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I want to call the ulimit shell command through a C program. right now, I have: 

execlp("ulimit", "ulimit", "-u", "%d", ulimit_a);

It compiles fine, but does not actually edit the user-id max process value when I later check it with ulimit -a (both the dash and bash version). The same was happening with system() as well, yet this seems to be a known issue for system()

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  • And execlp() is not a system call, and you're lucky if that call works at all instead of crashing (as expected) -- the argument list of execlp should be terminated by a null pointer. Commented May 5, 2020 at 0:16

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The ulimit shell command only applies to the current shell and its descendants. It's a built-in not a separate command.

You cannot start a separate shell process, run ulimit in it, and expect it to have any effect on the parent process.

From C you should use the getrlimit(2) and setrlimit(2) functions to get and set the resource limits for the current process and its descendants. Though, judging by your approach, I guess they won't do what you expect them to do, either.

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  • Is there a way to limit the maximum amount of processes a user can make permanently? IE not getrlimit/setrlimit because, like you said, those effect descendants of the process. I basically just want to make an interface for the root user to manage this. Commented May 5, 2020 at 2:01
  • @Connor Setting resource limits persistently (not "permanently", there is no such thing) is a completely different question that you may want to ask separately. Commented May 5, 2020 at 6:55
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Take a look at pam_limits, a PAM (Pluggable Authentication Modules) module for setting resource limits when a user session is created. You will have to configure PAM to use this module, and then you will have to configure the limits in /etc/security/limits.d/. See https://linux.die.net/man/8/pam_limits for more information.

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