The getrlimit() and setrlimit() system calls get and set resource
limits. Each resource has an associated soft and hard limit, as
defined by the rlimit structure:
struct rlimit {
rlim_t rlim_cur; /* Soft limit */
rlim_t rlim_max; /* Hard limit (ceiling for rlim_cur) */
};
The soft limit is the value that the kernel enforces for the
corresponding resource. The hard limit acts as a ceiling for the
soft limit: an unprivileged process may set only its soft limit
to a value in the range from 0 up to the hard limit, and
(irreversibly) lower its hard limit. A privileged process (under
Linux: one with the CAP_SYS_RESOURCE capability in the initial
user namespace) may make arbitrary changes to either limit value.
The value RLIM_INFINITY denotes no limit on a resource (both in
the structure returned by getrlimit() and in the structure passed
to setrlimit()).
The resource argument must be one of:
RLIMIT_AS
This is the maximum size of the process's virtual memory
(address space). The limit is specified in bytes, and is
rounded down to the system page size. This limit affects
calls to brk(2), mmap(2), and mremap(2), which fail with
the error ENOMEM upon exceeding this limit. In addition,
automatic stack expansion fails (and generates a SIGSEGV
that kills the process if no alternate stack has been made
available via sigaltstack(2)). Since the value is a long,
on machines with a 32-bit long either this limit is at
most 2 GiB, or this resource is unlimited.
RLIMIT_CORE
This is the maximum size of a core file (see core(5)) in
bytes that the process may dump. When 0 no core dump
files are created. When nonzero, larger dumps are
truncated to this size.
RLIMIT_CPU
This is a limit, in seconds, on the amount of CPU time
that the process can consume. When the process reaches
the soft limit, it is sent a SIGXCPU signal. The default
action for this signal is to terminate the process.
However, the signal can be caught, and the handler can
return control to the main program. If the process
continues to consume CPU time, it will be sent SIGXCPU
once per second until the hard limit is reached, at which
time it is sent SIGKILL. (This latter point describes
Linux behavior. Implementations vary in how they treat
processes which continue to consume CPU time after
reaching the soft limit. Portable applications that need
to catch this signal should perform an orderly termination
upon first receipt of SIGXCPU.)
RLIMIT_DATA
This is the maximum size of the process's data segment
(initialized data, uninitialized data, and heap). The
limit is specified in bytes, and is rounded down to the
system page size. This limit affects calls to brk(2),
sbrk(2), and (since Linux 4.7) mmap(2), which fail with
the error ENOMEM upon encountering the soft limit of this
resource.
RLIMIT_FSIZE
This is the maximum size in bytes of files that the
process may create. Attempts to extend a file beyond this
limit result in delivery of a SIGXFSZ signal. By default,
this signal terminates a process, but a process can catch
this signal instead, in which case the relevant system
call (e.g., write(2), truncate(2)) fails with the error
EFBIG.
RLIMIT_LOCKS (Linux 2.4.0 to 2.4.24)
This is a limit on the combined number of flock(2) locks
and fcntl(2) leases that this process may establish.
RLIMIT_MEMLOCK
This is the maximum number of bytes of memory that may be
locked into RAM. This limit is in effect rounded down to
the nearest multiple of the system page size. This limit
affects mlock(2), mlockall(2), and the mmap(2) MAP_LOCKED
operation. Since Linux 2.6.9, it also affects the
shmctl(2) SHM_LOCK operation, where it sets a maximum on
the total bytes in shared memory segments (see shmget(2))
that may be locked by the real user ID of the calling
process. The shmctl(2) SHM_LOCK locks are accounted for
separately from the per-process memory locks established
by mlock(2), mlockall(2), and mmap(2) MAP_LOCKED; a
process can lock bytes up to this limit in each of these
two categories.
In Linux kernels before 2.6.9, this limit controlled the
amount of memory that could be locked by a privileged
process. Since Linux 2.6.9, no limits are placed on the
amount of memory that a privileged process may lock, and
this limit instead governs the amount of memory that an
unprivileged process may lock.
RLIMIT_MSGQUEUE (since Linux 2.6.8)
This is a limit on the number of bytes that can be
allocated for POSIX message queues for the real user ID of
the calling process. This limit is enforced for
mq_open(3). Each message queue that the user creates
counts (until it is removed) against this limit according
to the formula:
Since Linux 3.5:
bytes = attr.mq_maxmsg * sizeof(struct msg_msg) +
min(attr.mq_maxmsg, MQ_PRIO_MAX) *
sizeof(struct posix_msg_tree_node)+
/* For overhead */
attr.mq_maxmsg * attr.mq_msgsize;
/* For message data */
Linux 3.4 and earlier:
bytes = attr.mq_maxmsg * sizeof(struct msg_msg *) +
/* For overhead */
attr.mq_maxmsg * attr.mq_msgsize;
/* For message data */
where attr is the mq_attr structure specified as the
fourth argument to mq_open(3), and the msg_msg and
posix_msg_tree_node structures are kernel-internal
structures.
