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Libtool does this deliberately so that it is possible todistinguish between a static object build and a Libtool object build,from within the source code.</P><P>Lets add a data export to the DLL source to illustrate:</P><P>The <TT>`hello.h'</TT> header must be changed quite significantly:<TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre>#ifndef HELLO_H#define HELLO_H 1#if HAVE_CONFIG_H# include <config.h>#endif#ifdef _WIN32# ifdef DLL_EXPORT# define HELLO_SCOPE __declspec(dllexport)# else# ifdef LIBHELLO_DLL_IMPORT# define HELLO_SCOPE extern __declspec(dllimport)# endif# endif#endif#ifndef HELLO_SCOPE# define HELLO_SCOPE extern#endifHELLO_SCOPE const char *greet;extern int hello (const char *who);#endif /* !HELLO_H */</pre></td></tr></table></P><P>The nasty block of preprocessor would need to be shared among all thesource files which comprise the <TT>`libhello.la'</TT> Libtool library,which in this example is just <TT>`hello.c'</TT>. It needs to take care offive different cases:</P><P><DL COMPACT><DT>compiling <TT>`hello.lo'</TT><DD>When compiling the Libtool object which will be included in theDLL, we need to tell the compiler which symbols are exported dataso that it can do the automatic extra dereference required to refer tothat data from a program which uses this DLL. We need to flag thedata with <CODE>__declspec(dllexport)</CODE>, See section <A HREF="autobook_251.html#SEC251">25.4 DLLs with Libtool</A>.<P><DT>compilation unit which will link with <TT>`libhello-0-0-0.dll'</TT><DD>When compiling an object which will import data from the DLL, again weneed to tell the compiler so that it can perform the extra dereference,except this time we use <CODE>extern __declspec(dllimport)</CODE>. From thepreprocessor block, you will see that we need to define<SAMP>`LIBHELLO_DLL_IMPORT'</SAMP> to get this define, which I will describeshortly.<P><DT>compiling <TT>`hello.o'</TT><DD>When compiling the object for inclusion in the static archive, we must becareful to hide the <CODE>__declspec()</CODE> declarations from the compiler,or else it will start dereferencing variables for us by mistake atruntime, and in all likelihood cause a segmentation fault. In this casewe want the compiler to see a simple <CODE>extern</CODE> declaration.<P><DT>compilation unit which will link with <TT>`libhello.a'</TT><DD>Similarly, an object which references a data symbol which will bestatically linked into the final binary from a static archive must notsee any of the <CODE>__declspec()</CODE> code, and requires a simple<CODE>extern</CODE>.<P>⌨️ 快捷键说明
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