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(Private file-scope functions and
variables
should
also
be declared
<TT>static</TT>.
See also question <a href="faqcat6b6b.html?sec=struct#opaquetypes">2.4</a>.)
</p><p>Finally,
you should not put actual code
(i.e. function
bodies)
or global variable definitions
(that is, defining
or initializing
instances)
in header files.
Also,
when you are building a project
out of multiple source files,
you should compile each of them separately
(using a compiler option
to request compilation only),
and use the linker
to link the resultant object files together.
(In an integrated development environment,
all of this may be taken care of for you.)
Don't try to ``link'' all of your source files together
with <TT>#include</TT>;
the <TT>#include</TT> directive should be used
to pull in
header files,
not other <TT>.c</TT> files.

</p><p>See also
questions <a href="faqcatd3c2.html?sec=decl#decldef">1.7</a>, <a href="faqcat204f.html?sec=cpp#nestincl">10.7</a>, and <a href="faqcataae2.html?sec=style#srcfiles">17.2</a>.
</p><p>Additional links:
<a href="../../cpp/sd7.html" rel=subdocument>more explanation</a>
</p>



<p>References:

K&amp;R2 Sec. 4.5 pp. 81-2
<br>
H&amp;S Sec. 9.2.3 p. 267
<br>
CT&amp;P Sec. 4.6 pp. 66-7
<hr><hr><hr>
<a name="nestincl">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/nestincl.html"><!-- qtag -->Question 10.7</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
Is it acceptable for one header file to <TT>#include</TT> another?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
It's a question of style,
and thus receives considerable debate.
Many people believe that
``nested <TT>#include</TT> files''
are to be avoided:

the prestigious Indian Hill Style Guide
(see question
<a href="faqcataae2.html?sec=style#guides">17.9</a>)
disparages them;
they can make it harder to find relevant definitions;
they can lead to multiple-definition errors if a file is <TT>#include</TT>d twice;
they can lead to
increased compilation time;
and
they make manual Makefile maintenance very difficult.
On the other hand,
they make it possible to use header files in a modular way (a
header file can <TT>#include</TT> what it needs itself,
rather than


requiring each <TT>#include</TT>r to do so);
a tool like <TT>grep</TT>
(or a tags file)
makes it easy to find definitions no matter where they are;
a popular trick
along the lines of:


<pre>
	#ifndef HFILENAME_USED
	#define HFILENAME_USED
	...header file contents...
	#endif
</pre>
(where a different
bracketing
macro name
is used for each header file)
makes a header file ``idempotent''
so that it can safely be <TT>#include</TT>d multiple times;
a clever compiler can avoid expending any more time
on later instances of
an already-included header;
and automated Makefile maintenance tools
(which are a virtual necessity in large projects anyway;
see question <a href="faqcatccbd.html?sec=resources#tools">18.1</a>)
handle dependency generation in the face of nested <TT>#include</TT> files easily.
See also question <a href="faqcataae2.html?sec=style#stylewars">17.10</a>.
</p>
<p>References:

Rationale Sec. 4.1.2
<hr><hr><hr>
<a name="inclkinds">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/inclkinds.html"><!-- qtag -->Question 10.8a</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What's the difference between
<TT>#include&nbsp;&lt;&gt;</TT>
and
<TT>#include&nbsp;""</TT>
?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The
<TT>&lt;&gt;</TT> syntax is
typically used with
Standard or system-supplied headers,
while <TT>""</TT> is typically used for a program's own header files.
See also question <a href="faqcat204f.html?sec=cpp#cppsearchpath">10.8b</a>.
</p><p>Additional links:
<br>
<br>
<a href="../../cpp/inclk.kaz.html">nice explanation</a>
by Kaz Kylheku
<br>
<br>
An




<a href="../../cpp/incl.970507.html">article</a>
and a longer




<a href="../../cpp/incl.970605.html">followup</a>
I posted in 1997 exploring some of the finer points which
sometimes arise in deciding between <TT>#include&nbsp;""</TT>and <TT>#include&nbsp;&lt;&gt;</TT>.
(Warning:
among the possibilities explored
is that of using <TT>#include&nbsp;&lt;&gt;</TT> for your own header files
under certain circumstances,
a practice which is <em>not</em> standard
and is not condoned by the readership of <TT>comp.lang.c</TT>.)
<hr><hr><hr>
<a name="cppsearchpath">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/cppsearchpath.html"><!-- qtag -->Question 10.8b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
What are the complete rules for header file searching?
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
The exact behavior is implementation-defined
(which means that it is supposed to be documented;
see question <a href="faqcat7d4b.html?sec=ansi#undef">11.33</a>).
Typically, headers
named with <TT>&lt;&gt;</TT>
syntax
are searched for
in one or more standard places.
<a href="../../cpp/bihdr.html" rel=subdocument>[footnote]</a>
Header files named with <TT>""</TT>
syntax
are
first searched for
in the ``current directory,''
then
(if not found)
in the same standard places.
(This last rule,
that
<TT>""</TT> files
are additionally searched
for as if they were <TT>&lt;&gt;</TT> files,
is
the
only rule
specified by the Standard.)
</p><p>Another distinction is the definition of ``current directory''
for <TT>""</TT> files.
Traditionally
(especially under Unix compilers),
the current directory is
taken to be
the directory
containing the file
containing
the <TT>#include</TT> directive.
Under other compilers, however,
the current directory
is the directory in which the compiler was initially invoked.
(Compilers running on systems without directories
or
without
the notion of a current directory
may of course use still different rules.)
</p><p>It is also common for there to be a way
(usually a command line option involving capital I,
or maybe an environment variable)
to
add additional directories
to the list of standard places to
search.
Check your
compiler
documentation.
</p><p>Additional links:
<br>
<br>




