# 11.3. Extracting flags from the wrapper compilers

If you cannot use the wrapper compiler for some reason, there are multiple supported ways to extract the compiler/linker flags that you will need.

## 11.3.1. Using the `--showme` option

The wrapper compiler supports a `--showme` command line option that will show what commands would have been invoked.

Note

If you pass `--showme` and additional command line parameters to the wrapper compiler, be sure to also pass in a filename. Otherwise, the `--showme` functionality will not display output as expected.

Warning

It is almost never a good idea to hard-code these results in a `Makefile` (or other build system). It is almost always best to run (for example) `pmixcc --showme:compile` in a dynamic fashion to find out what you need. For example, GNU Make allows running commands and assigning their results to variables:

    PMIX_COMPILE_FLAGS = $(shell pmixcc --showme:compile)
    PMIX_LINK_FLAGS = $(shell pmixcc --showme:link)

    my_app: my_app.c
            $(CC) $(PMIX_COMPILE_FLAGS) my_app.c $(PMIX_LINK_FLAGS) -o my_app

## 11.3.2. Using `pkg-config`

Alternatively, PMIx also installs `pkg-config(1)` configuration files under `$libdir/pkgconfig`. If `pkg-config` is configured to find these files (e.g., if you add `$libdir/pkgconfig` — which is usually `$prefix/lib/pkgconfig` — to the `PKG_CONFIG_PATH` environment variable), then compiling / linking PMIx programs can be performed like this:

    shell$ export PKG_CONFIG_PATH=/opt/pmix/lib/pkgconfig
    shell$ gcc hello_world.c -o hello_world -g \
                `pkg-config pmix --cflags --libs`
    shell$

Note

PMIx’s `pkg-config` file works properly, but it probably isn’t technically correct.

Specifically: PMIx will list all of its dependent libraries that are necessary to link PMIx-based application, even if a given dependency has a `.pc` file and should therefore be listed as a `Requires` and/or `Requires.private` in PMIx’s `.pc` files.

For example, PMIx lists `-lhwloc` in both `Libs` and `Libs.private`. But since HWLOC provides its own `pmix.pc` file, it would be more correct for PMIx to not list `-lhwloc` in `Libs` / `Libs.private`, and instead include:

    Requires: hwloc
    Requires.private: hwloc

The end result is likely immaterial, but we document this just in case it ever becomes an issue someday.

## 11.3.3. Using `pmix_info`

This method is not directly suitable for getting all the compiler / linker flags needed to compile PMIx-based applications because it does not include the relevant flags to find PMIx’s headers and libraries. But it does show a breakdown of all other flags.

    shell$ pmix_info --all | grep Build
             Build CFLAGS:
            Build LDFLAGS:
               Build LIBS: -ldl -levent_core -levent_pthreads -lhwloc

This installation is only adding options in the `xLIBS` areas of the wrapper compilers; all other values are blank (remember: the `-I`’s and `-L`’s are implicit).

Note that the `--parsable` option can be used to obtain machine-parsable versions of this output. For example:

    shell$ ompi_info --all --parsable | grep option:build
         option:build:cflags:
         option:build:ldflags:
         option:build:libs:-ldl -levent_core -levent_pthreads -lhwloc