File: libtool.info,  Node: Creating object files,  Next: Linking libraries,  Up: Using libtool

3.1 Creating object files
=========================

To create an object file from a source file, the compiler is invoked
with the ‘-c’ flag (and any other desired flags):

     burger$ gcc -I. -g -O -c main.c
     burger$

   The above compiler command produces an object file, usually named
‘main.o’, from the source file ‘main.c’.

   For most library systems, creating object files that become part of a
static library is as simple as creating object files that are linked to
form an executable:

     burger$ gcc -I. -g -O -c foo.c
     burger$ gcc -I. -g -O -c hello.c
     burger$

   Shared libraries, however, may only be built from
“position-independent code” (PIC). So, special flags must be passed to
the compiler to tell it to generate PIC rather than the standard
position-dependent code.

   Since this is a library implementation detail, libtool hides the
complexity of PIC compiler flags and uses separate library object files
(the PIC one lives in the ‘.libs’ subdirectory and the static one lives
in the current directory).  On systems without shared libraries, the PIC
library object files are not created, whereas on systems where all code
is PIC, such as AIX, the static ones are not created.

   To create library object files for ‘foo.c’ and ‘hello.c’, simply
invoke libtool with the standard compilation command as arguments (*note
Compile mode::):

     a23$ libtool --mode=compile gcc -I. -g -O -c foo.c
     gcc -I. -g -O -c foo.c -o foo.o
     a23$ libtool --mode=compile gcc -I. -g -O -c hello.c
     gcc -I. -g -O -c hello.c -o hello.o
     a23$

   Note that libtool silently creates an additional control file on each
‘compile’ invocation.  The ‘.lo’ file is the libtool object, which
Libtool uses to determine what object file may be built into a shared
library.  On ‘a23’, only static libraries are supported so the library
objects look like this:

     # foo.lo - a libtool object file
     # Generated by ltmain.sh (GNU libtool) 2.5.4
     #
     # Please DO NOT delete this file!
     # It is necessary for linking the library.

     # Name of the PIC object.
     pic_object=none

     # Name of the non-PIC object.
     non_pic_object='foo.o'

   On shared library systems, libtool automatically generates an
additional PIC object by inserting the appropriate PIC generation flags
into the compilation command:

     burger$ libtool --mode=compile gcc -I. -g -O -c foo.c
     mkdir .libs
     gcc -I. -g -O -c foo.c  -fPIC -DPIC -o .libs/foo.o
     gcc -I. -g -O -c foo.c -o foo.o >/dev/null 2>&1
     burger$

   Note that Libtool automatically created the ‘.libs’ directory upon
its first execution, where PIC library object files will be stored.

   Since ‘burger’ supports shared libraries, and requires PIC objects to
build them, Libtool has compiled a PIC object and made a note of it in
the libtool object:

     # foo.lo - a libtool object file
     # Generated by ltmain.sh (GNU libtool) 2.5.4
     #
     # Please DO NOT delete this file!
     # It is necessary for linking the library.

     # Name of the PIC object.
     pic_object='.libs/foo.o'

     # Name of the non-PIC object.
     non_pic_object='foo.o'

   Notice the second run of GCC has its output discarded.  This is done
so the compiler warnings aren't annoyingly duplicated.  If you need to
see both sets of warnings (you might have conditional code inside
‘#ifdef PIC’ for example), you can turn off suppression by passing the
‘-no-suppress’ option to libtool's compile mode:

     burger$ libtool --mode=compile gcc -no-suppress -I. -g -O -c hello.c
     gcc -I. -g -O -c hello.c  -fPIC -DPIC -o .libs/hello.o
     gcc -I. -g -O -c hello.c -o hello.o
     burger$