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</p><p>To make the code fragment above work,
you'd have to use an intermediate <TT>void&nbsp;*</TT> variable:
<pre>
	double *dp;
	void *vp = dp;
	f(&amp;vp);
	dp = vp;
</pre>
The assignments to and from <TT>vp</TT>
give the compiler the opportunity to perform any conversions,
if necessary.
</p><p>Again,

the discussion so far assumes that different pointer types
might have different sizes or representations,
which is rare today,
but not unheard of.
To
appreciate the
problem with <TT>void&nbsp;**</TT> more clearly,
compare the situation to an analogous one involving, say,
types <TT>int</TT> and <TT>double</TT>,
which probably have different sizes
and certainly have different representations.
If we have a function
<pre>
	void incme(double *p)
	{
		*p += 1;
	}
</pre>
then we can do something like
<pre>
	int i = 1;
	double d = i;
	incme(&amp;d);
	i = d;
</pre>
and <TT>i</TT> will be incremented by 1.
(This is analogous to the correct <TT>void&nbsp;**</TT> code 
involving the auxiliary <TT>vp</TT>.)
If, on the other hand,
we were to attempt something like
<pre>
	int i = 1;
	incme((double *)&amp;i);	/* WRONG */
</pre>
(this code is analogous to the fragment in the question),
it would be highly unlikely to work.
<hr><hr><hr>
<a name="refconst">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/refconst.html"><!-- qtag -->Question 4.10</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I have a function
<pre>
	extern int f(int *);
</pre>
which accepts a pointer to an <TT>int</TT>.
How can I pass a constant
by reference?
A call like
<pre>
	f(&amp;5);
</pre>
doesn't seem to work.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
In C99, you can use a ``compound literal'':
<pre>
	f((int[]){5});
</pre>

</p><p>Prior to C99,
you couldn't
do
this
directly;
you had to
declare a temporary variable,
and then pass its address to the function:
<pre>
	int five = 5;
	f(&amp;five);
</pre>
In C, a function that accepts a pointer to a value
(rather than
simply
accepting
the value itself)
probably intends to modify the pointed-to value,
so it may be a bad idea to pass pointers to constants.
<a href="../../ptrs/fn17.html" rel=subdocument>[footnote]</a>
Indeed,
if <TT>f</TT> is in fact declared as accepting an 
<TT>int&nbsp;*</TT>,
a diagnostic is required if you attempt to pass it a 
pointer to a <TT>const&nbsp;int</TT>.
(<TT>f</TT> could be declared as accepting a
<TT>const&nbsp;int&nbsp;*</TT> if it promises not to modify the 
pointed-to value.)
</p><p>See also questions <a href="faqcat6b6b.html?sec=struct#anonstruct">2.10</a>,
<a href="faqcatabdc.html?sec=ptrs#passptrinit">4.8</a>,
and
<a href="faqcat38c2.html?sec=misc#multretval">20.1</a>.
<hr><hr><hr>
<a name="passbyref">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/passbyref.html"><!-- qtag -->Question 4.11</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Does C even have ``pass by reference''?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Not really.
</p><p>Strictly speaking, C always uses pass by value.
You can simulate pass by reference yourself,
by defining functions which accept pointers
and then using the <TT>&amp;</TT> operator when calling,
and

the
compiler will
essentially
simulate it

for you when you pass an array to a function
(by passing a pointer instead,
see question <a href="faqcatca65.html?sec=aryptr#aryptrparam">6.4</a> et al.).
</p><p>Another way of looking at it is that




if an parameter has type, say, <TT>int&nbsp;*</TT>
then an integer is being passed by reference
and a pointer to an integer is being passed by value.
</p><p>Fundamentally,
C has nothing truly

equivalent to formal pass by reference
or C++ reference parameters.
(On the other hand,



function-like preprocessor macros
can
provide a form of ``pass by name''.)
</p><p>See also
questions <a href="faqcatabdc.html?sec=ptrs#passptrinit">4.8</a>, <a href="faqcatbafd.html?sec=malloc#mymallocretp">7.9</a>, <a href="faqcat1d60.html?sec=stdio#fopenfp">12.27</a>, and <a href="faqcat38c2.html?sec=misc#multretval">20.1</a>.
</p><p>Additional links:




A
<a href="faqcatabdc.html?sec=ptrs#passbyref2">message of mine</a>
further explaining how a function can modify a caller's passed array.
</p>







<p>References:

K&amp;R1 Sec. 1.8 pp. 24-5, Sec. 5.2 pp. 91-3
<br>
K&amp;R2 Sec. 1.8 pp. 27-8, Sec. 5.2 pp. 95-7
<br>
ISO Sec. 6.3.2.2
<br>
H&amp;S Sec. 9.5 pp. 273-4
<hr><hr><hr>
<a name="funccall">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/funccall.html"><!-- qtag -->Question 4.12</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I've seen different syntax used for calling functions via pointers.
What's the story?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Originally, a pointer to a function had to be
``turned into'' a ``real''

function,
with the <TT>*</TT> operator,
before calling:
<pre>
	int r, (*fp)(), func();
	fp = func;
	r = (*fp)();
</pre>
The interpretation of the

last line is clear:
<TT>fp</TT> is a pointer to function,
so <TT>*fp</TT> is the function;
append an argument list in parentheses
(and extra parentheses around <TT>*fp</TT> to get the precedence right),
and you've got a function call.
</p><p>It can also be argued
that functions are always called via pointers,
and
that ``real'' function
names

always
decay implicitly into pointers
(in expressions, as they do in initializations;
see question <a href="faqcatd3c2.html?sec=decl#ptrfuncinit">1.34</a>).
This reasoning
means that
<pre>
	r = fp();
</pre>
is legal and works correctly,
whether <TT>fp</TT> is
the name of
a function or a pointer to one.
(The usage has always been unambiguous;
there is nothing you ever could have done with a function pointer
followed by an argument list
except call
the function pointed to.)
</p><p>The ANSI C Standard essentially adopts the latter interpretation,
meaning that the
explicit <TT>*</TT> is
not required,
though it is
still allowed.
</p><p>See also question <a href="faqcatd3c2.html?sec=decl#ptrfuncinit">1.34</a>.
</p>





