Dynamic linking on Mac OS X, a tiny example
Steps:
- create a library libmylib.dylib containing mymod.o
- compile and link a "callmymod" which calls it
- call mymod from callmymod, using DYLD_LIBRARY_PATH and DYLD_PRINT_LIBRARIES
Problem: you "just" want to create a library for other modules to use.
However there's a daunting pile of programs -- gcc, ld, macosx libtool, dyld --
with zillions of options, some well-rotted compost, and differences between MacOSX and Linux.
There are tons of man pages (I count 7679 + 1358 + 228 + 226 lines in 10.4.11 ppc)
but not much in the way of examples, or programs with a "tell me what you're doing" mode.
(The most important thing in understanding is to make a simplified
OVERVIEW for yourself: draw some pictures, run some small examples,
explain it to someone else).
Background: apple OverviewOfDynamicLibraries,
Wikipedia Dynamic_library
Step 1, create libmylib.dylib --
mymod.c:
#include <stdio.h>
void mymod( int x )
{
printf( "mymod: %d\n", x );
}
gcc -c mymod.c # -> mymod.o
gcc -dynamiclib -current_version 1.0 mymod.o -o libmylib.dylib
# calls libtool with many options -- see man libtool
# -compatibility_version is used by dyld, see also cmpdylib
file libmylib.dylib # Mach-O dynamically linked shared library ppc
otool -L libmylib.dylib # versions, refs /usr/lib/libgcc_s.1.dylib
Step 2, compile and link callmymod --
callmymod.c:
extern void mymod( int x );
int main( int argc, char** argv )
{
mymod( 42 );
}
gcc -c callmymod.c
gcc -v callmymod.o ./libmylib.dylib -o callmymod
# == gcc callmymod.o -dynamic -L. -lmylib
otool -L callmymod # refs libmylib.dylib
nm -gpv callmymod # U undef _mymod: just a reference, not mymod itself
Step 3, run callmymod linking to libmylib.dylib --
export DYLD_PRINT_LIBRARIES=1 # see what dyld does, for ALL programs
./callmymod
dyld: loaded: libmylib.dylib ...
mymod: 42
mv libmylib.dylib /tmp
export DYLD_LIBRARY_PATH=/tmp # dir:dir:...
./callmymod
dyld: loaded: /tmp/libmylib.dylib ...
mymod: 42
unset DYLD_PRINT_LIBRARIES
unset DYLD_LIBRARY_PATH
That ends one tiny example; hope it helps understand the steps.
(If you do this a lot, see GNU Libtool
which is glibtool on macs,
and SCons.)
