faqcat38c2.html

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

<font color=blue>&middot;</font>
<a href="../../misc/circshift.html"><!-- qtag -->Question 20.21a</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Does C have circular shift operators?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
No.
(Part of the reason why is that the sizes of C's types aren't
precisely defined--see question <a href="faqcatd3c2.html?sec=decl#exactsizes">1.2</a>--but
a circular shift makes most sense when applied to a word of a
particular known size.)
</p><p>You can implement a circular shift
using two regular shifts and a bitwise OR:
<pre>
	(x &lt;&lt; 13) | (x &gt;&gt; 3)	/* circular shift left 13 in 16 bits */
</pre>
<hr><hr><hr>
<a name="quintplus">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/quintplus.html"><!-- qtag -->Question 20.21b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Is C a great language, or what?
Where else could you write something like
<TT>a+++++b</TT>
?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Well, you can't
meaningfully
write it in C, either.
The rule for lexical analysis
is that
at each point during a straightforward left-to-right scan,
the longest possible token is determined,
without regard to whether the
resulting sequence of tokens makes sense.
The fragment in the question is therefore
interpreted as
<pre>
a ++ ++ + b</pre>

and
cannot be parsed as a valid expression.
</p>



<p>References:

K&amp;R1 Sec. A2 p. 179
<br>
K&amp;R2 Sec. A2.1 p. 192
<br>
ISO Sec. 6.1
<br>
H&amp;S Sec. 2.3 pp. 19-20
<hr><hr><hr>
<a name="colonequal">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/colonequal.html"><!-- qtag -->Question 20.22</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
If the assignment operator were <TT>:=</TT>,
wouldn't it then be harder to accidentally write things like
<TT>if(a&nbsp;=&nbsp;b)</TT> ?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Yes, but it would also be
just a
little bit
more cumbersome

to type all of the assignment statements
which a typical program contains.
</p><p>In any case,
it's really too late to be worrying about this sort of thing now.
The choices of <TT>=</TT> for assignment and <TT>==</TT> for comparison
were made,
rightly or wrongly,
over two decades ago,
and are not likely to be changed.
(With respect to the question,
many compilers and versions of <TT>lint</TT>
will warn about <TT>if(a&nbsp;=&nbsp;b)</TT>
and similar expressions;
see also question <a href="faqcataae2.html?sec=style#revtest">17.4</a>.)
</p><p>As a point of historical interest,
the choices were made based on the observation that assignment
is more frequent than comparison,
and so deserves fewer keystrokes.
In fact,
using <TT>=</TT> for assignment in



C and its predecessor B
represented a change from
B's own predecessor
BCPL,
which did use <TT>:=</TT>
as its assignment operator.

(See also question <a href="faqcat38c2.html?sec=misc#nameofc">20.38</a>).
<hr><hr><hr>
<a name="with">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/with.html"><!-- qtag -->Question 20.23</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Does C have an equivalent to Pascal's
<TT>with</TT> statement?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
No.
The way
in C
to get quick and easy access to the fields of a structure
is to declare a little local structure pointer variable
(which,
it must be admitted,
is not quite as notationally convenient
as a <TT>with</TT> statement
and doesn't save quite as many
keystrokes,
though it is probably safer).
That is,
if you have something unwieldy like
<pre>
	structarray[complex_expression].a =
		structarray[complex_expression].b +
			structarray[complex_expression].c;
</pre>
you can replace it with
<pre>
	struct whatever *p = &amp;structarray[complex_expression];
	p-&gt;a = p-&gt;b + p-&gt;c;
</pre>
<hr><hr><hr>
<a name="nestfcns">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/nestfcns.html"><!-- qtag -->Question 20.24</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Why doesn't C have nested functions?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's not trivial to implement nested functions
such that they have the proper access to local variables
in the containing function(s),
so
they
were deliberately left out of C as a simplification.



