[lttng-dev] dlopen, gcc -l and LD_PRELOAD

[Thibault, Daniel]

   In the lttng-ust man pages, there is a paragraph that reads:

Note about dlopen() usage: Due to locking side-effects caused by the way libc lazily resolves Thread-Local Storage (TLS) symbols when a library is dlopened, linking the tracepoint probe or liblttng-ust with dlopen() is discouraged.  They should be linked with the application using "-llibname" or loaded with LD_PRELOAD.

   I'm confused by the LD_PRELOAD bit: as far as I know, LD_PRELOAD serves only to substitute a dynamic library (the LD_PRELOAD argument) for another (the one the executable was linked against).  I don't see how one could compile an instrumented application and leave its tracepoint provider unlinked, which is what the stated -llibname <-> LD_PRELOAD opposition implies.

   (Also, shouldn't that paragraph drop 'lazily', assuming the locking problems also arise if dlopen is used with RTLD_NOW?  Because if RTLD_NOW fixed the locking problem, that's what would be recommended here, no?)

Daniel U. Thibault
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