 11:05 AM

Re: function system() from stdlib.h

santosh wrote:

Chris Dollin wrote:

>>
> void waitForRETURN(v oid)
> {
> fprintf( stderr, "press RETURN to continue:" );
> fflush( stderr );
> while (getchar() != '\n') {}
> }

>
Is there a specific reason for doing while( ... ) {} instead of
while( ... ); other than readability? I consider the latter form to be
more readable, but I may be in the minority regarding that.

Readability. I find `;` to be more missable (and more likely to
be an accident) than {}. One's parsecage may vary.

>Pickier people might want to handle EOF. Prettier people might
>want to parametrise with a reason message. Perfectionist people
>will probably see five things wrong I won't notice until after
>I post, or after my coffee.

>
What if stdin is already at end-of-file, when getchar() is invoked?

"Pickier people might want to handle EOF."

--
Chris "electric hedgehog" Dollin
"How am I to understand if you won't teach me?" - Trippa, /Falling/

**=?utf-8?B?SGFyYWxkIHZhbiBExLNr?=** · Feb 16 '07, 11:05 AM

Re: function system() from stdlib.h

santosh wrote:

Chris Dollin wrote:

[...]

void waitForRETURN(v oid)
{
fprintf( stderr, "press RETURN to continue:" );
fflush( stderr );
while (getchar() != '\n') {}
}

>
Is there a specific reason for doing while( ... ) {} instead of
while( ... ); other than readability? I consider the latter form to be
more readable, but I may be in the minority regarding that.

There are no benefits other than readability. I'm in the group that
considers {} more readable where a statement without effect is
required. Better yet to me would be a comment, with either form.

while (getchar() != '\n')
/* do nothing */ ;

while (getchar() != '\n')
{ /* do nothing */ }

Pickier people might want to handle EOF. Prettier people might
want to parametrise with a reason message. Perfectionist people
will probably see five things wrong I won't notice until after
I post, or after my coffee.

>
What if stdin is already at end-of-file, when getchar() is invoked?

Then it may loop forever comparing EOF to '\n' and doing nothing. It's
also possible that EOF is reached during the loop, since input is not
necessarily terminated by a newline character.

**Richard Heathfield** · Feb 16 '07, 11:05 AM

Re: function system() from stdlib.h

santosh said:

>
Chris Dollin wrote:

<snip>

>>
> void waitForRETURN(v oid)
> {
> fprintf( stderr, "press RETURN to continue:" );
> fflush( stderr );
> while (getchar() != '\n') {}
> }

>
Is there a specific reason for doing while( ... ) {} instead of
while( ... ); other than readability? I consider the latter form to be
more readable, but I may be in the minority regarding that.

Obviously both forms are correct; it's a style issue. Personally, I
prefer

while(...)
{
continue;
}

which is even more verbose. But Chris's version is fine and so is yours.
Some compilers, however, will issue a diagnostic message for your
version, not because it is required but because it is helpful to
newbies who plaster semicolons all over their code in sheer
desperation, to not-quite-newbies who aren't completely sure of their
syntax rules, and even to oldbies - we all tismype occasionally.

>Pickier people might want to handle EOF. [...]

>
What if stdin is already at end-of-file, when getchar() is invoked?

That makes you a pickier people (which is good, and it's why Chris
specifically covered his back with all those spiky bits).

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at the above domain, - www.

**santosh** · Feb 16 '07, 11:15 AM

Re: function system() from stdlib.h

Richard Heathfield wrote:

santosh said:

Chris Dollin wrote:

>
<snip>

>
void waitForRETURN(v oid)
{
fprintf( stderr, "press RETURN to continue:" );
fflush( stderr );
while (getchar() != '\n') {}
}

Is there a specific reason for doing while( ... ) {} instead of
while( ... ); other than readability? I consider the latter form to be
more readable, but I may be in the minority regarding that.

