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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()  performs the control operation specified by cmd on the shared
       memory segment whose identifier is given in shmid.

       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(2)/shmdt(2) */
               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(2) */
               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 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; /* Maximum segment size */
                         unsigned long shmmin; /* Minimum segment size;
                                                  always 1 */
                         unsigned long shmmni; /* Maximum number of segments */
                         unsigned long shmseg; /* Maximum number of segments
                                                  that a process can attach;
                                                  unused within kernel */
                         unsigned long shmall; /* Maximum number 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.)

CONFORMING TO

       SVr4, POSIX.1-2001.

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(2)) 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 typed as short under Linux 2.2
       and have become long 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.)

SEE ALSO

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

COLOPHON

       This page is part of release 3.21 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.