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 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 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).
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!
so without an argument to -j the number of concurrent jobs is unbounded???
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).