Question

I’m building a simple C++ program and I want to temporarily substitute a system supplied shared library with a more recent version of it, for development and testing.

I tried setting the LD_LIBRARY_PATH variable but the linker (ld) failed with:

/usr/bin/ld: cannot find -lyaml-cpp

I expected that to work because according to the ld man page:

The linker uses the following search
paths to locate required shared
libraries: … For a native linker,
the contents of the environment variable
“LD_LIBRARY_PATH”…

I then tried setting the LIBRARY_PATH, and that worked.

According to the GCC manual:

The value of
LIBRARY_PATH is a colon-separated list
of directories, much like PATH. When
configured as a native compiler, GCC
tries the directories thus specified
when searching for special linker
files, if it can’t find them using
GCC_EXEC_PREFIX. Linking using GCC
also uses these directories when
searching for ordinary libraries for
the -l option (but directories
specified with -L come first).

As the (GCC) manual suggests, LIBRARY_PATH works because I link with GCC.

But..

• Since I link with gcc why ld is
  being called, as the error message
  suggests?
• What’s the point of
  having two variables serving the same
  purpose? Are there any other
  differences?

Answer 1

LIBRARY_PATH is used by gcc before compilation to search directories containing static and shared libraries that need to be linked to your program.

LD_LIBRARY_PATH is used by your program to search directories containing shared libraries after it has been successfully compiled and linked.

EDIT:
As pointed below, your libraries can be static or shared. If it is static then the code is copied over into your program and you don’t need to search for the library after your program is compiled and linked. If your library is shared then it needs to be dynamically linked to your program and that’s when LD_LIBRARY_PATH comes into play.