getrlimit(2) — Linux manual page

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GETRLIMIT(2)            Linux Programmer's Manual           GETRLIMIT(2)

NAME         top

       getrlimit, setrlimit, prlimit - get/set resource limits

SYNOPSIS         top

       #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         top

       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 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.

              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).

   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 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.

RETURN VALUE         top

       On success, these system calls return 0.  On error, -1 is
       returned, and errno is set to indicate the error.

ERRORS         top

       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         top

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

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌──────────────────────────────────────┬───────────────┬─────────┐
       │Interface                             Attribute     Value   │
       ├──────────────────────────────────────┼───────────────┼─────────┤
       │getrlimit(), setrlimit(), prlimit()   │ Thread safety │ MT-Safe │
       └──────────────────────────────────────┴───────────────┴─────────┘

CONFORMING TO         top

       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         top

       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 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         top

       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.

   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 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().

EXAMPLES         top

       The program below demonstrates the use of prlimit().

       #define _GNU_SOURCE
       #define _FILE_OFFSET_BITS 64
       #include <stdint.h>
       #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=%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)
               errExit("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         top

       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)

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2021-03-22                   GETRLIMIT(2)

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