QNX Neutrino Programmer's Guide [6.5.0 SP1] - QNX Software ...

© 2012, QNX Software Systems Limited Optimizing the runtime linker
• In the lazy-bound case, it doesn’t fail right away (since it didn’t check to see if it
could resolve all the symbols) but will still fail on the first call to an unresolved
symbol. This doesn’t change even if the application later calls dlopen() to load an
object that defines that symbol, because the application can’t change the resolution
scope. The only exceptions to this rule are objects loaded using dlopen() with the
RTLD_LAZY flag (see below).
Lazy binding is controlled by the -z option to the linker, ld. This option takes
keywords as an argument; the keywords include (among others):
lazy
now
When generating an executable or shared library, mark it to tell the dynamic
linker to defer function-call resolution to the point when the function is
called (lazy binding), rather than at load time.
When generating an executable or shared library, mark it to tell the dynamic
linker to resolve all symbols when the program is started, or when the
shared library is linked to using dlopen(), instead of deferring function-call
resolution to the point when the function is first called.
Lazy binding is the default. If you’re using qcc (as we recommend), use the -W option
to pass the -z option to ld. For example, specify -Wl,-zlazy or -Wl,-znow.
There are cases where the default lazy binding isn’t desired. For example:
• While the system is under development, you might want to fully resolve all
symbols right away, to catch library mismatches; your application would fail to
load if a referenced function couldn’t be resolved.
• You might want to fully resolve the symbols for a particular object at load time.
• You might want only a given program to be always bound right away.
There’s a way to do each of these:
• To change the default lazy binding to the “bind now” behavior for all processes
started from a given shell, set the LD_BIND_NOW environment variable to a
non-null value. For example:
LD_BIND_NOW=1 ./foobar
By default, pdebug sets LD_BIND_NOW to 1. If you start gdb from the command
line on a self-hosted QNX Neutrino system, gdb also sets LD_BIND_NOW to 1.
Without LD_BIND_NOW, you’d see a different backtrace for the first function call
into the shared object as the runtime linker resolves the symbol. On subsequent calls
to the same function, the backtrace would be as expected. You can prevent pdebug
from setting LD_BIND_NOW by specifying the -l (“el”) option.
• To override the binding strategy for a given shared object, link it with the -znow
linker option:
June 14, 2012 Chapter 1 • Compiling and Debugging 17