preprocessing statements

[Bill Cunningham]

Re: preprocessing statements


"Walter Roberson" <roberson@ibd.n rc-cnrc.gc.cawrote in message
news:g1hn6u$nhj $1@canopus.cc.u manitoba.ca...

In this thread you did not ask about #if vs #ifdef . They do not mean
the same thing. For example,
>
#define FOO 0
#ifdef FOO
/* this section *will* be compiled, because FOO -is- defined. */
#endif
#if FOO
/* this section will *not* be compiled, because FOO's value is 0,
and #if 0 is false */
#endif
#ifdef if
/* this section will *not* be compiled, because the macro if is -not-
defined. */
#endif
#if !if
/* this section *will* be compiled. In a #if line, after all known
macros are expanded, all remaining identifiers have the value 0
substituted, without any consideration as to whether the identifiers
might be C keywords or library functions. With no macro named if,
the line would be equivilent to #if !0 which is #if 1 which is true.
*/
#endif

Priceless.

Bill

[Keith Thompson]

Re: preprocessing statements

"Bill Cunningham" <nospam@nspam.c omwrites:

I have been thinking about hiding headers from my compiler's
preprocessor and allowing others to be shown. I want to use the most used
and add others is necessary. Would this be how it is properly done. I want
to ask ahead of time because what I do might work but it might not be the
"correct" or standard way.
>
#include <stdio.h>
#include <stdlib.h>
#ifdef LINUX
#include <unistd.h>
#include <sys/types>
#include <sys/stats.h>
#include <fctnl.h>
#endif
>
Is the #endif needed at the end of this case ?

One more thing that you didn't ask about (but probably should have):

How do you expect the symbol LINUX to be defined? Since that
identifier must be available for use in programs, an implementation is
not allowed to pre-define it. There may be other symbols that are
predefined (if you're trying to determine whether you're on a Linux
system); which symbols those might be is a question for another forum.

If you're defining it yourself, that's fine, but you didn't show that.

Also, you appear to be trying to write code that will use things
declared in those implementation-defined headers if you're on a Linux
system, but will still compile and work properly otherwise. Such
things are doable, but the nature of the questions you've been asking
here suggest to me that this is a more advanced topic than you're
ready for.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Keith Thompson]

Re: preprocessing statements

"Bill Cunningham" <nospam@nspam.c omwrites:

"santosh" <santosh.k83@gm ail.comwrote in message
news:g1hl26$rje $1@registered.m otzarella.org.. .
>

>>>Seriously though, no preprocessing directives are perfectly topical.
>>>vippstar was talking about the various system specific headers that
>>>you had included in your code. Those _are_ OT and I believe better
>>>addressed in comp.unix.progr ammer. But your actual question can be
>>>answered here and Walter Roberson did so. Also your method to
>>>conditionall y compile the include directives is perfectly fine.
>>>That's how it's done.

[snip]
>
So then is it ok to talk about and use examples of #if and #ifdef and so
on. Or is preprocessing directives OT ?

Preprocessing directives are topical. In my opinion, the person who
complained that your orginal post was off-topic overreacted.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[D. Power]

Re: preprocessing statements

In article <g1hn6u$nhj$1@c anopus.cc.umani toba.ca>,
roberson@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:

<snip>

#ifdef if
/* this section will *not* be compiled, because the macro if is -not-
defined. */
#endif

This seems to be an odd use of a keyword, but it seems to work.

#include <stdio.h>

#define if

int main (void)
{
#ifdef if
puts ("It seems to be ok.");
#else
puts ("No go.");
#endif

return 0;
}

Compiles with gcc with the -ansi -pedantic -W -Wall -Wextra switches
with no complaints; it just doesn't look right.

