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10.2.4 Creating Convenience Libraries
The third type of library which can be built with libtool is the
convenience library. Modern compilers are able to create
partially linked objects: intermediate compilation units which
comprise several compiled objects, but are neither an executable or a
library. Such partially linked objects must be subsequently linked
into a library or executable to be useful. Libtool convenience
libraries are partially linked objects, but are emulated by
libtool on platforms with no native implementation.
If you want to try this to see what libtool does on your
machine, put the following code in a file `trim.c', in the same
directory as `hello.c' and `libhello.la', and run the example
shell commands from there:
| | #include <string.h>
#define WHITESPACE_STR " \f\n\r\t\v"
/**
* Remove whitespace characters from both ends of a copy of
* '\0' terminated STRING and return the result.
**/
char *
trim (char *string)
{
char *result = 0;
/* Ignore NULL pointers. */
if (string)
{
char *ptr = string;
/* Skip leading whitespace. */
while (strchr (WHITESPACE_STR, *ptr))
++ptr;
/* Make a copy of the remainder. */
result = strdup (ptr);
/* Move to the last character of the copy. */
for (ptr = result; *ptr; ++ptr)
/* NOWORK */;
--ptr;
/* Remove trailing whitespace. */
for (--ptr; strchr (WHITESPACE_STR, *ptr); --ptr)
*ptr = '\0';
}
return result;
}
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To compile the convenience library with libtool, you would do
this:
| | $ libtool gcc -c trim.c
rm -f .libs/trim.lo
gcc -c -fPIC -DPIC trim.c -o .libs/trim.lo
gcc -c trim.c -o trim.o >/dev/null 2>&1
mv -f .libs/trim.lo trim.lo
$ libtool gcc -o libtrim.la trim.lo
rm -fr .libs/libtrim.la .libs/libtrim.* .libs/libtrim.*
ar cru .libs/libtrim.al trim.lo
ranlib .libs/libtrim.al
creating libtrim.la
(cd .libs && rm -f libtrim.la && ln -s ../libtrim.la libtrim.la)
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Additionally, you can use a convenience library as an alias for a set of
zero or more object files and some dependent libraries. If you need to
link several objects against a long list of libraries, it is much more
convenient to create an alias:
| | $ libtool gcc -o libgraphics.la -lpng -ltiff -ljpeg -lz
rm -fr .libs/libgraphics.la .libs/libgraphics.* .libs/libgraphics.*
ar cru .libs/libgraphics.al
ranlib .libs/libgraphics.al
creating libgraphics.la
(cd .libs && rm -f libgraphics.la && \
ln -s ../libgraphics.la libgraphics.la)
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Having done this, whenever you link against `libgraphics.la' with
libtool, all of the dependent libraries will be linked too.
In this case, there are no actual objects compiled into the convenience
library, but you can do that too, if need be.
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