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<H2><A NAME="SECTION04121000000000000000"></A><A NAME="ss:compilebootcamp"></A>
<BR>
Basics of Compilation and Linkage
</H2>
<P>
What we generally think of as a compiler is really an ensemble of related
tools. Generally there is a preprocessing step where very simple transformations
occur (e.g. #define, #include directives and others). Next, the compiler
proper executes and typically transforms your sourcecode into assembler or
some other intermediate form. Optimizers work on this intermediate form and
do perform additional transformations. Most big vendors of C, C++, and Fortran
compilers have a common optimizer for all languages. Next, assemblers transform
the optimized codes into platform-specific binaries. But this is not the end.
The binaries may be linked together into libraries or programs. Libraries can
be linked against other libraries, and eventually multiple programs.
The main difference is that a program has additional instructions to bootstrap
itself, do some interaction with the operating system, receive an argument list,
and call <TT>main()</TT>. To see all this in action, try building a ``hello world''
type program in your favorite language, and run the ``compiler'' with an additional
flag such as <TT><B>-v</B></TT>, <TT><B><code>--</code>verbose</B></TT>, or whatever.
<P>
For example, this is what I get from a g77 compiler.
<BLOCKQUOTE>
<TT> <TT>%</TT> <TT><B>g77 hello_world.f</B></TT>
<BR><TT>%</TT> <TT><B>./a.out</B></TT>
<BR>
Hello World!
<TT>%</TT> <TT><B>g77 -v hello_world.f</B></TT>
<BR>
Driving: g77 -v hello_world.f -lfrtbegin -lg2c -lm -shared-libgcc
<BR>
Reading specs from /usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/specs
<BR>
Configured with: ../gcc-3.2/configure -prefix=/usr/local/gcc/3.2
<BR>
Thread model: posix
<BR>
gcc version 3.2
<BR> /usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/f771 hello_world.f -quiet -dumpbase hello_world.f -version -o /tmp/ccp2GBGE.s
<BR>
GNU F77 version 3.2 (i686-pc-linux-gnu)
<BR>
compiled by GNU C version 3.2.
<BR>
as -traditional-format -V -Qy -o /tmp/ccEiIsHc.o /tmp/ccp2GBGE.s
<BR>
GNU assembler version 2.11.90.0.8 (i386-redhat-linux) using BFD version 2.11.90.0.8
<BR> /usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/collect2 -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o /usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/crtbegin.o -L/usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2 -L/usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/../../.. /tmp/ccEiIsHc.o -lfrtbegin -lg2c -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/local/gcc/3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/crtend.o /usr/lib/crtn.o
<BR>
</TT></BLOCKQUOTE>
<P>
For the purposes of this discussion, we will make a big distinction between
linking to build a library and linking to build and executable. Even though
these transformations have similar names, they perform very different kinds
of transformations to the code.
<P>
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