NAME
getrlimit, setrlimit - get/set resource limits
SYNOPSIS
#include <sys/time.h>
#include <sys/resource.h>
int getrlimit(int resource, struct rlimit *rlim);
int setrlimit(int resource, const struct rlimit *rlim);
DESCRIPTION
getrlimit() and setrlimit() get and set resource limits respectively.
Each resource has an associated soft and hard limit, as defined by the
rlimit structure (the rlim argument to both getrlimit() and
setrlimit()):
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 only set 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) 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()).
resource must be one of:
RLIMIT_AS
The maximum size of the process’s virtual memory (address space)
in bytes. This limit affects calls to brk(2), mmap(2) and
mremap(2), which fail with the error ENOMEM upon exceeding this
limit. Also automatic stack expansion will fail (and generate 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
Maximum size of core file. When 0 no core dump files are
created. When nonzero, larger dumps are truncated to this size.
RLIMIT_CPU
CPU time limit in seconds. 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 2.2 through 2.6 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
The maximum size of the process’s data segment (initialized
data, uninitialized data, and heap). This limit affects calls
to brk(2) and sbrk(2), which fail with the error ENOMEM upon
encountering the soft limit of this resource.
RLIMIT_FSIZE
The maximum size 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 (Early Linux 2.4 only)
A limit on the combined number of flock(2) locks and fcntl(2)
leases that this process may establish.
RLIMIT_MEMLOCK
The maximum number of bytes of memory that may be locked into
RAM. In effect this limit is rounded down to the nearest
multiple of the system page size. This limit affects mlock(2)
and 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)
Specifies the 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:
bytes = attr.mq_maxmsg * sizeof(struct msg_msg *) +
attr.mq_maxmsg * attr.mq_msgsize
where attr is the mq_attr structure specified as the fourth
argument to mq_open(3).
The first addend in the formula, which includes sizeof(struct
msg_msg *) (4 bytes on Linux/i386), 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)
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. (This
strangeness occurs 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.)
RLIMIT_NOFILE
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.)
RLIMIT_NPROC
The maximum number of processes (or, more precisely on Linux,
threads) that can be created for the real user ID of the calling
process. Upon encountering this limit, fork(2) fails with the
error EAGAIN.
RLIMIT_RSS
Specifies the limit (in pages) of the process’s resident set
(the number of virtual pages resident in RAM). This limit only
has effect in Linux 2.4.x, x < 30, and there only affects calls
to madvise(2) specifying MADV_WILLNEED.
RLIMIT_RTPRIO (Since Linux 2.6.12, but see BUGS)
Specifies a ceiling on the real-time priority that may be set
for this process using sched_setscheduler(2) and
sched_setparam(2).
RLIMIT_RTTIME (Since Linux 2.6.25)
Specifies a limit 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.
RLIMIT_SIGPENDING (Since Linux 2.6.8)
Specifies the 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 only enforced for sigqueue(2); 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
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).
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
EFAULT rlim points outside the accessible address space.
EINVAL resource is not valid; or, for setrlimit(): rlim->rlim_cur was
greater than rlim->rlim_max.
EPERM An unprivileged process tried to use setrlimit() to increase a
soft or hard limit above the current hard limit; the
CAP_SYS_RESOURCE capability is required to do this. Or, the
process tried to use setrlimit() to increase the soft or hard
RLIMIT_NOFILE limit above the current kernel maximum (NR_OPEN).
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001. RLIMIT_MEMLOCK and RLIMIT_NPROC derive
from BSD and are not specified in POSIX.1-2001; they are present on the
BSDs and Linux, but on few other implementations. RLIMIT_RSS derives
from BSD and is not specified in POSIX.1-2001; it is nevertheless
present on most implementations. RLIMIT_MSGQUEUE, RLIMIT_NICE,
RLIMIT_RTPRIO, RLIMIT_RTTIME, and RLIMIT_SIGPENDING are Linux-specific.
NOTES
A child process created via fork(2) inherits its parent’s resource
limits. Resource limits are preserved across execve(2).
One can set the resource limits of the shell using the built-in ulimit
command (limit in csh(1)). The shell’s resource limits are inherited
by the processes that it creates to execute commands.
BUGS
In older Linux kernels, the SIGXCPU and SIGKILL signals delivered when
a process encountered the soft and hard RLIMIT_CPU limits were
delivered one (CPU) second later than they should have been. This was
fixed in kernel 2.6.8.
In 2.6.x kernels before 2.6.17, a RLIMIT_CPU limit of 0 is wrongly
treated as "no limit" (like RLIM_INFINITY). Since Linux 2.6.17,
setting a limit of 0 does have an effect, but is actually treated as a
limit of 1 second.
A kernel bug means that RLIMIT_RTPRIO does not work in kernel 2.6.12;
the problem is fixed in kernel 2.6.13.
In kernel 2.6.12, there was an off-by-one mismatch between the priority
ranges returned by getpriority(2) and RLIMIT_NICE. This had the effect
that actual ceiling for the nice value was calculated as 19 - rlim_cur.
This was fixed in kernel 2.6.13.
Kernels before 2.4.22 did not diagnose the error EINVAL for setrlimit()
when rlim->rlim_cur was greater than rlim->rlim_max.
SEE ALSO
dup(2), fcntl(2), fork(2), getrusage(2), mlock(2), mmap(2), open(2),
quotactl(2), sbrk(2), shmctl(2), sigqueue(2), malloc(3), ulimit(3),
core(5), capabilities(7), signal(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.