<a href="../../cpp/include.dmr.html">further elaboration by Dennis Ritchie</a>
<br>
<br>




A
<a href="../../cpp/incl.970605.html">long article</a>
of mine exploring some of the bore subtle implications of the search rules
</p>


<p>References:

K&amp;R2 Sec. A12.4 p. 231
<br>
ISO Sec. 6.8.2
<br>
H&amp;S Sec. 3.4 p. 55
<hr><hr><hr>
<a name="headerglom">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/headerglom.html"><!-- qtag -->Question 10.9</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm getting strange
syntax errors
on the very first
declaration in
a file,
but it looks fine.

</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
Perhaps there's a missing semicolon at the end of the last
declaration in the last header file you're #including.
See also
questions <a href="faqcat6b6b.html?sec=struct#retcrash">2.18</a>, <a href="faqcat7d4b.html?sec=ansi#preansi">11.29a</a>, <a href="faqcat5e04.html?sec=strangeprob#emptyloop">16.1</a>, and <a href="faqcat5e04.html?sec=strangeprob#unclosed">16.1b</a>.
<hr><hr><hr>
<a name="def3rdparty">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/def3rdparty.html"><!-- qtag -->Question 10.10</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
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><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
This is indeed an annoying situation.
It's a classic namespace problem;
see questions <a href="faqcatd3c2.html?sec=decl#semiglobal">1.9</a> and <a href="faqcatd3c2.html?sec=decl#namespace">1.29</a>.
Ideally,
third-party vendors would be conscientious when defining symbols
(both preprocessor <TT>#define</TT>s and global variable and function names)
to assure that namespace collisions were

unlikely.
The best solution

is
to get the vendor(s)
to fix their header files.
</p><p>As a workaround,



you can
sometimes
undefine or redefine
the
offending
macros
between the conflicting <TT>#include</TT> directives.
<hr><hr><hr>
<a name="extlibs">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/extlibs.html"><!-- qtag -->Question 10.10b</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
I'm #including the
right
header file for
the library function I'm using,
but the linker keeps saying it's undefined.
</p><p><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
See question <a href="faqcat1067.html?sec=lib#extlibs">13.25</a>.
<hr><hr><hr>
<a name="missinghdr">
<h1>
comp.lang.c FAQ list
<font color=blue>&middot;</font>
<a href="../../cpp/missinghdr.html"><!-- qtag -->Question 10.11</a>
</h1>
<p>
<font face=Helvetica size=8 color=blue><b>Q:</b></font>
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><hr>
<p>
<font face=Helvetica size=8 color=blue><b>A:</b></font>
There are several situations, depending on what sort of header
file it is that's ``missing''.
</p><p>If the missing header file is
truly
a standard one
(that is,
one defined by the ANSI C Standard,
such as <TT>&lt;stdio.h&gt;</TT>),
there's a problem with your compiler.
Either
the compiler
wasn't installed properly,
or your project is somehow not configured
to find the standard header files.
You'll need to contact your vendor,
or someone knowledgeable about your particular compiler,
for help.
</p><p>In
the case of nonstandard headers,
the situation is considerably more complicated.
Some
headers
(such as <TT>&lt;dos.h&gt;</TT>)
are completely system- or compiler-specific.
Some
are completely unnecessary,
and should be replaced by
their Standard equivalents.
(For example,
instead of
the nonstandard
<TT>&lt;malloc.h&gt;</TT>
and
<TT>&lt;memory.h&gt;</TT>,
portable
source code should instead <TT>#include</TT>
<TT>&lt;stdlib.h&gt;</TT>
and
<TT>&lt;string.h&gt;</TT>,
respectively.)
Other
headers,
such as those associated with popular add-on libraries,
may be reasonably portable.
</p><p>If the missing header file is an OS-specific one,
such as
<TT>&lt;sgtty.h&gt;</TT>,
<TT>&lt;sys/stat.h&gt;</TT>,
<TT>&lt;netinet/in.h&gt;</TT>,
or
<TT>&lt;dos.h&gt;</TT>,
it may be that the program you're compiling was
tailored to
an operating system other than the one you're using.
(It's also possible that
some conditional compilation settings need to be adjusted.)




It likely won't be possible to get the program working
without rewriting its system-dependent portions.
Simply getting copies of the missing header files wouldn't help--they
typically contain declarations of the interfaces
to the various system calls and libraries.
Obtaining copies of the header files wouldn't get you copies of the libraries,
and the libraries wouldn't be of any use
(that is, they wouldn't work)
without the underlying system calls, anyway.
See also
questions <a href="faqcatea63.html?sec=osdep#cbreak">19.1</a>, <a href="faqcatea63.html?sec=osdep#termcap">19.4</a>, <a href="faqcatea63.html?sec=osdep#modtime">19.12b</a>, and <a href="faqcatea63.html?sec=osdep#int86">19.40c</a>.
</p><p>If the missing header file is for
some external, add-on, third-party
library,
look for the header where you got the library.
If you have some source code
which #includes what appears to be an add-on header,
but
which

you don't have,
you probably don't have the library, either,
and you'll need both.
See question <a href="faqcat1067.html?sec=lib#extlibs">13.25</a>;
see also question <a href="faqcatccbd.html?sec=resources#sources">18.16</a>.
</p><p>If the header file is unique to