RLIMITNPROC

RLIMIT_NPROC is a per-user resource limit that governs the maximum number of processes a single user may have running on a system. It is a limit type used with the getrlimit and setrlimit interfaces and is represented in the kernel by both a soft limit (rlim_cur) and a hard limit (rlim_max). The soft limit is what the kernel enforces at run time, while the hard limit sets the ceiling for possible soft-limit values and can only be raised by privileged processes.

Enforcement and behavior: When a process attempts to create a new process via fork, vfork, or clone, the kernel checks how many processes are currently associated with the real user ID. If the addition would exceed the allowed total, the creation fails with the error code EAGAIN. If the soft limit is not exceeded, process creation proceeds. A process may increase its soft limit up to the hard limit using setrlimit, typically requiring appropriate privileges to raise the hard limit. An unlimited limit is expressed as RLIM_INFINITY.

Usage considerations: RLIMIT_NPROC serves to prevent fork bombs and excessive resource use by a single user. In Linux, threads are counted as separate tasks, so a high thread count can contribute to reaching the limit. Root or privileged processes may experience different behavior depending on system configuration and capabilities.

Container and virtualization: In containerized environments, the per-user limit interacts with container cgroups and namespace isolation. Some runtimes also impose their own limits, which may override or supplement host limits to restrict process creation inside containers.

See also: getrlimit, setrlimit, RLIMIT_CPU, RLIMIT_AS.