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NAME

       shmctl - shared memory control

SYNOPSIS

       #include <sys/ipc.h>

       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION

       shmctl()  allows the caller to obtain information about a shared memory
       segment, set the owner, group, and permissions of a segment, or destroy
       a segment.

       The  buf  argument  is  a  pointer  to a shmid_ds structure, defined in
       <sys/shm.h> as follows:

           struct shmid_ds {
               struct ipc_perm shm_perm;    /* Ownership and permissions */
               size_t          shm_segsz;   /* Size of segment (bytes) */
               time_t          shm_atime;   /* Last attach time */
               time_t          shm_dtime;   /* Last detach time */
               time_t          shm_ctime;   /* Last change time */
               pid_t           shm_cpid;    /* PID of creator */
               pid_t           shm_lpid;    /* PID of last shmat()/shmdt() */
               shmatt_t        shm_nattch;  /* No. of current attaches */
               ...
           };

       The ipc_perm structure  is  defined  in  <sys/ipc.h>  as  follows  (the
       highlighted fields are settable using IPC_SET):

           struct ipc_perm {
               key_t key;            /* Key supplied to shmget() */
               uid_t uid;            /* Effective UID of owner */
               gid_t gid;            /* Effective GID of owner */
               uid_t cuid;           /* Effective UID of creator */
               gid_t cgid;           /* Effective GID of creator */
               unsigned short mode;  /* Permissions + SHM_DEST and
                                        SHM_LOCKED flags */
               unsigned short seq;   /* Sequence number */
           };

       Valid values for cmd are:

       IPC_STAT    Copy  information from the kernel data structure associated
                   with shmid into the shmid_ds structure pointed to  by  buf.
                   The  caller  must have read permission on the shared memory
                   segment.

       IPC_SET     Write the values of some members of the shmid_ds  structure
                   pointed   to  by  arg.buf  to  the  kernel  data  structure
                   associated with this shared memory segment,  updating  also
                   its shm_ctime member.  The following fields can be changed:
                   shm_perm.uid, shm_perm.gid, and (the  least  significant  9
                   bits  of)  shm_perm.mode.  The effective UID of the calling
                   process must match  the  owner  (shm_perm.uid)  or  creator
                   (shm_perm.cuid) of the shared memory segment, or the caller
                   must be privileged.

       IPC_RMID    Mark the segment to be destroyed.  The  segment  will  only
                   actually  be  destroyed  after the last process detaches it
                   (i.e.,  when  the  shm_nattch  member  of  the   associated
                   structure  shmid_ds is zero).  The caller must be the owner
                   or creator, or be privileged.  If a segment has been marked
                   for  destruction,  then the (non-standard) SHM_DEST flag of
                   the shm_perm.mode field in the  associated  data  structure
                   retrieved by IPC_STAT will be set.

       The  caller  must  ensure  that  a  segment  is  eventually  destroyed;
       otherwise its pages that were faulted in will remain in memory or swap.

       IPC_INFO (Linux specific)
              Returns  information  about system-wide shared memory limits and
              parameters in the structure pointed to by buf.   This  structure
              is  of  type  shminfo  (thus,  a  cast  is required), defined in
              <sys/shm.h> if the _GNU_SOURCE feature test macro is defined:

                struct  shminfo {
                    unsigned long shmmax; /* Max. segment size */
                    unsigned long shmmin; /* Min. segment size; always 1 */
                    unsigned long shmmni; /* Max. # of segments */
                    unsigned long shmseg; /* Max. # of segments that a
                                             process can attach; unused */
                    unsigned long shmall; /* Max. # of pages of shared
                                             memory, system-wide */
                };

              The shmmni, shmmax, and shmall settings can be changed via /proc
              files of the same name; see proc(5) for details.

       SHM_INFO (Linux specific)
              Returns  a  shm_info  structure whose fields contain information
              about  system  resources  consumed  by  shared   memory.    This
              structure  is  defined in <sys/shm.h> if the _GNU_SOURCE feature
              test macro is defined:

                struct shm_info {
                    int used_ids;           /* # of currently existing
                                               segments */
                    unsigned long shm_tot;  /* Total number of shared
                                               memory pages */
                    unsigned long shm_rss;  /* # of resident shared
                                               memory pages */
                    unsigned long shm_swp;  /* # of swapped shared
                                               memory pages */
                    unsigned long swap_attempts;  /* Unused since Linux 2.4 */
                    unsigned long swap_successes; /* Unused since Linux 2.4 */
                };

       SHM_STAT (Linux specific)
              Returns a shmid_ds structure  as  for  IPC_STAT.   However,  the
              shmid argument is not a segment identifier, but instead an index
              into the kernel’s  internal  array  that  maintains  information
              about all shared memory segments on the system.

