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

       shmctl - shared memory control

       #include <sys/ipc.h>
       #include <sys/shm.h>

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

       shmctl()  performs the control operation specified by cmd on the shared memory seg-
       ment whose identifier is given in shmid.

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

           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 seg-
                 ment, 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 sys-
                 tem 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 argu-
                 ment is not a segment identifier, but instead an index into the  kernel's
                 internal  array  that  maintains information about all shared memory seg-
                 ments on the system.

       The caller can prevent or allow swapping of a shared memory segment with  the  fol-
       lowing 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_MEM-
       LOCK resource limit (see setrlimit(2)).

       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.

       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

       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  seg-
              ments  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  setr-

              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  capabil-
              ity).   (Since  Linux 2.6.9, this error can also occur if the RLIMIT_MEMLOCK
              is 0 and the caller is not privileged.)

       SVr4, POSIX.1-2001.

       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.)

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

       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-

Linux                             2008-08-07                         SHMCTL(2)

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