getrlimit, setrlimit, prlimit - get/set resource limits
#include <sys/time.h>
#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 *new_limit,
struct rlimit *old_limit);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
prlimit(): _GNU_SOURCE
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 (early Linux 2.4 only)
- 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).
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.
On success, these system calls return 0. On error, -1 is returned, and
errno is set appropriately.
- 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.
The
prlimit() system call is available since Linux 2.6.36. Library
support is available since glibc 2.13.
For an explanation of the terms used in this section, see
attributes(7).
| Interface |
Attribute |
Value |
| getrlimit (), setrlimit (), prlimit () |
Thread safety |
MT-Safe |
getrlimit(),
setrlimit(): POSIX.1-2001, POSIX.1-2008, SVr4,
4.3BSD.
prlimit(): Linux-specific.
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
implementations.
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.
A child process created via
fork(2) inherits its parent's resource
limits. 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 consumption
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().
Since version 2.13, the glibc
getrlimit() and
setrlimit() wrapper
functions no longer invoke the corresponding system calls, but instead employ
prlimit(), for the reasons described in BUGS.
The name of the glibc wrapper function is
prlimit(); the underlying
system call is
prlimit64().
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 the actual ceiling for the nice value was calculated as
19 - rlim_cur. This was fixed in kernel 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 implementations
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 2.4.22 did not diagnose the error
EINVAL for
setrlimit() 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.
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 (assuming a program compiled with
_FILE_OFFSET_BITS=64).
To work around this kernel limitation, if a program tried to set a resource
limit to a value larger than can be represented in a 32-bit
unsigned
long, then the glibc
setrlimit() wrapper function silently
converted the limit value to
RLIM_INFINITY. In other words, the
requested resource limit setting was silently ignored.
Since version 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().
The program below demonstrates the use of
prlimit().
#define _GNU_SOURCE
#define _FILE_OFFSET_BITS 64
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/resource.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \
} while (0)
int
main(int argc, char *argv[])
{
struct rlimit old, new;
struct rlimit *newp;
pid_t pid;
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)
errExit("prlimit-1");
printf("Previous limits: soft=%lld; hard=%lld\n",
(long long) old.rlim_cur, (long long) old.rlim_max);
/* Retrieve and display new CPU time limit */
if (prlimit(pid, RLIMIT_CPU, NULL, &old) == -1)
errExit("prlimit-2");
printf("New limits: soft=%lld; hard=%lld\n",
(long long) old.rlim_cur, (long long) old.rlim_max);
exit(EXIT_SUCCESS);
}
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),
signal(7)