The "overhead" addend in the formula accounts for overhead
bytes required by the implementation and ensures that the
user cannot create an unlimited number of zero-length
messages (such messages nevertheless each consume some
system memory for bookkeeping overhead).
RLIMIT_NICE (since Linux 2.6.12, but see BUGS below)
This specifies a ceiling to which the process's nice value
can be raised using setpriority(2) or nice(2). The actual
ceiling for the nice value is calculated as 20 - rlim_cur.
The useful range for this limit is thus from 1
(corresponding to a nice value of 19) to 40 (corresponding
to a nice value of -20). This unusual choice of range was
necessary because negative numbers cannot be specified as
resource limit values, since they typically have special
meanings. For example, RLIM_INFINITY typically is the
same as -1. For more detail on the nice value, see
sched(7).
RLIMIT_NOFILE
This specifies a value one greater than the maximum file
descriptor number that can be opened by this process.
Attempts (open(2), pipe(2), dup(2), etc.) to exceed this
limit yield the error EMFILE. (Historically, this limit
was named RLIMIT_OFILE on BSD.)
Since Linux 4.5, this limit also defines the maximum
number of file descriptors that an unprivileged process
(one without the CAP_SYS_RESOURCE capability) may have "in
flight" to other processes, by being passed across UNIX
domain sockets. This limit applies to the sendmsg(2)
system call. For further details, see unix(7).
RLIMIT_NPROC
This is a limit on the number of extant process (or, more
precisely on Linux, threads) for the real user ID of the
calling process. So long as the current number of
processes belonging to this process's real user ID is
greater than or equal to this limit, fork(2) fails with
the error EAGAIN.
The RLIMIT_NPROC limit is not enforced for processes that
have either the CAP_SYS_ADMIN or the CAP_SYS_RESOURCE
capability.
RLIMIT_RSS
This is a limit (in bytes) on the process's resident set
(the number of virtual pages resident in RAM). This limit
has effect only in Linux 2.4.x, x < 30, and there affects
only calls to madvise(2) specifying MADV_WILLNEED.
RLIMIT_RTPRIO (since Linux 2.6.12, but see BUGS)
This specifies a ceiling on the real-time priority that
may be set for this process using sched_setscheduler(2)
and sched_setparam(2).
For further details on real-time scheduling policies, see
sched(7)
RLIMIT_RTTIME (since Linux 2.6.25)
This is a limit (in microseconds) on the amount of CPU
time that a process scheduled under a real-time scheduling
policy may consume without making a blocking system call.
For the purpose of this limit, each time a process makes a
blocking system call, the count of its consumed CPU time
is reset to zero. The CPU time count is not reset if the
process continues trying to use the CPU but is preempted,
its time slice expires, or it calls sched_yield(2).
Upon reaching the soft limit, the process is sent a
SIGXCPU signal. If the process catches or ignores this
signal and continues consuming CPU time, then SIGXCPU will
be generated once each second until the hard limit is
reached, at which point the process is sent a SIGKILL
signal.
The intended use of this limit is to stop a runaway real-
time process from locking up the system.
For further details on real-time scheduling policies, see
sched(7)
RLIMIT_SIGPENDING (since Linux 2.6.8)
This is a limit on the number of signals that may be
queued for the real user ID of the calling process. Both
standard and real-time signals are counted for the purpose
of checking this limit. However, the limit is enforced
only for sigqueue(3); it is always possible to use kill(2)
to queue one instance of any of the signals that are not
already queued to the process.
RLIMIT_STACK
This is the maximum size of the process stack, in bytes.
Upon reaching this limit, a SIGSEGV signal is generated.
To handle this signal, a process must employ an alternate
signal stack (sigaltstack(2)).
Since Linux 2.6.23, this limit also determines the amount
of space used for the process's command-line arguments and
environment variables; for details, see execve(2).
prlimit()
The Linux-specific prlimit() system call combines and extends the
functionality of setrlimit() and getrlimit(). It can be used to
both set and get the resource limits of an arbitrary process.
The resource argument has the same meaning as for setrlimit() and
getrlimit().
If the new_limit argument is a not NULL, then the rlimit
structure to which it points is used to set new values for the
soft and hard limits for resource. If the old_limit argument is
a not NULL, then a successful call to prlimit() places the
previous soft and hard limits for resource in the rlimit
structure pointed to by old_limit.
The pid argument specifies the ID of the process on which the
call is to operate. If pid is 0, then the call applies to the
calling process. To set or get the resources of a process other
than itself, the caller must have the CAP_SYS_RESOURCE capability
in the user namespace of the process whose resource limits are
being changed, or the real, effective, and saved set user IDs of
the target process must match the real user ID of the caller and
the real, effective, and saved set group IDs of the target
process must match the real group ID of the caller.
