questions.html

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

(i.e. not <TT>sizeof(char)</TT>).
</p><p><a href="charstring/wchar.html" rel=subdocument>8.10</a>
I'm starting to think about multinational character sets,
and I'm worried about the implications
of making <TT>sizeof(char)</TT> be 2
so that 16-bit character sets can be represented.
</p><hr>
<H4>9. Boolean Expressions and Variables</H4>
<p><a href="bool/booltype.html" rel=subdocument>9.1</a>
What is the right type to use for Boolean values in C?
Is there a standard type?
Should
I use
<TT>#define</TT>s or enums
for the true and false values?
</p><p><a href="bool/bool2.html" rel=subdocument>9.2</a>
Isn't #defining <TT>TRUE</TT> to be 1 dangerous, since any nonzero value
is considered ``true'' in C?
What if a built-in logical or relational operator ``returns''
something other than 1?
</p><p><a href="bool/ifptr.html" rel=subdocument>9.3</a>
Is <TT>if(p)</TT>,
where <TT>p</TT> is a pointer,
a
valid
and portable test?
</p><p><a href="bool/macros.html" rel=subdocument>9.4</a>
Should I use symbolic names like <TT>TRUE</TT>
and <TT>FALSE</TT> for Boolean constants,
or plain 1 and 0?
</p><p><a href="bool/thirdparty.html" rel=subdocument>9.5</a>
A
third-party
header file
I just started using
is defining its own <TT>TRUE</TT>
and <TT>FALSE</TT> values
incompatibly with
the code I've already developed.
What can I do?
</p><hr>
<H4>10. C Preprocessor</H4>
<p><a href="cpp/safemacros.html" rel=subdocument>10.1</a>
I'm trying to define
a few simple little function-like macros
such as
<pre>
	#define square(x) x * x
</pre>
but they're not always working.
</p><p><a href="cpp/slm.html" rel=subdocument>10.2</a>
Here are
some cute preprocessor macros:
<pre>
	#define begin	{
	#define end	}
</pre>
With these,
I can write C code that looks more like Pascal.
What do y'all think?
</p><p><a href="cpp/swapmacro.html" rel=subdocument>10.3</a>
How can I write a generic macro to swap two values?
</p><p><a href="cpp/multistmt.html" rel=subdocument>10.4</a>
What's the best way to write a multi-statement macro?
</p><p><a href="cpp/typedefvsdefine.html" rel=subdocument>10.5</a>
What's the difference between using a <TT>typedef</TT>
or

a <TT>#define</TT>
for a user-defined type?

</p><p><a href="cpp/constvsdefine.html" rel=subdocument>10.5b</a>
What's the difference between
<pre>
	const MAXSIZE = 100;
</pre>
<pre>
and
</pre>
<pre>
	#define MAXSIZE 100
</pre>
</p><p><a href="cpp/hfiles.html" rel=subdocument>10.6</a>
I'm splitting up a program
into multiple source files
for the first time,

and I'm wondering what to put in <TT>.c</TT> files
and what to put in <TT>.h</TT> files.
(What does ``<TT>.h</TT>''
mean,
anyway?)
</p><p><a href="cpp/nestincl.html" rel=subdocument>10.7</a>
Is it acceptable for one header file to <TT>#include</TT> another?
</p><p><a href="cpp/inclkinds.html" rel=subdocument>10.8a</a>
What's the difference between
<TT>#include&nbsp;&lt;&gt;</TT>
and
<TT>#include&nbsp;""</TT>
?
</p><p><a href="cpp/cppsearchpath.html" rel=subdocument>10.8b</a>
What are the complete rules for header file searching?
</p><p><a href="cpp/headerglom.html" rel=subdocument>10.9</a>
I'm getting strange
syntax errors
on the very first
declaration in
a file,
but it looks fine.

</p><p><a href="cpp/def3rdparty.html" rel=subdocument>10.10</a>
I'm using header files
which
accompany two different third-party libraries,
and they are ``helpfully'' defining
common macros such as <TT>TRUE</TT>,
<TT>FALSE</TT>,
<TT>Min()</TT>,
and <TT>Max()</TT>,

but the definitions clash with each other
and with definitions I'd

already established in my own header files.
What can I do?
</p><p><a href="cpp/extlibs.html" rel=subdocument>10.10b</a>
I'm #including the
right
header file for
the library function I'm using,
but the linker keeps saying it's undefined.
</p><p><a href="cpp/missinghdr.html" rel=subdocument>10.11</a>
I'm compiling a program, and
I seem to be missing
one
of the
header files it
requires.
Can someone send me
a copy?
</p><p><a href="cpp/ifstrcmp.html" rel=subdocument>10.12</a>
How can I construct preprocessor <TT>#if</TT> expressions
which compare strings?
</p><p><a href="cpp/ifexpr.html" rel=subdocument>10.13</a>
Does the <TT>sizeof</TT> operator work in preprocessor <TT>#if</TT> directives?
</p><p><a href="cpp/ifddef.html" rel=subdocument>10.14</a>
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><a href="cpp/iftypedef.html" rel=subdocument>10.15</a>
Is there 



anything like
an <TT>#ifdef</TT> for <TT>typedef</TT>s?
</p><p><a href="cpp/ifendian.html" rel=subdocument>10.16</a>
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><a href="cpp/ifdefsyntax.html" rel=subdocument>10.17</a>
I'm getting strange syntax errors inside
lines
I've
<TT>#ifdef</TT>fed out.

