faqcat204f.html

this is a mirrored site c-faq. thought might need offline

(that is, part of)
the program you're trying to compile,
your search will obviously begin
at the spot
where you found the program.
(Again, see question <a href="faqcatccbd.html?sec=resources#sources">18.16</a>.)
</p><p>In general, however, asking if someone can
``send you a copy''
of a missing header file is not likely to be productive.
Standard headers exist in part so that definitions appropriate to your
compiler,
operating system,
and
processor
can be supplied.
You cannot
just
pick up a copy of someone else's header file
and expect it to work,
unless that person is using exactly the same environment.
Nonstandard headers--such as
those specific to
a particular operating system
or third-party library--aren't
generally any more portable;
the OS-specific ones are likely to be <em>very</em> specific
to a particular version and release of the OS,
and the third-party ones are likely to be equally tied
to a particular version of the library.
</p><p>The bottom line is that
a random person on the net
is unlikely to be able to send you a working copy
of the header file you
(seem to)
need.
You may actually have a
portability problem

(see section <a href="faqcatea63.html?sec=osdep#index">19</a>),
or a compiler problem
(in which case you might be able to
ask your compiler vendor why the file was not provided,
or to send a replacement copy).
Otherwise
(if the header is third-party or application-specific),
see question <a href="faqcatccbd.html?sec=resources#sources">18.16</a>.
<hr><hr><hr>
<a name="ifstrcmp">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/ifstrcmp.html"><!-- qtag -->Question 10.12</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I construct preprocessor <TT>#if</TT> expressions
which compare strings?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You can't do it directly;
preprocessor <TT>#if</TT> arithmetic uses only

integers.
An alternative is to
<TT>#define</TT>
several
macros with symbolic names and distinct
integer
values,
and implement conditionals

on those:
<pre>
	#define RED	1
	#define BLUE	2
	#define GREEN	3

	#if COLOR == RED
	/* red case */
	#else
	#if COLOR == BLUE
	/* blue case */
	#else
	#if COLOR == GREEN
	/* green case */
	#else
	/* default case */
	#endif
	#endif
	#endif
</pre>
(Standard C
specifies a new <TT>#elif</TT> directive
which makes if/else chains like these a bit cleaner.)
</p><p>See also question <a href="faqcat38c2.html?sec=misc#strswitch">20.17</a>.
</p>



<p>References:

K&amp;R2 Sec. 4.11.3 p. 91
<br>
ISO Sec. 6.8.1
<br>
H&amp;S Sec. 7.11.1 p. 225
<hr><hr><hr>
<a name="ifexpr">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/ifexpr.html"><!-- qtag -->Question 10.13</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Does the <TT>sizeof</TT> operator work in preprocessor <TT>#if</TT> directives?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
No.
Preprocessing happens during an earlier
phase of compilation,
before type names have been parsed.

Instead
of <TT>sizeof</TT>,
consider

using the predefined constants in ANSI's <TT>&lt;limits.h&gt;</TT>,
if applicable,

or
perhaps
a ``configure'' script.
(Better yet,
try to write code which is inherently insensitive to type sizes;
see also
questions <a href="faqcatd3c2.html?sec=decl#inttypes">1.1</a> and <a href="faqcatd3c2.html?sec=decl#int16">1.3</a>.)
</p><p>Additional links:




<a href="../../cpp/ifsizeof.ct.html">nice explanation by Chris Torek</a>
</p>




<p>References:

ISO Sec. 5.1.1.2, Sec. 6.8.1
<br>
H&amp;S Sec. 7.11.1 p. 225
<hr><hr><hr>
<a name="ifddef">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/ifddef.html"><!-- qtag -->Question 10.14</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Can I use an <TT>#ifdef</TT> in a <TT>#define</TT> line,
to define something two different
ways,
like this?
<pre>
	#define a b \
	#ifdef whatever
		c d
	#else
		e f g
	#endif
</pre>
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
No.
You can't
``run the preprocessor on itself,''
so to speak.
What you can do is
use one of two
completely separate
<TT>#define</TT> lines,
depending on
the
<TT>#ifdef</TT> setting:
<pre>
	#ifdef whatever
	#define a b c d
	#else
	#define a b e f g
	#endif
</pre>


</p>


<p>References:

ISO Sec. 6.8.3, Sec. 6.8.3.4
<br>
H&amp;S Sec. 3.2 pp. 40-1
<hr><hr><hr>
<a name="iftypedef">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/iftypedef.html"><!-- qtag -->Question 10.15</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Is there 



anything like
an <TT>#ifdef</TT> for <TT>typedef</TT>s?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Unfortunately, no.
(There can't be,
because types and <TT>typedef</TT>s
haven't been parsed at preprocessing time.)
You may have to keep
sets of preprocessor macros
(e.g. <TT>MY_TYPE_DEFINED</TT>)
recording whether
certain
typedefs have been declared.
</p><p>See
also
questions <a href="faqcatd3c2.html?sec=decl#typedefvsdefine">1.13</a> and <a href="faqcat204f.html?sec=cpp#ifexpr">10.13</a>.
</p>