>
Obviously both forms are correct; it's a style issue. Personally, I
prefer
>
while(...)
{
continue;
}
>
which is even more verbose. But Chris's version is fine and so is yours.
Some compilers, however, will issue a diagnostic message for your
version, not because it is required but because it is helpful to
newbies who plaster semicolons all over their code in sheer
desperation, to not-quite-newbies who aren't completely sure of their
syntax rules, and even to oldbies - we all tismype occasionally.

Out of curiosity, can you name a compiler that emits this diagnostic?
I haven't encountered it in gcc, even with the -Wall and -Wextra
switches.

**Richard Heathfield** · Feb 16 '07, 11:25 AM

Re: function system() from stdlib.h

santosh said:

<snip>

>
Out of curiosity, can you name a compiler that emits this diagnostic?

[for the while(condition ); construct]

Not off the top of my head, no.

I just tested it with gcc, and it doesn't seem to do this by default, by
which I mean my usual plethora of bizarre gcc switches.

I could test with Borland and Microsoft, but that would involve effort.
:-)

Maybe it was some lint or other (shrug), or maybe I'm just
disremembering. If so, then I am also disremembering that one way to
quell the warning was to add a space: while(condition ) ;

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999

http://www.cpax.org.uk

email: rjh at the above domain, - www.

**Christopher Layne** · Feb 16 '07, 12:25 PM

Re: function system() from stdlib.h

britzkrieg wrote:

Hi,
>
I had write a program for Windows OS and put the following line:
>
system("pause")
>
that use the "pause" command from DOS to pause the screen until I push
a button. How to get the same result in Linux's bash ?
>
thanks a lot

"read"

Literally:

#!/bin/bash
read -p "press enter key when ready"
<whatever else>

Now if it's a modern version of bash and you're commited to being bash
specific:

read -s -n 1 -p "press any key when ready"

Silent echo, 1 char max

**Christopher Layne** · Feb 16 '07, 12:25 PM

Re: function system() from stdlib.h

Christopher Layne wrote:

>that use the "pause" command from DOS to pause the screen until I push
>a button. How to get the same result in Linux's bash ?
>>
>thanks a lot

>
"read"
>
Literally:

Also my answer was jsut in general for bash scripts. IF you're trying to
attain the same result on both DOS/Unix platforms, follow the advice already
presented. Definitely do not use system() for this.

**britzkrieg** · Feb 16 '07, 01:35 PM

Re: function system() from stdlib.h

Christopher Layne wrote:

#!/bin/bash
read -p "press enter key when ready"
<whatever else>
>
Now if it's a modern version of bash and you're commited to being bash
specific:
>
read -s -n 1 -p "press any key when ready"
>
Silent echo, 1 char max

I haven't test the Christopher's solution yet. But the getchar()
solution isn't work; I dont' know what's going on, but my program get
off without I push a button.

**Chris Dollin** · Feb 16 '07, 01:55 PM

Re: function system() from stdlib.h

britzkrieg wrote:

I haven't test the Christopher's solution yet. But the getchar()
solution isn't work; I dont' know what's going on, but my program get
off without I push a button.

Well, we can't see your code, so we can't see what you've done
wrong.

But I /guess/ that you did some input earlier and didn't
eat all of it, so your getchar getted a char without you
having to type any more. Solution: don't leave random input
unread. (I have a small bet with my evil twin that it's your
`scanf` that's the root of the problem.)

--
Chris "fgets!" Dollin
The "good old days" used to be much better.

**CBFalconer** · Feb 16 '07, 04:25 PM

Re: function system() from stdlib.h

Chris Dollin wrote:

santosh wrote:
>

.... snip ...

>>
>Is there a specific reason for doing while( ... ) {} instead of
>while( ... ); other than readability? I consider the latter form
>to be more readable, but I may be in the minority regarding that.

>
Readability. I find `;` to be more missable (and more likely to
be an accident) than {}. One's parsecage may vary.

Try:

while (side_effects) continue;

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>

"A man who is right every time is not likely to do very much."
-- Francis Crick, co-discover of DNA
"There is nothing more amazing than stupidity in action."
-- Thomas Matthews

function system() from stdlib.h

function system() from stdlib.h

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