GETRLIMIT(2)

NAME
getrlimit, setrlimit - control maximum system resource consumption

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <sys/resource.h>
int
getrlimit(int resource, struct rlimit *rlp);
int
setrlimit(int resource, const struct rlimit *rlp);

DESCRIPTION
Limits on the consumption of system resources by the current process and each process it creates may be obtained with the getrlimit() system call, and set with the setrlimit() system call.

The resource argument is one of the following:

RLIMIT_AS
    The maximum amount (in bytes) of virtual memory the process is allowed to map.

RLIMIT_CORE
    The largest size (in bytes) core(5) file that may be created.

RLIMIT_CPU
    The maximum amount of cpu time (in seconds) to be used by each process.

RLIMIT_DATA
    The maximum size (in bytes) of the data segment for a process; this defines how far a program may extend its break with the sbrk(2) function.

RLIMIT_FSIZE
    The largest size (in bytes) file that may be created.

RLIMIT_KQUEUES
    The maximum number of kqueues this user id is allowed to create.

RLIMIT_MEMLOCK
    The maximum size (in bytes) which a process may lock into memory using the mlock(2) system call.

RLIMIT_NOFILE
    The maximum number of open files for this process.

RLIMIT_NPROC
    The maximum number of simultaneous processes for this user id.

RLIMIT_NPTS
    The maximum number of pseudo-terminals this user id is allowed to create.

RLIMIT_PIPEBUF
    The maximum total size of in-kernel buffers for bi-directional pipes/fifos that this user id is allowed to consume. The buffers for kernel FIFOs created on the first open of a filesystem object created by (mkfifo(2)) are also charged to the user ID of the process opening it, not the FIFO's filesystem owner. Despite somewhat unexpected, this is in fact fair, since user of the fifo is not necessary its creator.

RLIMIT_RSS
    When there is memory pressure and swap is available, prioritize eviction of a process' resident pages beyond this amount (in bytes). When memory is not under pressure, this rlimit is effectively ignored. Even when there is memory pressure, the amount of available swap space and some sysctl settings like vm.swap_enabled and vm.swap_idle_enabled can affect what happens to processes that have exceeded this size.

    Processes that exceed their set RLIMIT_RSS are not signalled or halted. The limit is merely a hint to the VM daemon to prefer to deactivate pages from processes that have exceeded their set RLIMIT_RSS.

RLIMIT_SBSIZE
    The maximum size (in bytes) of socket buffer usage for this user. This limits the amount of network memory, and hence the amount of mbufs, that this user may hold at any time.

RLIMIT_STACK
    The maximum size (in bytes) of the stack segment for a process; this defines how far a program's stack segment may be extended. Stack extension is performed automatically by the system.

RLIMIT_SWAP
    The maximum size (in bytes) of the swap space that may be reserved or used by all of this user id's processes. This limit is enforced only if bit 1 of the vm.overcommit sysctl is set. Please see tuning(7) for a complete description of this sysctl.

RLIMIT_UMTXP
    The limit of the number of process-shared posix thread library objects allocated by user id.

RLIMIT_VMEM
    An alias for RLIMIT_AS.

A resource limit is specified as a soft limit and a hard limit. When a soft limit is exceeded, a process might or might not receive a signal. For example, signals are generated when the cpu time or file size is exceeded, but not if the address space or RSS limit is exceeded. A program that exceeds the soft limit is allowed to continue execution until it reaches the hard limit, or modifies its own resource limit. Even reaching the hard limit does not necessarily halt a process. For example, if the RSS hard limit is exceeded, nothing happens.

The rlimit structure is used to specify the hard and soft limits on a resource.

    struct rlimit {
    	rlim_t	rlim_cur;	/* current (soft) limit */
    	rlim_t	rlim_max;	/* maximum value for rlim_cur */
    };

Only the super-user may raise the maximum limits. Other users may only alter rlim_cur within the range from 0 to rlim_max or (irreversibly) lower rlim_max.

An “infinite” value for a limit is defined as RLIM_INFINITY.

Because this information is stored in the per-process information, this system call must be executed directly by the shell if it is to affect all future processes created by the shell; limit is thus a built-in command to csh(1).

The system refuses to extend the data or stack space when the limits would be exceeded in the normal way: a brk(2) function fails if the data space limit is reached. When the stack limit is reached, the process receives a segmentation fault (SIGSEGV); if this signal is not caught by a handler using the signal stack, this signal will kill the process.

A file I/O operation that would create a file larger that the process' soft limit will cause the write to fail and a signal SIGXFSZ to be generated; this normally terminates the process, but may be caught. When the soft cpu time limit is exceeded, a SIGXCPU signal is sent to the offending process.

When most operations would allocate more virtual memory than allowed by the soft limit of RLIMIT_AS, the operation fails with ENOMEM and no signal is raised. A notable exception is stack extension, described above. If stack extension would allocate more virtual memory than allowed by the soft limit of RLIMIT_AS, a SIGSEGV signal will be delivered. The caller is free to raise the soft address space limit up to the hard limit and retry the allocation.

RETURN VALUES

ERRORS
The getrlimit() and setrlimit() system calls will fail if:

[EFAULT]
    The address specified for rlp is invalid.

[EPERM]
    The limit specified to setrlimit() would have raised the maximum limit value, and the caller is not the super-user.

SEE ALSO
csh(1), quota(1), quotactl(2), sigaction(2), sigaltstack(2), sysctl(3), ulimit(3)

HISTORY
The getrlimit() system call appeared in 4.2BSD.