rlimit

rlimit refers to the resource limit mechanism used by POSIX-compliant operating systems to cap a process's consumption of certain resources. It is accessed through the getrlimit and setrlimit system calls and uses a structure called struct rlimit, which stores two values: rlim_cur (the soft limit) and rlim_max (the hard limit). The soft limit is the current enforcement threshold, while the hard limit acts as a ceiling that soft limits cannot exceed. The hard limit can typically be raised only by privileged processes.

Resources are identified by constants named RLIMIT_*, with common examples including RLIMIT_CPU (CPU time), RLIMIT_FSIZE (maximum file size), RLIMIT_DATA (data segment size), RLIMIT_STACK (stack size), RLIMIT_CORE (core file size), RLIMIT_NOFILE (maximum number of open file descriptors), and RLIMIT_AS (address space). RLIMIT_NPROC restricts the number of processes for a user. Limits are inherited by child processes created via fork and persist across exec unless changed.

A process may lower its soft limit at any time; raising a soft limit up to the hard limit is typically allowed, but increasing the hard limit itself generally requires appropriate privileges (such as root or capabilities like CAP_SYS_RESOURCE).

Enforcement varies by resource and system. When a soft CPU time limit is reached, the process receives a signal (often SIGXCPU); depending on handling, the process may continue until the hard limit is reached, at which point it may be terminated. For other resources, violations may result in signals, errors, or termination, depending on the limit type and kernel behavior.

rlimit is used for resource control, sandboxing, and system stability, enabling administrators and applications to prevent a single process from exhausting system resources.