       The  caller  can  prevent  or allow swapping of a shared memory segment
       with the following cmd values:

       SHM_LOCK (Linux specific)
                   Prevent swapping of the shared memory segment.  The  caller
                   must  fault  in  any  pages that are required to be present
                   after locking is enabled.  If a segment  has  been  locked,
                   then    the   (non-standard)   SHM_LOCKED   flag   of   the
                   shm_perm.mode  field  in  the  associated  data   structure
                   retrieved by IPC_STAT will be set.

       SHM_UNLOCK (Linux specific)
                   Unlock the segment, allowing it to be swapped out.

       In  kernels  before  2.6.10,  only  a  privileged  process could employ
       SHM_LOCK and SHM_UNLOCK.  Since kernel 2.6.10, an unprivileged  process
       can  employ  these operations if its effective UID matches the owner or
       creator UID of the segment, and (for SHM_LOCK) the amount of memory  to
       be   locked   falls  within  the  RLIMIT_MEMLOCK  resource  limit  (see
       setrlimit(2)).

RETURN VALUE

       A successful IPC_INFO or SHM_INFO operation returns the  index  of  the
       highest used entry in the kernel’s internal array recording information
       about all shared memory segments.  (This information can be  used  with
       repeated  SHM_STAT  operations  to  obtain information about all shared
       memory segments  on  the  system.)   A  successful  SHM_STAT  operation
       returns  the  identifier  of  the shared memory segment whose index was
       given in shmid.  Other operations return 0 on success.

       On error, -1 is returned, and errno is set appropriately.

ERRORS

       EACCES     IPC_STAT or SHM_STAT is requested and shm_perm.mode does not
                  allow  read  access  for shmid, and the calling process does
                  not have the CAP_IPC_OWNER capability.

       EFAULT     The argument cmd has  value  IPC_SET  or  IPC_STAT  but  the
                  address pointed to by buf isn’t accessible.

       EIDRM      shmid points to a removed identifier.

       EINVAL     shmid  is  not  a  valid  identifier,  or cmd is not a valid
                  command.  Or: for a  SHM_STAT  operation,  the  index  value
                  specified  in  shmid  referred  to  an  array  slot  that is
                  currently unused.

       ENOMEM     (In kernels since 2.6.9), SHM_LOCK  was  specified  and  the
                  size  of  the to-be-locked segment would mean that the total
                  bytes in locked shared  memory  segments  would  exceed  the
                  limit  for  the  real  user ID of the calling process.  This
                  limit is defined by the RLIMIT_MEMLOCK soft  resource  limit
                  (see setrlimit(2)).

       EOVERFLOW  IPC_STAT is attempted, and the GID or UID value is too large
                  to be stored in the structure pointed to by buf.

       EPERM      IPC_SET or IPC_RMID is attempted, and the effective user  ID
                  of  the calling process is not that of the creator (found in
                  shm_perm.cuid), or the owner (found  in  shm_perm.uid),  and
                  the  process  was  not  privileged  (Linux: did not have the
                  CAP_SYS_ADMIN capability).

                  Or (in kernels before 2.6.9),  SHM_LOCK  or  SHM_UNLOCK  was
                  specified,  but  the  process was not privileged (Linux: did
                  not have the CAP_IPC_LOCK capability).  (Since Linux  2.6.9,
                  this error can also occur if the RLIMIT_MEMLOCK is 0 and the
                  caller is not privileged.)

NOTES

       The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the  ipcs(8)
       program  to  provide information on allocated resources.  In the future
       these may modified or moved to a /proc file system interface.

       Linux permits a process to attach (shmat())  a  shared  memory  segment
       that has already been marked for deletion using shmctl(IPC_RMID).  This
       feature is  not  available  on  other  Unix  implementations;  portable
       applications should avoid relying on it.

       Various  fields  in  a  struct shmid_ds were shorts under Linux 2.2 and
       have become longs under  Linux  2.4.  To  take  advantage  of  this,  a
       recompilation  under glibc-2.1.91 or later should suffice.  (The kernel
       distinguishes old and new calls by an IPC_64 flag in cmd.)

CONFORMING TO

       SVr4, SVID.  SVr4 documents additional error conditions EINVAL, ENOENT,
       ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor SVID documents an EIDRM error
       condition.

SEE ALSO

       mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7)