oracular (2) getrlimit.2.gz

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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.

ATTRIBUTES

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

       ┌──────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │InterfaceAttributeValue   │
       ├──────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │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 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.

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 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);
       }

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),
       signal(7)