<p>References:

ISO Sec. 5.1.1.2, Sec. 6.8.1
<br>
H&amp;S Sec. 7.11.1 p. 225
<hr><hr><hr>
<a name="ifendian">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/ifendian.html"><!-- qtag -->Question 10.16</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I use a preprocessor <TT>#if</TT> expression to
tell
whether a machine's byte order
is big-endian or little-endian?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
You probably can't.
The
usual techniques for detecting endianness
involve pointers or arrays of <TT>char</TT>,
or maybe unions,
but
preprocessor
arithmetic uses only long integers,
and


there is no concept of

addressing.
Another tempting possibility is something like




<pre>
	#if 'ABCD' == 0x41424344
</pre>
but this isn't reliable, either.

At any rate,
the integer formats used in preprocessor <TT>#if</TT> expressions 
are not necessarily
the same as
those that will be used at run time.
</p><p>Are you
sure you need to know the machine's endianness
explicitly?
Usually it's better to write code which doesn't care
(see for example the code fragment in question <a href="faqcat1d60.html?sec=stdio#extconform">12.42</a>).
See also question <a href="faqcat38c2.html?sec=misc#endiantest">20.9</a>.
</p>

<p>References:

ISO Sec. 6.8.1
<br>
H&amp;S Sec. 7.11.1 p. 225
<hr><hr><hr>
<a name="ifdefsyntax">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/ifdefsyntax.html"><!-- qtag -->Question 10.17</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm getting strange syntax errors inside
lines
I've
<TT>#ifdef</TT>fed out.
</p>
<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcat7d4b.html?sec=ansi#ifdefsyntax">11.19</a>.
<hr><hr><hr>
<a name="unifdef">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/unifdef.html"><!-- qtag -->Question 10.18</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I inherited some code
which contains far too many <TT>#ifdef</TT>'s for my taste.
How can I preprocess
the
code
to
leave only one conditional compilation set,
without running it through the preprocessor
and expanding all of the <TT>#include</TT>'s and <TT>#define</TT>'s as well?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There are programs
floating around
called <TT>unifdef</TT>,


<TT>rmifdef</TT>,
and


<TT>scpp</TT>
(``selective C preprocessor'')
which do exactly this.
See question
<a href="faqcatccbd.html?sec=resources#sources">18.16</a>.
<hr><hr><hr>
<a name="listpredef">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/listpredef.html"><!-- qtag -->Question 10.19</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I list all of the predefined identifiers?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There's no standard way, although it is a common need.
<TT>gcc</TT> provides a <TT>-dM</TT> option which works with <TT>-E</TT>,
and other compilers may provide something similar.
If the compiler documentation
is unhelpful,

the
most expedient way is probably to extract
printable strings
from the compiler or preprocessor executable
with something like the
Unix

<TT>strings</TT> utility.
Beware that many traditional
system-specific
predefined identifiers
(e.g. ``<TT>unix</TT>'')
are non-Standard
(because they clash with the user's namespace)
and are being removed or renamed.
(In any case,
as a general rule,
it's
considered wise to keep conditional compilation to a minimum.)
</p><p>Additional links:
<a href="../../cpp/sd8.html" rel=subdocument>sources!</a>
<hr><hr><hr>
<a name="oldpaste">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/oldpaste.html"><!-- qtag -->Question 10.20</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I have some old code that tries to construct identifiers with a
macro like
<pre>
#define Paste(a, b) a/**/b</pre>

but it doesn't work any more.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>

It
was an undocumented feature of some early
preprocessor implementations
(notably

Reiser's)
that comments
disappeared entirely and could therefore be used
for token pasting.
ANSI affirms
(as did K&amp;R1)
that comments are replaced with white space,
so they cannot portably be used in a <TT>Paste()</TT> macro.
However, since the need for pasting tokens was

demonstrated and real,
ANSI

introduced a well-defined token-pasting operator, <TT>##</TT>,
which can be used like this:
<pre>
	#define Paste(a, b) a##b
</pre>
</p><p>Here is one other



method you could try for
pasting tokens under
a pre-ANSI compiler:
<pre>
	#define	XPaste(s)	s
	#define	Paste(a, b)	XPaste(a)b
</pre>
See also question
<a href="faqcat7d4b.html?sec=ansi#stringize">11.17</a>.
</p>


<p>References:

ISO Sec. 6.8.3.3
<br>