<snip>
--
D. Power
"I hold it to be the inalienable right of anybody to go to hell in his
own way."
-Robert Frost

[Keith Thompson]

Re: preprocessing statements

"D. Power" <powerd@pcisys. net.invalidwrit es:

In article <g1hn6u$nhj$1@c anopus.cc.umani toba.ca>,
roberson@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:
<snip>

>#ifdef if
> /* this section will *not* be compiled, because the macro if is -not-
> defined. */
>#endif

>
This seems to be an odd use of a keyword, but it seems to work.
>
#include <stdio.h>
>
#define if
>
int main (void)
{
#ifdef if
puts ("It seems to be ok.");
#else
puts ("No go.");
#endif
>
return 0;
}
>
Compiles with gcc with the -ansi -pedantic -W -Wall -Wextra switches
with no complaints; it just doesn't look right.

"if" isn't a keyword as far as the preprocessor is concerned; it's
just another identifier. Though I agree that the use of "if" in this
context is ugly.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Walter Roberson]

Re: preprocessing statements

In article <lnd4n7qx3n.fsf @nuthaus.mib.or g>,
Keith Thompson <kst-u@mib.orgwrote:

>"D. Power" <powerd@pcisys. net.invalidwrit es:

>In article <g1hn6u$nhj$1@c anopus.cc.umani toba.ca>,
> roberson@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:

>>#ifdef if
>> /* this section will *not* be compiled, because the macro if is -not-
>> defined. */
>>#endif

>This seems to be an odd use of a keyword, but it seems to work.

>"if" isn't a keyword as far as the preprocessor is concerned; it's
>just another identifier. Though I agree that the use of "if" in this
>context is ugly.

Keith is correct on both counts -- the preprocesor doesn't know
anything about "keywords" or library functions, or about any of
the types other than the basic arithmetic types of various
sizes (and, I suppose, size_t).

And yes, it is ugly -- its name was chosen specifically to illustrate
the point that *everything* that looks like an identifier (but which
is not a macro), will have 0 substituted in an #if preprocessor statement.

Further example:

#if the + world + is *round* why + do + we + ~ fall + off + while + we + stand + on + our + heads + at + night ? Author : unknown
/* something */
#endif


If I haven't overlooked anything, that should be compilable even if
nothing is #define'd, even though it appears to use the C keywords
if and do and while .
--
"When a scientist is ahead of his times, it is often through
misunderstandin g of current, rather than intuition of future truth.
In science there is never any error so gross that it won't one day,
from some perspective, appear prophetic." -- Jean Rostand

[Bill Cunningham]

Re: preprocessing statements


"Keith Thompson" <kst-u@mib.orgwrote in message
news:lnprr7r0xp .fsf@nuthaus.mi b.org...

"Bill Cunningham" <nospam@nspam.c omwrites:

> I have been thinking about hiding headers from my compiler's
>preprocessor and allowing others to be shown. I want to use the most used
>and add others is necessary. Would this be how it is properly done. I
>want
>to ask ahead of time because what I do might work but it might not be the
>"correct" or standard way.
>>
>#include <stdio.h>
>#include <stdlib.h>
>#ifdef LINUX
>#include <unistd.h>
>#include <sys/types>
>#include <sys/stats.h>
>#include <fctnl.h>
>#endif
>>
> Is the #endif needed at the end of this case ?

>
One more thing that you didn't ask about (but probably should have):
>
How do you expect the symbol LINUX to be defined? Since that
identifier must be available for use in programs, an implementation is
not allowed to pre-define it. There may be other symbols that are
predefined (if you're trying to determine whether you're on a Linux
system); which symbols those might be is a question for another forum.
>
If you're defining it yourself, that's fine, but you didn't show that.
>
Also, you appear to be trying to write code that will use things
declared in those implementation-defined headers if you're on a Linux
system, but will still compile and work properly otherwise. Such
things are doable, but the nature of the questions you've been asking
here suggest to me that this is a more advanced topic than you're
ready for.
>

Perhaps so but the code above didn't work. This code I wrote did. The
question is did I do it correctly.

