getrlimit(2) System Calls Manual getrlimit(2)
NAME
getrlimit, setrlimit, prlimit - get/set resource limits
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <sys/resource.h>
int getrlimit(int resource, struct rlimit *rlim);
int setrlimit(int resource, const struct rlimit *rlim);
int prlimit(pid_t pid, int resource,
const struct rlimit *_Nullable new_limit,
struct rlimit *_Nullable old_limit);
struct rlimit {
rlim_t rlim_cur; /* Soft limit */
rlim_t rlim_max; /* Hard limit (ceiling for rlim_cur) */
};
typedef /* ... */ rlim_t; /* Unsigned integer type */
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
prlimit():
_GNU_SOURCE
DESCRIPTION
The getrlimit() and setrlimit() system calls get and set resource lim-
its. Each resource has an associated soft and hard limit, as defined by
the rlimit structure.
The soft limit is the value that the kernel enforces for the correspond-
ing 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 capabil-
ity 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 setr-
limit()).
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 ex-
ceeding 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 cre-
ated. 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 ap-
plications that need to catch this signal should perform an or-
derly termination upon first receipt of SIGXCPU.)
RLIMIT_DATA
This is the maximum size of the process's data segment (initial-
ized 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 Linux 2.4.24)
This is a limit on the combined number of flock(2) locks and fc-
ntl(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 sepa-
rately 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.
Before Linux 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 argu-
ment 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 descrip-
tor 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 de-
tails, see unix(7).
RLIMIT_NPROC
This is a limit on the number of extant process (or, more pre-
cisely 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, or
run with real user ID 0.
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 mad-
vise(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_set-
param(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 con-
sume 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 sig-
nal. If the process catches or ignores this signal and continues
consuming CPU time, then SIGXCPU will be generated once each sec-
ond 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 sig-
nals 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 environ-
ment variables; for details, see execve(2).
prlimit()
The Linux-specific prlimit() system call combines and extends the func-
tionality 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 getr-
limit().
If the new_limit argument is 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 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.
RETURN VALUE
On success, these system calls return 0. On error, -1 is returned, and
errno is set to indicate the error.
ERRORS
EFAULT A pointer argument points to a location outside the accessible
address space.
EINVAL The value specified in resource is not valid; or, for setrlimit()
or prlimit(): rlim->rlim_cur was greater than rlim->rlim_max.
EPERM An unprivileged process tried to raise the hard limit; the
CAP_SYS_RESOURCE capability is required to do this.
EPERM The caller tried to increase the hard RLIMIT_NOFILE limit above
the maximum defined by /proc/sys/fs/nr_open (see proc(5))
EPERM (prlimit()) The calling process did not have permission to set
limits for the process specified by pid.
ESRCH Could not find a process with the ID specified in pid.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│ Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│ getrlimit(), setrlimit(), prlimit() │ Thread safety │ MT-Safe │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
getrlimit()
setrlimit()
POSIX.1-2008.
prlimit()
Linux.
RLIMIT_MEMLOCK and RLIMIT_NPROC derive from BSD and are not specified in
POSIX.1; they are present on the BSDs and Linux, but on few other imple-
mentations. RLIMIT_RSS derives from BSD and is not specified in
POSIX.1; it is nevertheless present on most implementations.
RLIMIT_MSGQUEUE, RLIMIT_NICE, RLIMIT_RTPRIO, RLIMIT_RTTIME, and
RLIMIT_SIGPENDING are Linux-specific.
HISTORY
getrlimit()
setrlimit()
POSIX.1-2001, SVr4, 4.3BSD.
prlimit()
Linux 2.6.36, glibc 2.13.
NOTES
A child process created via fork(2) inherits its parent's resource lim-
its. Resource limits are preserved across execve(2).
Resource limits are per-process attributes that are shared by all of the
threads in a process.
Lowering the soft limit for a resource below the process's current con-
sumption of that resource will succeed (but will prevent the process
from further increasing its consumption of the resource).
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.
Since Linux 2.6.24, the resource limits of any process can be inspected
via /proc/pid/limits; see proc(5).
Ancient systems provided a vlimit() function with a similar purpose to
setrlimit(). For backward compatibility, glibc also provides vlimit().
All new applications should be written using setrlimit().
C library/kernel ABI differences
Since glibc 2.13, the glibc getrlimit() and setrlimit() wrapper func-
tions no longer invoke the corresponding system calls, but instead em-
ploy prlimit(), for the reasons described in BUGS.
