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



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 fol-
                 lowing 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 cre-
                 ator (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  identi-
                 fier,  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  inter-
       nal  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  call-
              ing 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  privi-
              leged (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 allo-
       cated 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 distin-
       guishes 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 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-07                         SHMCTL(2)