<p>References:

K&amp;R1 Sec. 5.12 p. 116
<br>
K&amp;R2 Sec. 5.11 p. 120
<br>
ISO Sec. 6.3.2.2
<br>
Rationale Sec. 3.3.2.2
<br>
H&amp;S Sec. 5.8 p. 147, Sec. 7.4.3 p. 190
<hr><hr><hr>
<a name="generic">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/generic.html"><!-- qtag -->Question 4.13</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's the total generic pointer type?
My compiler complained
when I tried to stuff function pointers into a <TT>void&nbsp;*</TT>.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There
is no
``total generic pointer type.''
</p><p><TT>void&nbsp;*</TT>'s are only guaranteed to hold object
(i.e. data)
pointers;
it is not portable to convert a function pointer to
type
<TT>void&nbsp;*</TT>.
(On some machines, function addresses
can be very large,
bigger

than any

data pointers.)
</p><p>It is guaranteed,
however,
that all function pointers
can be interconverted,
as long as they are converted back
to an appropriate type 
before calling.
Therefore,
you can pick any function type
(usually <TT>int&nbsp;(*)()</TT>
or <TT>void&nbsp;(*)()</TT>,
that is,
pointer to function
of unspecified arguments
returning <TT>int</TT> or <TT>void</TT>)
as a generic function pointer.
When you need
a place
to hold object and function pointers 
interchangeably,
the portable solution is to use a union of a
<TT>void&nbsp;*</TT>
and a
generic function pointer
(of whichever type you choose).
</p><p>See also questions <a href="faqcatd3c2.html?sec=decl#recurfuncp">1.22</a>
and <a href="faqcat1f1a.html?sec=null#fcnptr">5.8</a>.
</p>




<p>References:

ISO Sec. 6.1.2.5, Sec. 6.2.2.3, Sec. 6.3.4
<br>
Rationale Sec. 3.2.2.3
<br>
H&amp;S Sec. 5.3.3 p. 123
<hr><hr><hr>
<a name="int2ptr">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/int2ptr.html"><!-- qtag -->Question 4.14</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How are integers converted to and from pointers?
Can I temporarily stuff an integer into a pointer,
or vice versa?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Once upon a time,
it was guaranteed that a pointer could be converted to an integer
(though one never knew
whether an <TT>int</TT> or a <TT>long</TT> might be required),
and that an integer could be converted to a pointer,
and that a pointer remained unchanged when converted to a 
(large enough)
integer and back again,
and

that the conversions
(and any

mapping)
were intended to be
``unsurprising to those who know the addressing structure 
of the machine.''
In other words,
there is
some
precedent and
support for
integer/pointer
conversions,
but
they
have always been machine dependent,
and hence nonportable.
Explicit casts have always been required
(though early compilers rarely complained if you left them out).
</p><p>The ANSI/ISO C Standard,
in order to ensure that C is widely implementable,
has
weakened
those earlier guarantees.
Pointer-to-integer and integer-to-pointer conversions
are implementation-defined
(see question <a href="faqcat7d4b.html?sec=ansi#undef">11.33</a>),
and
there
is no longer any guarantee that pointers can be converted 
to integers and back, without change.
</p><p>Forcing
pointers into integers,
or integers into pointers,
has never been good practice.
When you need a generic slot that can hold either
kind of data,
a union is a much better

idea.
</p><p>See also questions
<a href="faqcatabdc.html?sec=ptrs#int2charp">4.15</a>,
<a href="faqcat1f1a.html?sec=null#runtime0">5.18</a>,
and <a href="faqcatea63.html?sec=osdep#rawmemadr">19.25</a>.
</p>





<p>References:

K&amp;R1 Sec. A14.4 p. 210
<br>
K&amp;R2 Sec. A6.6 p. 199
<br>
ISO Sec. 6.3.4
<br>
Rationale Sec. 3.3.4
<br>
H&amp;S Sec. 6.2.3 p. 170, Sec. 6.2.7 pp. 171-2
<hr><hr><hr>
<a name="int2charp">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/int2charp.html"><!-- qtag -->Question 4.15</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
How do I convert an <TT>int</TT> to a <TT>char&nbsp;*</TT>?
I tried a cast, but it's not working.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It depends
on what you're trying to do.
If you tried a cast
but it's not working,
you're probably trying to convert an integer to a string,
in which case
see question <a href="faqcat1067.html?sec=lib#itoa">13.1</a>.
If you're trying to convert an integer to a character,
see question <a href="faqcate107.html?sec=charstring#asciivals">8.6</a>.
If you're trying to set a pointer to point
to
a particular memory
address,
see question <a href="faqcatea63.html?sec=osdep#rawmemadr">19.25</a>.
<hr><hr><hr>
<a name="charstarws3">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/charstarws3.html"><!-- qtag -->Question 4.16</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's wrong with
this
declaration?
<pre>
char* p1, p2;</pre>




I get errors when I try to
use
<TT>p2</TT>.

<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcatd3c2.html?sec=decl#charstarws">1.5</a>.
<hr><hr><hr>
<a name="nearfar2">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../ptrs/nearfar2.html"><!-- qtag -->Question 4.17</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What are ``near'' and ``far'' pointers?

<p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcatea63.html?sec=osdep#nearfar">19.40d</a>.
<hr><hr><hr>
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