</p><p><a href="cpp/unifdef.html" rel=subdocument>10.18</a>
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><a href="cpp/listpredef.html" rel=subdocument>10.19</a>
How can I list all of the predefined identifiers?
</p><p><a href="cpp/oldpaste.html" rel=subdocument>10.20</a>
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><a href="cpp/charize.html" rel=subdocument>10.21</a>
I have an old macro
<pre>
#define&nbsp;CTRL(c)&nbsp;('c'&nbsp;&amp;&nbsp;037)</pre>


that doesn't
seem to
work any more.
</p><p><a href="cpp/macstrexp2.html" rel=subdocument>10.22</a>
Why is the macro
<pre>
	#define TRACE(n) printf("TRACE: %d\n", n)
</pre>
giving me the warning ``macro replacement within a string literal''?
It seems to be expanding
<pre>
	TRACE(count);
</pre>
as
<pre>
	printf("TRACE: %d\count", count);
</pre>
</p><p><a href="cpp/macstrexp1.html" rel=subdocument>10.23</a>
How can I
use
a macro argument
inside
a string
literal
in the macro expansion?
</p><p><a href="cpp/stringize.html" rel=subdocument>10.24</a>
I'm trying to use
the ANSI ``stringizing'' preprocessing operator
`<TT>#</TT>'
to insert the value of a symbolic constant into a message,
but it keeps
stringizing the macro's name rather than its
value.

</p><p><a href="cpp/notgeneral.html" rel=subdocument>10.25</a>
I've got this tricky preprocessing I want to do
and
I can't figure out a way
to do it.
</p><p><a href="cpp/varargs.html" rel=subdocument>10.26</a>
How can I write a macro which takes a variable number of
arguments,
or use the preprocessor to ``turn off'' a function call with a
variable number of arguments?
</p><p><a href="cpp/debugmacs.html" rel=subdocument>10.27</a>
How can I include expansions of the <TT>__FILE__</TT>
and <TT>__LINE__</TT> macros
in a general-purpose debugging macro?
</p><hr>
<H4>11. ANSI/ISO Standard C</H4>
<p><a href="ansi/ansi1.html" rel=subdocument>11.1</a>
What is the ``ANSI C Standard?''
</p><p><a href="ansi/avail.html" rel=subdocument>11.2</a>
How can I get a copy of the Standard?
</p><p><a href="ansi/updates.html" rel=subdocument>11.2b</a>
Where can I get information about updates to the Standard?
</p><p><a href="ansi/argpromos.html" rel=subdocument>11.3</a>
My ANSI compiler
complains about a mismatch when it sees
<pre>
	extern int func(float);

	int func(x)
	float x;
	{ ...
</pre>
</p><p><a href="ansi/mixoldandnew.html" rel=subdocument>11.4</a>
Can you mix old-style and new-style
function syntax?
</p><p><a href="ansi/structinproto.html" rel=subdocument>11.5</a>
Why does the declaration
<pre>
extern int f(struct x *p);</pre>

give me
an obscure
warning message
about
``struct x declared inside parameter list''?
</p><p><a href="ansi/varargproto.html" rel=subdocument>11.6</a>
I had a frustrating problem which turned out to be caused by the line
<pre>
	printf("%d", n);
</pre>
where <TT>n</TT> was actually a <TT>long&nbsp;int</TT>.
I thought that ANSI function prototypes
were supposed to guard against argument type mismatches like this.

</p><p><a href="ansi/varargproto2.html" rel=subdocument>11.7</a>
I heard that you have to <TT>#include</TT> <TT>&lt;stdio.h&gt;</TT>
before calling <TT>printf</TT>.
Why?

</p><p><a href="ansi/constasconst.html" rel=subdocument>11.8</a>
I don't understand why I can't
use <TT>const</TT> values in initializers and array
dimensions,
as in
<pre>
	const int n = 5;
	int a[n];
</pre>
</p><p><a href="ansi/strlitnotconst.html" rel=subdocument>11.8b</a>
If you can't modify string literals,
why aren't they defined as being
arrays of <TT>const</TT> characters?
</p><p><a href="ansi/constptrconst.html" rel=subdocument>11.9</a>
What's the difference between
<TT>const&nbsp;char&nbsp;*p</TT>,
<TT>char&nbsp;const&nbsp;*p</TT>,
and <TT>char&nbsp;*&nbsp;const&nbsp;p</TT>?
</p><p><a href="ansi/constmismatch.html" rel=subdocument>11.10</a>



