I believe the left hand is a shell fork-bomb, on the assumption that anything that zany is probably malicious.
And the right hand is a way to tell Make to use up all available system resources:
"-j [jobs]’ ¶
‘--jobs[=jobs]’
Specifies the number of recipes (jobs) to run simultaneously. With noargument, make runs as many recipes simultaneously as possible. If there is more than one ‘-j’ option, the last one is effective. See Parallel Execution, for more information on how recipes are run. Note that this option is ignored on MS-DOS."
Edit: I think the make command is technically only a problem when run for a Makefile that tries to do too many things, and has at least one mistake in dependency controls. So… for every Makefile I ever encountered (or that I ever wrote!)
You are correct, left hand is a fork bomb. Specifically, it creates and then runs a function named “:”. What this function does is pipe its output into itself while running in a background process, which instantly spawns infinite copies of itself. Technically I believe the : character could be any character as its just a name. The creator just picked a colon for aesthetics.
I think it can also get weird when you call other makefiles, like if you go make -j64 at the top level and that thing goes on to call make on subprojects, that can be a looooot of threads of that -j gets passed down. So even on that 64 core machine, now you have possibly 4096 jobs going, and it surfaces bugs that might not have been a problem when we had 2-4 cores (oh no, make is running 16 jobs at once, the horror).
There could also be other problems, like not being able to assign jobs correctly with that many threads. I’ve encountered this when assigning more jobs than there are threads on the rig. A lot more, like 2x, 4x the threads available.
I believe the left hand is a shell
fork-bomb, on the assumption that anything that zany is probably malicious.And the right hand is a way to tell
Maketo use up all available system resources:"-j [jobs]’ ¶ ‘--jobs[=jobs]’ Specifies the number of recipes (jobs) to run simultaneously. With no argument, make runs as many recipes simultaneously as possible. If there is more than one ‘-j’ option, the last one is effective. See Parallel Execution, for more information on how recipes are run. Note that this option is ignored on MS-DOS."Edit: I think the
makecommand is technically only a problem when run for aMakefilethat tries to do too many things, and has at least one mistake in dependency controls. So… for everyMakefileI ever encountered (or that I ever wrote!)Yeah. They’re the same picture
You are correct, left hand is a fork bomb. Specifically, it creates and then runs a function named “:”. What this function does is pipe its output into itself while running in a background process, which instantly spawns infinite copies of itself. Technically I believe the : character could be any character as its just a name. The creator just picked a colon for aesthetics.
I always just kind of glazed over looking at that and just know “it’s a fork bomb” and basically what it does
With your explanation, I can now actually understand all the parts and how they work, it actually makes sense
Now I get why it does what it does and how it works. I never thought that the colon was the variable name but it makes so much sense!
I think it can also get weird when you call other makefiles, like if you go
make -j64at the top level and that thing goes on to call make on subprojects, that can be a looooot of threads of that-jgets passed down. So even on that 64 core machine, now you have possibly 4096 jobs going, and it surfaces bugs that might not have been a problem when we had 2-4 cores (oh no, make is running 16 jobs at once, the horror).There could also be other problems, like not being able to assign jobs correctly with that many threads. I’ve encountered this when assigning more jobs than there are threads on the rig. A lot more, like 2x, 4x the threads available.
so without an argument to -j the number of concurrent jobs is unbounded???
Yep.
It’s a “heck, here’s all I got” switch.
Remember, with great power comes great responsibility 😉.