This flag is named after the environment variable LD_PRELOAD used by ld-linix. Like LD_PRELOAD its value is a space-separated list of shared libraries to be preloaded and packaged with the application.
Note
This flag ONLY affects the packaging process, not running the Ermine-packaged executable.
But if --ld-preload doesn't affect running the Ermine-packed executable, then why bothering specifying it at all?
By default Ermine will only package the executable itself and all shared libraries reported by the ldd command. While this is a good start, ldd is unable to detect libraries that are dynamically loadded by an application via dlopen. So --ld_preload is a way to tell Ermine that certain libraries need to be packaged too.
Why use the --ld-preload option instead of just specifying those libraries in the config file? There are two reasons:
- ErmineLight doesn't offer the --config option.
- ErminePro has the --config option. While it's possible to use a config file instead of the --ld_preload option, it is usually inconvenient to do so: often one shared library needs another one, and then in turn they may need more, so tracking all of them can be time-consuming and error-prone.
Example 2.1. Adding libQtCore.so.4 to a packaged application using --config
First let's see what shared libraries libQtCore.so.4 depends on:$ /usr/lib/libQtCore.so.4 linux-gate.so.1 => (0x006c5000) libpthread.so.0 => /lib/libpthread.so.0 (0x00f5a000) libz.so.1 => /lib/libz.so.1 (0x00b57000) libdl.so.2 => /lib/libdl.so.2 (0x00f39000) libgthread-2.0.so.0 => /lib/libgthread-2.0.so.0 (0x00f0c000) librt.so.1 => /lib/librt.so.1 (0x0068d000) libglib-2.0.so.0 => /lib/libglib-2.0.so.0 (0x00d9c000) libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x00110000) libm.so.6 => /lib/libm.so.6 (0x006ec000) libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x009a3000) libc.so.6 => /lib/libc.so.6 (0x00716000) /lib/ld-linux.so.2 (0x005a6000)Next, a config file should be created:
# config.txt /lib/libpthread.so.0 internal /lib/libz.so.1 internal /lib/libdl.so.2 internal /lib/libgthread-2.0.so.0 internal /lib/librt.so.1 internal /lib/libglib-2.0.so.0 internal /usr/lib/libstdc++.so.6 internal /lib/libm.so.6 internal /lib/libgcc_s.so.1 internal /lib/libc.so.6 internal /lib/ld-linux.so.2 internalAnd finally Ermine should be invoked:
$ ErminePro --config=config.txt ...
Example 2.2. Adding libQtCore.so.4 to a packaged application using --ld-preload
$ ErminePro --ld_preload=libQtCore.so.4 ...That's it!
Personally I prefer the latter way, but of course you might find one that suits you better.