#include <stdio.h>
#include <stdlib.h>
#ifdef linux
#ifndef linux
#undef linux
#include <unistd.h>
#include <sys/types.h>
#include <sys/stats.h>
#include <fctnl.h>
#endif
#endif

I compiled a simple program asking sizeof to show through printf the
size of a long double. I haven't defined linux to use those sys call headers
yet though so that part of the header is untested. But the directives
blocked out the mentioned OT headers to compile a simple C program. I
must've did something right.

Bill

[Keith Thompson]

Re: preprocessing statements

"Bill Cunningham" <nospam@nspam.c omwrites:

"Keith Thompson" <kst-u@mib.orgwrote in message
news:lnprr7r0xp .fsf@nuthaus.mi b.org...

>"Bill Cunningham" <nospam@nspam.c omwrites:

>> I have been thinking about hiding headers from my compiler's
>>preprocesso r and allowing others to be shown. I want to use the most used
>>and add others is necessary. Would this be how it is properly done. I
>>want
>>to ask ahead of time because what I do might work but it might not be the
>>"correct" or standard way.
>>>
>>#include <stdio.h>
>>#include <stdlib.h>
>>#ifdef LINUX
>>#include <unistd.h>
>>#include <sys/types>
>>#include <sys/stats.h>
>>#include <fctnl.h>
>>#endif
>>>
>> Is the #endif needed at the end of this case ?

>>
>One more thing that you didn't ask about (but probably should have):
>>
>How do you expect the symbol LINUX to be defined? Since that
>identifier must be available for use in programs, an implementation is
>not allowed to pre-define it. There may be other symbols that are
>predefined (if you're trying to determine whether you're on a Linux
>system); which symbols those might be is a question for another forum.
>>
>If you're defining it yourself, that's fine, but you didn't show that.
>>
>Also, you appear to be trying to write code that will use things
>declared in those implementation-defined headers if you're on a Linux
>system, but will still compile and work properly otherwise. Such
>things are doable, but the nature of the questions you've been asking
>here suggest to me that this is a more advanced topic than you're
>ready for.
>>

Perhaps so but the code above didn't work. This code I wrote did. The
question is did I do it correctly.
>
#include <stdio.h>
#include <stdlib.h>
#ifdef linux
#ifndef linux
#undef linux
#include <unistd.h>
#include <sys/types.h>
#include <sys/stats.h>
#include <fctnl.h>
#endif
#endif
>
I compiled a simple program asking sizeof to show through printf the
size of a long double. I haven't defined linux to use those sys call headers
yet though so that part of the header is untested. But the directives
blocked out the mentioned OT headers to compile a simple C program. I
must've did something right.

Yes, it should work; it just doesn't make any sense.

You have a "#ifndef linux" nested within a "#ifdef linux". What is
this supposed to accomplish? What is the purpose of the "#undef
linux"?

Under what circumstances, if any, do you expect the symbol "linux" or
"LINUX" to be defined? Do you intend to define it yourself?

Do you intend to write code that uses things declared in those
headers? Is your program going to *depend* on those features?

Based on what you've told us so far, a much simpler solution would be
to omit the #include directives for the system-specific headers,
leaving just this:

#include <stdio.h>
#include <stdlib.h>

What exactly are you trying to accomplish?

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Bill Cunningham]

Re: preprocessing statements


"Keith Thompson" <kst-u@mib.orgwrote in message
news:ln4p8jqswy .fsf@nuthaus.mi b.org...

Yes, it should work; it just doesn't make any sense.
>
You have a "#ifndef linux" nested within a "#ifdef linux". What is
this supposed to accomplish? What is the purpose of the "#undef
linux"?
>
Under what circumstances, if any, do you expect the symbol "linux" or
"LINUX" to be defined? Do you intend to define it yourself?
>
Do you intend to write code that uses things declared in those
headers? Is your program going to *depend* on those features?

Yes I use them too. But they are OT here. I want to write something like
this.

#include "master.h"
#define linux

Then use those non-standard headers with stdio.h and stdlib.h. Otherwise if
linux is not defined.

#include "master.h"

int main(void) {

C code.

