SHMCTL(2)



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

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 Sys-
       tem 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;   /* Creation time/time of last
                                               modification via shmctl() */
               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 fields of the shmid_ds structure are as follows:

       shm_perm    This  is  an  ipc_perm structure (see below) that specifies
                   the access permissions on the shared memory segment.

       shm_segsz   Size in bytes of the shared memory segment.

       shm_cpid    ID of the process that created the shared memory segment.

       shm_lpid    ID of the last process that executed a shmat(2) or shmdt(2)
                   system call on this segment.

       shm_nattch  Number of processes that have this segment attached.

       shm_atime   Time  of  the  last shmat(2) system call that attached this
                   segment.

       shm_dtime   Time of the last shmdt(2) system call  that  detached  tgis
                   segment.

       shm_ctime   Time  of  creation  of segment or time of the last shmctl()
                   IPC_SET operation.

       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 */
           };

       The  least  significant 9 bits of the mode field of the ipc_perm struc-
       ture define the access permissions for the shared memory segment.   The
       permission bits are as follows:

       0400   Read by user
       0200   Write by user
       0040   Read by group
       0020   Write by group
       0004   Read by others
       0002   Write by others

       Bits  0100, 0010, and 0001 (the execute bits) are unused by the system.
       (It is not necessary to have execute permission on a segment  in  order
       to perform a shmat(2) call with the SHM_EXEC flag.)

       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 effec-
              tive  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 (nonstan-
              dard) 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  struc-
              ture  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.

       SHM_STAT_ANY (Linux-specific, since Linux 4.17)
              Return   a   shmid_ds   structure  as  for  SHM_STAT.   However,
              shm_perm.mode is not checked for read access for shmid,  meaning
              that  any  user  can employ this operation (just as any user may
              read /proc/sysvipc/shm to obtain the same information).

       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 (nonstan-
              dard) SHM_LOCKED flag of the shm_perm.mode field in the  associ-
              ated 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 setr-
       limit(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  or  SHM_STAT_ANY  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 in-
       dex 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 al-
              low 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 or SHM_STAT_ANY 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_AD-
              MIN capability).

              Or (in kernels before 2.6.9), SHM_LOCK or SHM_UNLOCK was  speci-
              fied,  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 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 re-
       compilation  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),
       sysvipc(7)

COLOPHON
       This  page  is  part of release 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-04-11                         SHMCTL(2)

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