RLIMITs

RLIMITs, or resource limits, are a mechanism in Unix-like operating systems to constrain the resources that a process may use. They are enforced on a per-process basis and are inherited by child processes. The limits are accessed with the getrlimit and setrlimit interfaces and are stored in a struct with two values: rlim_cur (the soft limit) and rlim_max (the hard limit). The soft limit is the current enforcement level, while the hard limit is the maximum value that the soft limit may be raised to. A process can decrease its soft limit, and it can raise it only up to the hard limit; increasing the hard limit generally requires elevated privileges.

Common RLIMITs control a range of resources. CPU time is limited with RLIMIT_CPU; file sizes with RLIMIT_FSIZE; data segment size with RLIMIT_DATA; stack size with RLIMIT_STACK; core file size with RLIMIT_CORE. Other limits include the number of open file descriptors (RLIMIT_NOFILE), the number of processes for a user (RLIMIT_NPROC), address space or virtual memory (RLIMIT_AS), memory locked (RLIMIT_MEMLOCK), and various IPC-related limits such as RLIMIT_MSGQUEUE and RLIMIT_SIGPENDING. Some systems also define RLIMIT_NICE (affecting scheduling priority) and RLIMIT_RTPRIO. A special value, RLIM_INFINITY, indicates no limit.

Behavior on limit violations varies by resource. When the soft limit is reached, the kernel may deliver a signal (for example SIGXCPU for CPU time or SIGXFSZ for file size). If the limit continues to be exceeded or the signal is handled in a certain way, the process may be terminated.

Limits can be viewed or changed at runtime by applications using setrlimit, or at the shell level with tools like ulimit. The limits apply to the process and are inherited by its children, providing a mechanism to enforce system stability and resource fairness.