}

[Keith Thompson]

Re: preprocessing statements

"Bill Cunningham" <nospam@nspam.c omwrites:

"Keith Thompson" <kst-u@mib.orgwrote in message
news:ln4p8jqswy .fsf@nuthaus.mi b.org...

>Yes, it should work; it just doesn't make any sense.
>>
>You have a "#ifndef linux" nested within a "#ifdef linux". What is
>this supposed to accomplish? What is the purpose of the "#undef
>linux"?
>>
>Under what circumstances, if any, do you expect the symbol "linux" or
>"LINUX" to be defined? Do you intend to define it yourself?
>>
>Do you intend to write code that uses things declared in those
>headers? Is your program going to *depend* on those features?

>
Yes I use them too. But they are OT here. I want to write something like
this.
>
#include "master.h"
#define linux
>
Then use those non-standard headers with stdio.h and stdlib.h. Otherwise if
linux is not defined.
>
#include "master.h"
>
int main(void) {
>
C code.
>
}

Based on what you've told us so far, I'm unable to figure out what
you're trying to do. I asked you several specific questions; you've
answered almost none of them. Rather than telling us what you're
trying to accomplish, you've shown us some pseudo-code of what I
presume is intended to be a solution.

What problem are you trying to solve?

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Bill Cunningham]

Re: preprocessing statements


"Keith Thompson" <kst-u@mib.orgwrote in message
news:lnwslfp9d2 .fsf@nuthaus.mi b.org...

What problem are you trying to solve?
>

In all compilation files include stdio.h and stdlib. When linux is
defined include several non-standard headers at compile time.

Bill

[D. Power]

Re: preprocessing statements

In article <lnd4n7qx3n.fsf @nuthaus.mib.or g>,
Keith Thompson <kst-u@mib.orgwrote:

"D. Power" <powerd@pcisys. net.invalidwrit es:

In article <g1hn6u$nhj$1@c anopus.cc.umani toba.ca>,
roberson@ibd.nr c-cnrc.gc.ca (Walter Roberson) wrote:
<snip>

#ifdef if
/* this section will *not* be compiled, because the macro if is -not-
defined. */
#endif

This seems to be an odd use of a keyword, but it seems to work.

#include <stdio.h>

#define if

int main (void)
{
#ifdef if
puts ("It seems to be ok.");
#else
puts ("No go.");
#endif

return 0;
}

Compiles with gcc with the -ansi -pedantic -W -Wall -Wextra switches
with no complaints; it just doesn't look right.

>
"if" isn't a keyword as far as the preprocessor is concerned; it's
just another identifier. Though I agree that the use of "if" in this
context is ugly.

I see, time to read more closely on translation phases in the Standard.
Thank you, and Walter Roberson, for the reply.

--
D. Power
"I hold it to be the inalienable right of anybody to go to hell in his
own way."
-Robert Frost

[Keith Thompson]

Re: preprocessing statements

"Bill Cunningham" <nospam@nspam.c omwrites:

"Keith Thompson" <kst-u@mib.orgwrote in message
news:lnwslfp9d2 .fsf@nuthaus.mi b.org...

>What problem are you trying to solve?
>>

In all compilation files include stdio.h and stdlib. When linux is
defined include several non-standard headers at compile time.

No, that's your proposed *solution*. What *problem* are you trying to
solve? How is your program supposed to behave?

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Bill Cunningham]

Re: preprocessing statements

No, that's your proposed *solution*. What *problem* are you trying to

solve? How is your program supposed to behave?

I want one header called master.h that includes all the headers that
have been typed. I am tired of typing them all in one at a time. I wish to
have a header with conditional compilation.

Bill

[vippstar@gmail.com]

Re: preprocessing statements

On May 28, 4:19 am, "Bill Cunningham" <nos...@nspam.c omwrote:

No, that's your proposed *solution*. What *problem* are you trying to
solve? How is your program supposed to behave?

>
I want one header called master.h that includes all the headers that
have been typed. I am tired of typing them all in one at a time.

Use a proper text editor that does this for you.
clc is not a group to ask about text editors.