bionic (2) shmctl.2.gz

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NAME

       shmctl - System V 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 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
                 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 actually be destroyed only 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 of the segment, or be privileged.  The buf
                 argument is ignored.

                 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.

                 See also the description of /proc/sys/kernel/shm_rmid_forced in proc(5).

       IPC_INFO (Linux-specific)
                 Return 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)
                 Return 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)
                 Return  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 in the user namespace that governs  its
              IPC namespace.

       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

       POSIX.1-2001, POSIX.1-2008, SVr4.

NOTES

       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on  Linux  or  by  any  version  of  POSIX.
       However,  some  old  implementations  required  the  inclusion  of  these header files, and the SVID also
       documented their inclusion.  Applications intended to be portable to such old systems 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
       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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