(<TT>gcc</TT> does allow them, as an extension.)
For many potential uses of nested functions
(e.g. <TT>qsort</TT>
comparison
functions),
an adequate if slightly
cumbersome
solution
is to use an adjacent

function with <TT>static</TT> declaration,
communicating if necessary
via a few <TT>static</TT>
variables.

(A
cleaner solution,
though unsupported by <TT>qsort</TT>,
is to pass around
a pointer to
a structure containing the necessary context.)

<hr><hr><hr>
<a name="assert">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/assert.html"><!-- qtag -->Question 20.24b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What is <TT>assert()</TT>
and when would I use it?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It is a macro,
defined in <TT>&lt;assert.h&gt;</TT>,
for testing ``assertions''.
An assertion essentially 
documents an assumption
being made by the programmer,
an assumption which, if violated,
would indicate a serious programming error.
For example, a function which was supposed to be called
with a non-null pointer could write
<pre>
	assert(p != NULL);
</pre>
A failed assertion terminates the program.
Assertions should <em>not</em> be used to catch expected errors,
such as <TT>malloc</TT> or <TT>fopen</TT> failures.
</p>


<p>References:

K&amp;R2 Sec. B6 pp. 253-4
<br>
ISO Sec. 7.2
<br>
H&amp;S Sec. 19.1 p. 406
<hr><hr><hr>
<a name="mixlang">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/mixlang.html"><!-- qtag -->Question 20.25</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How can I call FORTRAN
(C++, BASIC, Pascal,
Ada, LISP) functions from C?
(And vice versa?)
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The answer is entirely dependent

on the machine and the specific
calling sequences of the various compilers in
use, and may not be possible at all.
Read your compiler documentation very carefully;
sometimes there is a
``mixed-language programming guide,''
although the techniques for passing arguments
and ensuring correct run-time startup
are often arcane.
Besides arranging calling sequences correctly,
you may also have to conspire between the various languages
to get aggregate data structures declared compatibly.
</p><p>For FORTRAN,


more information may be found in


FORT.gz
by Glenn Geers,
available via anonymous ftp from suphys.physics.su.oz.au in
the src directory.



Burkhard Burow's header file
<TT>cfortran.h</TT>
simplifies C/FORTRAN interfacing on
many popular machines.
It is available via anonymous ftp from zebra.desy.de




or at
<a href="http://www-zeus.desy.de/~burow"><TT>http://www-zeus.desy.de/~burow</TT></a>.
</p><p>In C++, a <TT>"C"</TT> modifier
in an external function declaration
indicates that the function
is to be called
using C calling conventions.
</p><p>In Ada, 
you can use
the
<TT>Export</TT>
and
<TT>Convention</TT>
pragmas,
and
types from the package <TT>Interfaces.C</TT>,
to arrange for C-compatible calls, parameters, and data structures.
</p>
<p>References:

H&amp;S Sec. 4.9.8 pp. 106-7
<hr><hr><hr>
<a name="langtran">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/langtran.html"><!-- qtag -->Question 20.26</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Does anyone know of a program
for converting
Pascal or FORTRAN
(or
LISP,
Ada,
awk,
``Old'' C,
...)
to C?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Several freely distributable programs are
available:
<UL><li>p2c
A Pascal to C converter
written by Dave Gillespie,
posted to comp.sources.unix in
March, 1990 (Volume 21);
also available by anonymous ftp from
<a href="ftp://csvax.cs.caltech.edu/"><TT>csvax.cs.caltech.edu</TT></a>,
file
<a href="ftp://csvax.cs.caltech.edu/pub/p2c-1.20.tar.Z"><TT>pub/p2c-1.20.tar.Z</TT></a>&nbsp;.