The name of the glibc wrapper function is prlimit(); the underlying sys-
tem call is prlimit64().
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
Linux 2.6.8.
In Linux 2.6.x kernels before Linux 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 Linux 2.6.12; the
problem is fixed in Linux 2.6.13.
In Linux 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 the actual ceiling for the nice value was calculated as
19 - rlim_cur. This was fixed in Linux 2.6.13.
Since Linux 2.6.12, if a process reaches its soft RLIMIT_CPU limit and
has a handler installed for SIGXCPU, then, in addition to invoking the
signal handler, the kernel increases the soft limit by one second. This
behavior repeats if the process continues to consume CPU time, until the
hard limit is reached, at which point the process is killed. Other im-
plementations do not change the RLIMIT_CPU soft limit in this manner,
and the Linux behavior is probably not standards conformant; portable
applications should avoid relying on this Linux-specific behavior. The
Linux-specific RLIMIT_RTTIME limit exhibits the same behavior when the
soft limit is encountered.
Kernels before Linux 2.4.22 did not diagnose the error EINVAL for setr-
limit() when rlim->rlim_cur was greater than rlim->rlim_max.
Linux doesn't return an error when an attempt to set RLIMIT_CPU has
failed, for compatibility reasons.
Representation of "large" resource limit values on 32-bit platforms
The glibc getrlimit() and setrlimit() wrapper functions use a 64-bit
rlim_t data type, even on 32-bit platforms. However, the rlim_t data
type used in the getrlimit() and setrlimit() system calls is a (32-bit)
unsigned long. Furthermore, in Linux, the kernel represents resource
limits on 32-bit platforms as unsigned long. However, a 32-bit data
type is not wide enough. The most pertinent limit here is RLIMIT_FSIZE,
which specifies the maximum size to which a file can grow: to be useful,
this limit must be represented using a type that is as wide as the type
used to represent file offsets—that is, as wide as a 64-bit off_t (as-
suming a program compiled with _FILE_OFFSET_BITS=64).
To work around this kernel limitation, if a program tried to set a re-
source limit to a value larger than can be represented in a 32-bit un-
signed long, then the glibc setrlimit() wrapper function silently con-
verted the limit value to RLIM_INFINITY. In other words, the requested
resource limit setting was silently ignored.
Since glibc 2.13, glibc works around the limitations of the getrlimit()
and setrlimit() system calls by implementing setrlimit() and getrlimit()
as wrapper functions that call prlimit().
EXAMPLES
The program below demonstrates the use of prlimit().
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <err.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/resource.h>
#include <time.h>
int
main(int argc, char *argv[])
{
pid_t pid;
struct rlimit old, new;
struct rlimit *newp;
if (!(argc == 2 || argc == 4)) {
fprintf(stderr, "Usage: %s <pid> [<new-soft-limit> "
"<new-hard-limit>]\n", argv[0]);
exit(EXIT_FAILURE);
}
pid = atoi(argv[1]); /* PID of target process */
newp = NULL;
if (argc == 4) {
new.rlim_cur = atoi(argv[2]);
new.rlim_max = atoi(argv[3]);
newp = &new;
}
/* Set CPU time limit of target process; retrieve and display
previous limit */
if (prlimit(pid, RLIMIT_CPU, newp, &old) == -1)
err(EXIT_FAILURE, "prlimit-1");
printf("Previous limits: soft=%jd; hard=%jd\n",
(intmax_t) old.rlim_cur, (intmax_t) old.rlim_max);
/* Retrieve and display new CPU time limit */
if (prlimit(pid, RLIMIT_CPU, NULL, &old) == -1)
err(EXIT_FAILURE, "prlimit-2");
printf("New limits: soft=%jd; hard=%jd\n",
(intmax_t) old.rlim_cur, (intmax_t) old.rlim_max);
exit(EXIT_SUCCESS);
}
SEE ALSO
prlimit(1), dup(2), fcntl(2), fork(2), getrusage(2), mlock(2), mmap(2),
open(2), quotactl(2), sbrk(2), shmctl(2), malloc(3), sigqueue(3),
ulimit(3), core(5), capabilities(7), cgroups(7), credentials(7), sig-
nal(7)
Linux man-pages 6.9.1 2024-06-17 getrlimit(2)
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