(use_cases_macro_relauncher_mpi_standalone_cpp)=
# Macro "**relauncherCodeFlowrateMpiStandalone.C**"

## Objective

The goal of this macro is to show how to handle a code run on several threads with another memory 
paradigm: when the `TThreadedRun` instance is relying on shared memory (leading to possible 
thread-safe problem, as discussed in [](#relauncher_trun_tthreadedrun)), the MPI implementation is 
based on the separation of the memory. The communication is made through messages. In order to this, 
the usual sequential runner will be removed and another runner will be called to do the job. The 
`flowrate` code is provided with {{uranie}} and has been also used and discussed throughout these 
macros.

```{warning}
This macro is different from the one discussed previously in [](#use_cases_macro_relauncher_mpi) as 
here, one wants to handle MPI through standalone compilation (so C++ without using the {{root}} 
interface)
```
## Macro

{{
    "```{literalinclude} " + parent_dir
    + "/roottest/uranie/doc/relauncher/use_cases/cpp/relauncherCodeFlowrateMpiStandalone.C"
    + "\n"
    + ":language: cpp\n"
    + "```"
}}

Here there are few differences the most obvious one being the proper C++ structure:

- there are several includes that has to be included on top the macro in order to provided the 
needed headers;

- the main function has to follow a classical form `int main(int argc, char **argv)` which allows to 
modify parameters on the fly once compiled (as for the `initType` integer here through a dedicated 
loop). The consequence of this signature  will be discussed later on;

- a `usage` function has been written in order to provide interactive guidelines on how to deal with 
this code.

Apart from this, from the variable definition to the assessor description, the structure is similar 
to the one discussed previously (see [](#use_cases_macro_relauncher_threaded_macro)). Once this point 
is reached there are two ways to construct the runner:

- with the classical constructor. In this case, as one is not running a {{root}} session, the 
constructor will not found the interactive command parameters and the `MPI_Init` will not 
automatically be called, thus meaning that one needs to call this method with the proper parameters 
as done below:

{{
    "```{literalinclude} " + parent_dir
    + "/roottest/uranie/doc/relauncher/use_cases/cpp/relauncherCodeFlowrateMpiStandalone.C"
    + "\n"
    + ":language: cpp\n"
    + ":lines: 71,72\n"
    + "```"
}}

- with the new constructor provided with v4.6.0 whose argument are the usual assessor and the interactive parameters:

{{
    "```{literalinclude} " + parent_dir
    + "/roottest/uranie/doc/relauncher/use_cases/cpp/relauncherCodeFlowrateMpiStandalone.C"
    + "\n"
    + ":language: cpp\n"
    + ":lines: 75\n"
    + "```"
}}

The final difference is the fact that this code has to be properly compiled, using MPI-complient 
compilor in order to be able to produce an executable that will be run later on. This can be done 
like this:

```bash
mpicc -o relaunStand relauncherCodeFlowrateMpiStandalone.C `echo $URANIECPPFLAG $URANIELDFLAG` -lstdc++
```

where `mpicc` is the c++ mpi-complient compilor, the argument after `-o` is the executable name, 
then one can see the input file while the rest is provinding all compiling and linking flag in order 
to reach the goal. Finally, instead of using the classical `root -l` command, one will use this line

```bash
mpirun -np N ./relaunStand initType XX
```

where the *N* part should be replaced by the number of requested threads (always strickly greater 
than 1) and XX should be replaced either by 0 or 1 depending on whether one wants to use the 
classical `TMpiRun` constructor. Once run, this macro also leads to the following plots (both 
initialisation are shown).

## Graph

{{
    "```{" "figure" "} " + parent_dir +
        "/roottest/build/uranie/doc/relauncher/use_cases/cpp/standalone/relauncherCodeFlowrateMpiStandalone.png\n"
    ":align: center\n"
    ":name: usecases_relauncherCodeFlowrateMpiStandalone\n"
    + figure_scale + "\n"
    "\n"
    "Representation of the output as a function of the first input with a colZ option when using 
    either the classical or dedicated constructor\n"
    "```"
}}