Manpage of GETRLIMIT

GETRLIMIT

Section: Linux Programmer's Manual (2)
Updated: 2016-10-08
Index
 

NAME

getrlimit, setrlimit, prlimit - 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);

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  

DESCRIPTION

The getrlimit() and setrlimit() system calls get and set resource limits respectively. Each resource has an associated soft and hard limit, as defined by the rlimitstructure:

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_RESOURCEcapability) may make arbitrary changes to either limit value.

The value RLIM_INFINITYdenotes no limit on a resource (both in the structure returned by getrlimit() and in the structure passed to setrlimit()).

The resourceargument 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 ENOMEMupon exceeding this limit. Also automatic stack expansion will fail (and generate a SIGSEGVthat 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 longeither this limit is at most 2 GiB, or this resource is unlimited.
RLIMIT_CORE
Maximum size of a corefile (see core(5)). 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 SIGXCPUsignal. 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 SIGXCPUonce 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
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 ENOMEMupon 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 SIGXFSZsignal. 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_LOCKEDoperation. Since Linux 2.6.9 it also affects the shmctl(2) SHM_LOCKoperation, 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_LOCKlocks 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:
    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 attris the mq_attrstructure specified as the fourth argument to mq_open(3), and the msg_msgand posix_msg_tree_nodestructures 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)
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_INFINITYtypically 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_OFILEon 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. This limit is not enforced for processes that have either the CAP_SYS_ADMINor the CAP_SYS_RESOURCEcapability.
RLIMIT_RSS
Specifies the limit (in bytes) of 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)
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 (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 SIGXCPUsignal. If the process catches or ignores this signal and continues consuming CPU time, then SIGXCPUwill be generated once each second until the hard limit is reached, at which point the process is sent a SIGKILLsignal.

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 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
The maximum size of the process stack, in bytes. Upon reaching this limit, a SIGSEGVsignal 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 resourceargument has the same meaning as for setrlimit() and getrlimit().

If the new_limitargument is a not NULL, then the rlimitstructure to which it points is used to set new values for the soft and hard limits for resource. If the old_limitargument is a not NULL, then a successful call to prlimit() places the previous soft and hard limits for resourcein the rlimitstructure pointed to by old_limit.

The pidargument specifies the ID of the process on which the call is to operate. If pidis 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_RESOURCEcapability 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 andthe 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 errnois set appropriately.  

ERRORS

EFAULT
A pointer argument points to a location outside the accessible address space.
EINVAL
The value specified in resourceis not valid; or, for setrlimit() or prlimit(): rlim->rlim_curwas greater than rlim->rlim_max.
EPERM
An unprivileged process tried to raise the hard limit; the CAP_SYS_RESOURCEcapability is required to do this.
EPERM
The caller tried to increase the hard RLIMIT_NOFILElimit 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).
InterfaceAttributeValue
getrlimit(), setrlimit(), prlimit() Thread safetyMT-Safe

 

CONFORMING TO

getrlimit(), setrlimit(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
prlimit(): Linux-specific.

RLIMIT_MEMLOCKand RLIMIT_NPROCderive from BSD and are not specified in POSIX.1; they are present on the BSDs and Linux, but on few other implementations. RLIMIT_RSSderives 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_SIGPENDINGare Linux-specific.  

NOTES

A child process created via fork(2) inherits its parent's resource limits. Resource limits are preserved across execve(2).

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 ulimitcommand (limitin 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 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().  

BUGS

In older Linux kernels, the SIGXCPUand SIGKILLsignals delivered when a process encountered the soft and hard RLIMIT_CPUlimits 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_CPUlimit 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_RTPRIOdoes 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_CPUlimit 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_CPUsoft 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_RTTIMElimit exhibits the same behavior when the soft limit is encountered.

Kernels before 2.4.22 did not diagnose the error EINVALfor setrlimit() when rlim->rlim_curwas greater than rlim->rlim_max.  

Representation of large resource limit values on 32-bit platforms

The glibc getrlimit() and setrlimit() wrapper functions use a 64-bit rlim_tdata type, even on 32-bit platforms. However, the rlim_tdata type used in the getrlimit() and setrlimit() system calls is a (32-bit) unsigned long. Furthermore, in Linux versions before 2.6.36, 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.

This problem was addressed in Linux 2.6.36 with two principal changes:

*
the addition of a new kernel representation of resource limits that uses 64 bits, even on 32-bit platforms;
*
the addition of the prlimit() system call, which employs 64-bit values for its resource limit arguments.

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().  

EXAMPLE

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_FAILURE);
}
 

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), signal(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
prlimit()
RETURN VALUE
ERRORS
VERSIONS
ATTRIBUTES
CONFORMING TO
NOTES
C library/ kernel ABI differences
BUGS
Representation of large resource limit values on 32-bit platforms
EXAMPLE
SEE ALSO

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