setrlimit

Setrlimit is a POSIX-compatible API that sets resource limits for the calling process. It updates the soft limit (rlim_cur) and hard limit (rlim_max) for a specified resource, using a struct rlimit. The rlimit structure contains rlim_cur and rlim_max, both of type rlim_t. The resource parameter selects which limit to adjust, with common examples including RLIMIT_CPU (CPU time), RLIMIT_FSIZE (file size), RLIMIT_DATA (data segment size), RLIMIT_STACK (stack size), RLIMIT_CORE (core file size), RLIMIT_AS (address space), RLIMIT_NOFILE (number of open files), RLIMIT_NPROC (number of processes), and RLIMIT_MEMLOCK or RLIMIT_RSS on some systems.

Operation and effects: The soft limit is the enforcement threshold seen by the kernel at runtime, while the hard limit is the ceiling that the soft limit may be raised to. A process can increase its soft limit up to the hard limit using setrlimit, but increasing the hard limit generally requires elevated privileges (such as appropriate capabilities). A process can lower either limit without special privileges. Limits are inherited by child processes, which may independently adjust them within allowed bounds.

Behavior on limit breach: If a process attempts to exceed a limit, the kernel may take an action such as sending a signal or terminating the process, depending on the resource and system policy (for example, CPU time exceeding RLIMIT_CPU may trigger a SIGXCPU).

Usage notes: setrlimit is declared in header <sys/resource.h> and is commonly used for resource protection, sandboxing, or daemon startup to bound resource consumption. The function returns 0 on success and -1 on error, with errno describing the failure. Related functionality includes getrlimit, which retrieves current limits.