<li>ptoc
Another
Pascal to C converter,
this one written in Pascal
(comp.sources.unix, Volume 10, also patches in Volume 13?).
<li>f2c
A FORTRAN to C converter
jointly developed by people from Bell Labs, Bellcore, and Carnegie Mellon.
(f2c is really a FORTRAN compiler with a C backend--the
output is not intended to be maintainable.)
To find out more about f2c, send the mail message
``send index from f2c''
to [e-mail] or research!netlib.
(It is also available via anonymous ftp on
<a href="ftp://netlib.att.com/"><TT>netlib.att.com</TT></a>,
in directory
<a href="ftp://netlib.att.com/netlib/f2c/"><TT>netlib/f2c/</TT></a>.)

</UL><UL><li>A PL/M to C converter was posted to alt.sources in April, 1991.
</UL></p><p>The following companies sell various translation tools and services:
</p><UL><li><pre>

Cobalt Blue
2940 Union Ave., Suite C
San Jose, CA  95124  USA
([phone]
</pre>
<li><pre>

Promula Development Corp.
3620 N. High St., Suite 301
Columbus, OH  43214  USA
([phone]
</pre>
<li><pre>

Micro-Processor Services Inc.
92 Stone Hurst Lane
Dix Hills, NY  11746  USA
([phone]
</pre>
</UL><p>See also questions
<a href="faqcat7d4b.html?sec=ansi#cproto">11.31</a>
and
<a href="faqcatccbd.html?sec=resources#sources">18.16</a>.
<hr><hr><hr>
<a name="cplusplus">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../misc/cplusplus.html"><!-- qtag -->Question 20.27</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Is C++ a superset of C?
What are the differences between C and C++?
Can I use a C++ compiler to compile C code?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
C++ was derived from C,
and is largely based on it,
but there are some legal C constructs
which are not legal C++.
Conversely,
ANSI C inherited several features from C++,
including prototypes and <TT>const</TT>,
so neither language is really a subset or superset of the other;
the two also define the meaning of some common constructs differently.

</p><p>The most important feature of C++ not found in C is of course 
the extended structure
known as a <TT>class</TT>

which
along with operator overloading
makes object-oriented programming convenient.
There are
several
other differences and new features:
variables may be declared anywhere in a block;
<TT>const</TT> variables
may be true compile-time
constants;
structure tags are implicitly typedeffed;

an <TT>&amp;</TT> in a parameter declaration requests pass by reference;
and
the <TT>new</TT> and <TT>delete</TT> operators,
along with per-object constructors and destructors,
simplify dynamic data structure management.
There are
a host of
mechanisms tied up with 
classes and object-oriented programming:
inheritance,
<TT>friend</TT>s,
virtual functions,
templates,
etc.
(This list of C++ features is not intended to be complete;
C++ programmers will notice many omissions.)
</p><p>Some
features of C which keep it from being a strict subset of C++
(that is, which keep C programs from necessarily being 
acceptable to C++ compilers)
are that
<TT>main</TT>
may be called recursively,
character constants are of type <TT>int</TT>,
prototypes are not required,
and
<TT>void&nbsp;*</TT>
implicitly converts to other pointer types.

Also, every keyword in C++ which is not a keyword in C
is available in C as an identifier;
C programs which use words like <TT>class</TT> and <TT>friend</TT>
as ordinary identifiers will be rejected by C++ compilers.
</p><p>In spite of the differences,
many
C programs will
compile correctly in a C++ environment,
and many recent compilers
offer both C and C++ compilation modes.
(But it's usually a bad idea to compile straight C code as if it were C++;
the languages are different enough
that you'll generally get poor results.)
</p><p>See also
questions <a href="faqcate107.html?sec=charstring#sizeofchar">8.9</a> and <a href="faqcat38c2.html?sec=misc#nestcomment">20.20</a>.

</p><p>Additional links:
<br>
<br>

<a href="http://www.research.att.com/~bs/bs_faq.html#C-is-subset">Bjarne Stroustrup's answer</a>
on the subset/superset question
<br>
<br>
an




<a href="../../misc/cplusplus.rs.html">article by Richard Stamp</a>
listing some differences
<br>
<br>
an




<a href="../../misc/cplusplus.nr.html">article by ``Noone Really