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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  (nonstandard)  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 (nonstandard) 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(1) 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

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