How can I make an action run if build is successful or unneeded

[mark-summerfield]

I want to make an action run if a build is successful or if the build isn't needed (i.e., no source changes).

This will make an action run if the build is successful:

AddOption('--run', dest='run', action='store_true', default=False,
          help='Run once built')
# etc ...
if GetOption('run'):
    env.AddPostAction(app, Action('./$TARGET'))

This allows me to enter scons --run to build & run. But if I up-arrow in the terminal to rerun this command, scon says (rightly) that there's nothing to build and does not do the post option.

How can I make it do the post option in this second case?

[mwichmann]

You've already realized that a post (or pre) action attached to a target doesn't trigger if the target isn't rebuilt. From your description, it's not clear why you want to run something even if the target wasn't built... if you mean you want to actually execute the command (like might be the case for unit tests), you have to take a bit of a different approach. Don't take this as the definitive answer, but you could try this (there are several ways to skin this cat, and this may not be the best):

1. construct a Command builder call which runs the program you're interested in.
2. mark the target of the Command as depending on the original generated program.
3. add the target to an Alias called run
4. mark the alias as AlwaysBuild so as long that (alias) target is selected, it will build even if it doesn't seem "out of date"

Now build as scons run

[mark-summerfield]

I understand the theory of what you're saying, but I can't see how to do it in code (I'm an scons beginner).

[bdbaddog]

You could just setup an atexit.

See: https://docs.python.org/3/library/atexit.html for info
Basically you just set up a function to run your program, and then register it with atexit.register

@mwichmann's is another scons-only way to do this.

[mark-summerfield]

atexit won't help because I want to run if there is a successful build or if a build isn't needed, but not if a build fails.
This will allow me to write 'scons run' and then just up-arrow and Enter after each change I want to test.

[mwichmann]

At its very simplest, this should work:

prog = Program("hello.c")
pname = str(prog[0])
runit = Command(target=f'r_{pname}', source=prog, action=(f'-./{pname}'))

In this case, Command has a target that uses a made-up name, just prepending something to the name of the Program target, and its source is the built program, and since the action never "creates" that target, it's always out of date and so runs every time.

The more detailed recipe accounts for more complexity, such as when there are multiple programs to run (the accumulating nature of Alias is useful for that), or when the action actually produces something that looks like a target - like an output log (if you use a logfile or other tangible thing as the runner target, that's where AlwaysBuild would come into play). The Depends call from the recipe isn't needed if you are okay having the built program as the source, otherwise you'd need it.

SCons by default selects all targets under the project root, so in the minimal example both building the program and running it are selected. If you want the default to be "build" and "running" to have to be explicitly asked for, you can expand the recipe a bit:

prog = Program("hello.c")
Default(prog)
pname = str(prog[0])
runit = Command(target=f'r_{pname}', source=prog, action=(f'-./{pname}'))
alias = Alias("run", runit)

which might look like:

$ scons -Q
gcc -o hello.o -c hello.c
gcc -o hello hello.o
$ scons -Q
scons: `hello' is up to date.
$ scons -Q run
./hello
Hello world
$ rm hello
$ scons -Q run
gcc -o hello hello.o
./hello
Hello world

I'd recommend taking the simplest version and running some experiments, like build with --tree=all to make sure the plumbing looks right, and don't get more complicated unless you need to.

[mark-summerfield]

Based on your ideas I'm using the below (which only needs one line to be changed per project):

AddOption('--dbg', dest='dbg', action='store_true', default=False,
          help='Debug build')
env = Environment()
if GetOption('dbg'):
    env.Append(CCFLAGS='-Wall -g')
else:
    env.Append(CCFLAGS='-Wall -s -O3 -DNDEBUG')
env.Append(CPPPATH=['.', '../STC/include'])

app = env.Program('myapp', ['myapp.c']) # only this changes

Default(app)
appname = str(app[0])
runner = Command(target=f'run_{appname}', source=app,
                 action=f'-./{appname}')
alias = Alias('run', runner)

Now I can run scons -Q run to build & run, and to run.

Thanks.