On 27 March 2014 20:30, Donald Stufft firstname.lastname@example.org wrote:
When I interface with an external library that may not be in the same place on every system, I'd expect to say a bit more, but mainly just "look in location X, but the user can override this with a command line argument".
That's the job of the linker, it has built in locations to look for libraries and env vars/command line flags to override that. Someone packaging a C-ext that needs a library to compile shouldn't be specifying where to find it anyways.
Ideally, yes. But not in practice (at least not on Windows). This is getting off-topic, so I'll just give a quick example and leave it at that.
I recently wanted to build PyYAML on 64-bit Windows, which uses libyaml if that's available. First thing I had to do therefore was to build libyaml. That's not too hard (considering). But the build leaves include files in the project directory and a lib file somewhere in the bowels of the project ...\x64\Release\Output\Lib\libyaml.lib, IIRC). There is no install step, largely because there is nowhere standard to install include and lib files *to*.
OK, so now I build PyYAML. I don't have the compiler, linker etc on my PATH - the distutils code automatically detects that I have the right version of the compiler installed and sets up the environment for me. This is a *feature*, not a problem, as otherwise I would have to remember which compiler to use for which Python version, and activate it as an extra step. As a result of this fact, though, I can't set LIB or INCLUDE environment variables to point to the libyaml project directories (nor would I particularly want to, as this is purely a one-off build). So I need to tell the build system (distutils) where to find libyaml and yaml.h. There's no way setup.py can guess where they might be in the general case, so I use the build_ext -L and -I options to specify.
Consider "the linker can just find stuff" as the "look in location X" scenario. And the above as the reason we might need build-system support for adding compiler/linker flags or env vars at build time.