<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <HTML ><HEAD ><TITLE >Checking if a Library Provides a Function</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="What is a C library?" HREF="rtl.define.library.htm"><LINK REL="NEXT" TITLE="How Dost Thy Linker Link? 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On Linkers</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="linker.how.htm" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="LINKER.LIBRARY.CHECK" >Checking if a Library Provides a Function</A ></H1 ><P >A linker, any linker, knits together object files (some of which may be in libraries) such that every function needed by the program has a definition. If the linker fails to locate a definition for even one function, it will fail and the program will not run. </P ><P >Returning to <A HREF="rtl.htm#BSQLDB.UNRESOLVED" ><TT CLASS="FILENAME" >bsqldb.o</TT ></A >, we can use <B CLASS="COMMAND" >nm</B > to see which functions are unresolved, and determine whether or not a particular library contains them. We'll ignore the symbols that start with an underscore, marking them per the C standard as being provided by the implementation<A NAME="AEN6494" HREF="#FTN.AEN6494" ><SPAN CLASS="footnote" >[1]</SPAN ></A >, and focus on the last five in this abbreviated list. <P ></P ><DIV CLASS="VARIABLELIST" ><P ><B >Some unresolved functions in <TT CLASS="FILENAME" >bsqldb.o</TT ></B ></P ><DL ><DT ><CODE CLASS="FUNCTION" >asprintf</CODE >, <CODE CLASS="FUNCTION" >basename</CODE ></DT ><DD ><P >Normally provided by the standard C library, but if not by <SPAN CLASS="PRODUCTNAME" >FreeTDS</SPAN >'s replacements library: <PRE CLASS="SCREEN" ><SAMP CLASS="PROMPT" >$ </SAMP ><KBD CLASS="USERINPUT" >nm /usr/lib/libc.a | grep -w T | grep -E 'asprintf|basename'</KBD > <SAMP CLASS="COMPUTEROUTPUT" >0000000000000000 T _basename 0000000000000000 T _asprintf</SAMP ></PRE > </P ></DD ><DT ><CODE CLASS="FUNCTION" >calloc</CODE ></DT ><DD ><P >Provided by the standard C library: <PRE CLASS="SCREEN" ><SAMP CLASS="PROMPT" >$ </SAMP ><KBD CLASS="USERINPUT" >nm /usr/lib/libc.a | grep -w T | grep calloc </KBD > <SAMP CLASS="COMPUTEROUTPUT" >0000000000004240 T calloc</SAMP ></PRE > </P ></DD ><DT ><CODE CLASS="FUNCTION" >dbaltbind</CODE >, <CODE CLASS="FUNCTION" >dbaltcolid</CODE ></DT ><DD ><P >Provided by <SPAN CLASS="SYSTEMITEM" >DB-Library</SPAN >: <PRE CLASS="SCREEN" ><SAMP CLASS="PROMPT" >$ </SAMP ><KBD CLASS="USERINPUT" >nm libsybdb.a | grep -Ew 'dbaltbind|dbaltcolid'</KBD > <SAMP CLASS="COMPUTEROUTPUT" >0000000000007140 T dbaltbind 0000000000003590 T dbaltcolid</SAMP ></PRE > </P ></DD ></DL ></DIV > Although these examples refer to static libraries, <B CLASS="COMMAND" >nm</B > works just as well with dynamic libraries, too. </P ><P >There are other tools besides <B CLASS="COMMAND" >nm</B >. Windows®, for instance, has <B CLASS="COMMAND" >dumpbin</B >, and the GNU bintools include <B CLASS="COMMAND" >objdump</B >. </P ></DIV ><H3 CLASS="FOOTNOTES" >Notes</H3 ><TABLE BORDER="0" CLASS="FOOTNOTES" WIDTH="100%" ><TR ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="5%" ><A NAME="FTN.AEN6494" HREF="linker.library.check.htm#AEN6494" ><SPAN CLASS="footnote" >[1]</SPAN ></A ></TD ><TD ALIGN="LEFT" VALIGN="TOP" WIDTH="95%" ><P >Why and how leading underscores enter into this discussion is just one more example of arcane historical practices one needs to know to master the subject. For our purposes, though, it's enough to know that <SPAN CLASS="QUOTE" >"implementation-provided"</SPAN > functions like these — functions provided by the C standard library — often have an underscored prepended. </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.define.library.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.how.htm" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >What is a C library?</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="rtl.htm" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >How Dost Thy Linker Link?</TD ></TR ></TABLE ></DIV ></BODY ></HTML >
