<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML ><HEAD ><TITLE >What is a C library?</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REL="HOME" TITLE="FreeTDS User Guide" HREF="index.htm"><LINK REL="UP" TITLE="On Linkers" HREF="rtl.htm"><LINK REL="PREVIOUS" TITLE="On Linkers" HREF="rtl.htm"><LINK REL="NEXT" TITLE="Checking if a Library Provides a Function" HREF="linker.library.check.htm"><LINK REL="STYLESHEET" TYPE="text/css" HREF="userguide.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8"></HEAD ><BODY CLASS="SECTION" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" ><SPAN CLASS="PRODUCTNAME" >FreeTDS</SPAN > User Guide: A Guide to Installing, Configuring, and Running <SPAN CLASS="PRODUCTNAME" >FreeTDS</SPAN ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="rtl.htm" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Appendix A. On Linkers</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="linker.library.check.htm" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><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 — containing functions, of course — 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 — or the wrong versions of them — can lead to head-scratching and other amusing behavior. </P ><P >In Windows® 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 CLASS="NOTE" 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 > </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 & 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 BORDER="0" CLASS="FOOTNOTES" WIDTH="100%" ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN6435" HREF="rtl.define.library.htm#AEN6435" ><SPAN CLASS="footnote" >[1]</SPAN ></A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><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 ></TR ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN6446" HREF="rtl.define.library.htm#AEN6446" ><SPAN CLASS="footnote" >[2]</SPAN ></A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><P >Or, depending on how you look at it, the name of each address.</P ></TD ></TR ></TABLE ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="rtl.htm" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.htm" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="linker.library.check.htm" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >On Linkers</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="rtl.htm" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Checking if a Library Provides a Function</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