Why can't I pass a <TT>char&nbsp;**</TT> to a function which expects a
<TT>const&nbsp;char&nbsp;**</TT>?
</p><p><a href="ansi/typedefconst.html" rel=subdocument>11.11</a>
I've got
the declarations
<pre>
	typedef char *charp;
	const charp p;
</pre>
Why is <TT>p</TT> turning out <TT>const</TT>,
instead of the characters pointed to?
</p><p><a href="ansi/constvsdefine2.html" rel=subdocument>11.11b</a>
What's the difference between
<pre>
	const MAXSIZE = 100;
</pre>
<pre>
and
</pre>
<pre>
	#define MAXSIZE 100
</pre>

</p><p><a href="ansi/maindecl.html" rel=subdocument>11.12a</a>
What's the correct declaration of <TT>main()</TT>?
</p><p><a href="ansi/voidmain.html" rel=subdocument>11.12b</a>

Can I declare <TT>main</TT> as <TT>void</TT>,
to shut off these annoying
``main returns no value'' messages?
</p><p><a href="ansi/envp.html" rel=subdocument>11.13</a>
But what about <TT>main</TT>'s third argument,
<TT>envp</TT>?
</p><p><a href="ansi/voidmain3.html" rel=subdocument>11.14a</a>
I believe that declaring <TT>void&nbsp;main()</TT> can't fail,
since I'm calling <TT>exit</TT> instead of returning,
and anyway my operating system ignores a program's exit/return status.
</p><p><a href="ansi/voidmainexamp.html" rel=subdocument>11.14b</a>
So what could go wrong?
Are there
really any
systems where <TT>void&nbsp;main()</TT> doesn't work?
</p><p><a href="ansi/voidmainbooks.html" rel=subdocument>11.15</a>
The book I've been using,
<I>C Programing
for the Compleat Idiot</I>,
always uses <TT>void&nbsp;main()</TT>.
</p><p><a href="ansi/exitvsreturn.html" rel=subdocument>11.16</a>
Is <TT>exit(status)</TT> truly equivalent
to returning the same <TT>status</TT> from <TT>main</TT>?
</p><p><a href="ansi/stringize.html" rel=subdocument>11.17</a>
I'm trying to use
the ANSI ``stringizing'' preprocessing operator
`<TT>#</TT>'
to insert the value of a symbolic constant into a message,
but it keeps
stringizing the macro's name rather than its
value.
</p><p><a href="ansi/macstrexp.html" rel=subdocument>11.18</a>
What does the message
``warning: macro replacement within a string literal''
mean?
</p><p><a href="ansi/ifdefsyntax.html" rel=subdocument>11.19</a>
I'm getting strange syntax errors inside
lines
I've
<TT>#ifdef</TT>fed out.
</p><p><a href="ansi/pragma.html" rel=subdocument>11.20</a>
What are <TT>#pragma</TT>s 
and what are they good for?
</p><p><a href="ansi/pragmaonce.html" rel=subdocument>11.21</a>
What does ``<TT>#pragma&nbsp;once</TT>'' mean?
I found it in some header files.
</p><p><a href="ansi/nonstrings.html" rel=subdocument>11.22</a>
Is <TT>char&nbsp;a[3]&nbsp;=&nbsp;"abc";</TT> legal?
What does it mean?
</p><p><a href="ansi/aryvsadr.html" rel=subdocument>11.23</a>
Since array references decay into pointers,
if <TT>arr</TT> is an array,
what's
the difference between
<TT>arr</TT> and <TT>&amp;arr</TT>?

</p><p><a href="ansi/voidparith.html" rel=subdocument>11.24</a>
Why can't I perform arithmetic on a <TT>void&nbsp;*</TT> pointer?
</p><p><a href="ansi/memmove.html" rel=subdocument>11.25</a>
What's the difference between
<TT>memcpy</TT> and
<TT>memmove</TT>?

</p><p><a href="ansi/malloc0.html" rel=subdocument>11.26</a>
What should <TT>malloc(0)</TT> do?
Return a null pointer or a pointer to 0 bytes?
</p><p><a href="ansi/extidsignif.html" rel=subdocument>11.27</a>
Why does the ANSI Standard
place limits on
the length
and case-significance
of

external identifiers?
</p><p><a href="ansi/noalias.html" rel=subdocument>11.28</a>
What was <TT>noalias</TT> and what ever happened to it?
</p><p><a href="ansi/preansi.html" rel=subdocument>11.29a</a>
My compiler is rejecting the simplest possible test programs,
with all kinds of syntax errors.
It's complaining about the
first line of
<pre>
	main(int argc, char **argv)
	{
		return 0;
	}
</pre>
</p><p><a href="ansi/preansi2.html" rel=subdocument>11.29b</a>
What does the message

``Automatic aggregate intialization is an ANSI feature''
mean?
My compiler is complaining about valid ANSI code.
</p><p><a href="ansi/preansilib.html" rel=subdocument>11.30</a>
Why are some ANSI/ISO Standard library functions showing up as
undefined, even though I've got an ANSI compiler?
</p><p><a href="ansi/cproto.html" rel=subdocument>11.31</a>
Does anyone have a tool for converting old-style C programs to
ANSI C,