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18.2.5 Dynamic Module
Writing a module for use with the libltdl based dynamic module
loader is no more involved than before: It must provide the correct
entry points, as expected by the simple API I designed -- the
`run' entry point described in 17.4 A Simple GNU/Linux Module Loader. Here is such a module, `ltdl-module.c':
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#include <stdio.h>
#include <math.h>
#define run ltdl_module_LTX_run
int
run (const char *argument)
{
char *end = NULL;
long number;
if (!argument || *argument == '\0')
{
fprintf (stderr, "error: invalid argument, \"%s\".\n",
argument ? argument : "(null)");
return -1;
}
number = strtol (argument, &end, 0);
if (end && *end != '\0')
{
fprintf (stderr, "warning: trailing garbage \"%s\".\n",
end);
}
printf ("Square root of %s is %f\n", argument, sqrt (number));
return 0;
}
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To take full advantage of the new module loader, the module itself
must be compiled with Libtool. Otherwise dependent libraries
will not have been stored when libltdl tries to load the module
on an architecture that doesn't load them natively, or which doesn't
have shared libraries at all (see section 18.4 dlpreopen Loading).
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$ libtool --mode=compile gcc -c ltdl-module.c
rm -f .libs/ltdl-module.lo
gcc -c ltdl-module.c -fPIC -DPIC -o .libs/ltdl-module.lo
gcc -c ltdl-module.c -o ltdl-module.o >/dev/null 2>&1
mv -f .libs/ltdl-module.lo ltdl-module.lo
$ libtool --mode=link gcc -g -o ltdl-module.la -rpath `pwd` \
-no-undefined -module -avoid-version ltdl-module.lo -lm
rm -fr .libs/ltdl-module.la .libs/ltdl-module.* .libs/ltdl-module.*
gcc -shared ltdl-module.lo -lm -lc -Wl,-soname \
-Wl,ltdl-module.so -o .libs/ltdl-module.so
ar cru .libs/ltdl-module.a ltdl-module.o
creating ltdl-module.la
(cd .libs && rm -f ltdl-module.la && ln -s ../ltdl-module.la \
ltdl-module.la)
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You can see from the interaction below that `ltdl-loader' does not
load the math library, `libm', and that the shared part of the
Libtool module, `ltdl-module', does have a reference to it. The
pseudo-library also has a note of the `libm' dependency so that
libltdl will be able to load it even on architectures that can't
do it natively:
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$ libtool --mode=execute ldd ltdl-loader
libltdl.so.0 => /usr/lib/libltdl.so.0 (0x4001a000)
libdl.so.2 => /lib/libdl.so.2 (0x4001f000)
libc.so.6 => /lib/libc.so.6 (0x40023000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
$ ldd .libs/ltdl-module.so
libm.so.6 => /lib/libm.so.6 (0x40008000)
libc.so.6 => /lib/libc.so.6 (0x40025000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x80000000)
$ fgrep depend ltdl-module.la
# Libraries that this one depends upon.
dependency_libs=' -lm'
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This module is now ready to load from `ltdl-loader':
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$ ltdl-loader ltdl-module 9
Square root of 9 is 3.000000
=> 0
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