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Code Editor : rtl.define.library.htm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<HTML
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>What is a C library?</TITLE
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><H1
CLASS="SECTION"
><A
NAME="RTL.DEFINE.LIBRARY"
>What is a C library?</A
></H1
><P
>A C library is a set of named functions, for example <TT
CLASS="LITERAL"
>dbinit()</TT
> or <TT
CLASS="LITERAL"
>SQLConnect()</TT
>.  Or, for that matter, <TT
CLASS="LITERAL"
>fopen(3)</TT
><A
NAME="AEN6435"
HREF="#FTN.AEN6435"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>.  </P
><P
>Libraries come in two flavors: <I
CLASS="FIRSTTERM"
>static</I
> and <I
CLASS="FIRSTTERM"
>dynamic</I
>.  </P
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="RTL.DEFINE.LIBRARY.STATIC"
>Static libraries</A
></H2
><P
>Static libraries (also known as <I
CLASS="FIRSTTERM"
>archives</I
>) have been around as long as C itself.  Like a <TT
CLASS="LITERAL"
>.zip</TT
> file, they're just a bag of object files &mdash; containing functions, of course &mdash; with a table of contents in front giving the address of each name<A
NAME="AEN6446"
HREF="#FTN.AEN6446"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
>.  Static libraries are created from object files using a <I
CLASS="FIRSTTERM"
>librarian</I
> utility of some kind.  One such programs is <B
CLASS="COMMAND"
>ar</B
>, for <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>archive</I
></SPAN
>.  </P
><P
>Static libraries are part of the build environment.  Functions in static libraries are joined to a program's main module by a <I
CLASS="FIRSTTERM"
>static linker</I
> at build time to produce an executable program.  The executable incorporates the libraries' object code into its own body, making it completely self-sufficient.  </P
></DIV
><DIV
CLASS="SECTION"
><H2
CLASS="SECTION"
><A
NAME="RTL.DEFINE.LIBRARY.DYNAMIC"
>Dynamic libraries</A
></H2
><P
>Dynamic libraries are the new kid on the block, as these things go, arriving on the Unix scene circa 1985.   Like a static library, a dynamic library is a collection of functions with a table of contents.  They are referenced at build time to give the executatble information about how they will eventually be used, but they aren't <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>used</I
></SPAN
> until run time.  </P
><P
>Dynamic libraries are part of the run-time environment.  When a program is run, the run-time linker finds the dynamic libraries needed by the program, finds the addresses of the required functions, and assembles a runable image in memory.  Missing libraries and/or missing functions &mdash; or the wrong versions of them &mdash; can lead to head-scratching and other amusing behavior.  </P
><P
>In Windows&reg; dynamic libraries are called <I
CLASS="FIRSTTERM"
>dynamic link libraries</I
> (DLLs).  In Unix they're normally called <I
CLASS="FIRSTTERM"
>shared objects</I
>.  But they're roughly the same thing.    </P
><P
><DIV
CLASS="NOTE"
><P
></P
><TABLE
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WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="../images/note.gif"
HSPACE="5"
ALT="Note"></TD
><TH
ALIGN="LEFT"
VALIGN="MIDDLE"
><B
>What about <TT
CLASS="LITERAL"
>.h</TT
> files?</B
></TH
></TR
><TR
><TD
>&nbsp;</TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>C header files include <I
CLASS="FIRSTTERM"
>functional prototypes</I
>, declarations (not <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>definitions)</I
></SPAN
> of functions.  Functional prototypes describe to the compiler each function's parameters, allowing the compiler to confirm that the function is being called correctly.  </P
><P
>Most of the functions declared in header files are implemented in libraries.  However, there's <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>no mechanical or automatic relationship</I
></SPAN
> between the functional prototypes in the header files and their implementation in a library.  The <TT
CLASS="LITERAL"
>.h</TT
> file is maintained by hand, by the programmer, and is used to generate a library.  The header file and associated library are distributed and installed together (one hopes), but correct installation and subsequent use by the compiler &amp; linker require human beings to keep track of the pair.  Failure to do so leads to <SPAN
CLASS="QUOTE"
>"interesting"</SPAN
> development and even run-time problems, especially with libraries whose functions' parameters change from version to version. </P
><P
>For example, imagine a function <TT
CLASS="LITERAL"
>f(int g)</TT
> defined in library <TT
CLASS="FILENAME"
>libf.so</TT
> and declared in <TT
CLASS="FILENAME"
>f.h</TT
>.  In a later version of <TT
CLASS="FILENAME"
>libf.so</TT
>, the function's parameter is changed to use a pointer, <TT
CLASS="LITERAL"
>f(int *p)</TT
>, and <TT
CLASS="FILENAME"
>f.h</TT
> is likewise updated.  Possible errors that cannot be prevented by the linker include:

<P
></P
><OL
TYPE="1"
><LI
><P
>An old program could use the new library.  Probably the integer it passes will be interpreted as an out-of-bounds address, resulting in a segmentation violation.   </P
></LI
><LI
><P
>A new program could use the old library, passing an address that the library interprets as an integer.  Hillarity ensues.  </P
></LI
><LI
><P
>Existing source code could be compiled using the old header file but linked to the new library.  If you've never done that, give it time.   </P
></LI
></OL
>

These errors are possible because C functions are identified to the linker <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>by name only</I
></SPAN
>.  On the upside, that makes the tools simple and easy to implement and, by the same token, simplifies the use of C libraries by other languages.  The downside is that the work of ensuring that the right libraries are used becomes an administrative task instead of a technical one.   </P
></TD
></TR
></TABLE
></DIV
></P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
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><TD
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><A
NAME="FTN.AEN6435"
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><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
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><P
>The Unix convention is to put in parentheses behind the name the section of the manual in which the function is documented.  <SPAN
CLASS="PRODUCTNAME"
>FreeTDS</SPAN
> functions don't get numbers because they're not in the manual.  Yet.  </P
></TD
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><SPAN
CLASS="footnote"
>[2]</SPAN
></A
></TD
><TD
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><P
>Or, depending on how you look at it, the name of each address.</P
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Code Editor : rtl.define.library.htm