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           #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 System
           V 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 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
           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
                     The   caller   must  ensure  that  a  segment  is  eventually
                     destroyed; otherwise its pages  that  were  faulted  in  will
                     remain in memory or swap.
                     See  also the description of /proc/sys/kernel/shm_rmid_forced
                     in proc(5).
           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 fea-
                     ture 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
           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 setr-


           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 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)).
                  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).
           may need to include these header files.
           The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the  ipcs(1)
           program  to  provide information on allocated resources.  In the future
           these may modified or moved to a /proc filesystem 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 appli-
           cations 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)

    Linux 2014-02-07 SHMCTL(2)


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