Ubuntu Manpage: getrlimit, setrlimit, prlimit

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);

   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 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 Linux 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.

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 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, 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 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 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.

VERSIONS

       The  prlimit()  system call is available since Linux 2.6.36.  Library support is available
       since glibc 2.13.

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.1-2001, 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.

NOTES

       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().

   C library/kernel ABI differences
       Since glibc 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().

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 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 Linux 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.

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 (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 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);
       }