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

       shmget - allocates a System V shared memory segment

       #include <sys/ipc.h>
       #include <sys/shm.h>

       int shmget(key_t key, size_t size, int shmflg);

       shmget()  returns  the identifier of the System V shared memory segment
       associated with the value of the argument key.  It may be  used  either
       to  obtain the identifier of a previously created shared memory segment
       (when shmflg is zero and key does not have the value  IPC_PRIVATE),  or
       to create a new set.

       A  new  shared  memory  segment,  with  size equal to the value of size
       rounded up to a multiple of PAGE_SIZE, is created if key has the  value
       IPC_PRIVATE  or  key isn't IPC_PRIVATE, no shared memory segment corre-
       sponding to key exists, and IPC_CREAT is specified in shmflg.

       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
       segment  already  exists for key, then shmget() fails with errno set to
       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
       O_EXCL for open(2).)

       The value shmflg is composed of:

       IPC_CREAT   Create  a  new  segment.   If  this  flag is not used, then
                   shmget() will find the  segment  associated  with  key  and
                   check  to see if the user has permission to access the seg-

       IPC_EXCL    This flag is used with IPC_CREAT to ensure that  this  call
                   creates  the  segment.   If the segment already exists, the
                   call fails.

       SHM_HUGETLB (since Linux 2.6)
                   Allocate the segment using "huge  pages."   See  the  Linux
                   kernel  source  file  Documentation/admin-guide/mm/hugetlb-
                   page.rst for further information.

       SHM_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
                   Used in conjunction with SHM_HUGETLB to select  alternative
                   hugetlb page sizes (respectively, 2 MB and 1 GB) on systems
                   that support multiple hugetlb page sizes.

                   More generally, the desired huge page size can  be  config-
                   ured  by  encoding the base-2 logarithm of the desired page
                   size in the six bits at the offset  SHM_HUGE_SHIFT.   Thus,
                   the above two constants are defined as:

                       #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
                       #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)

                   For some additional details, see the discussion of the sim-
                   ilarly named constants in mmap(2).

       SHM_NORESERVE (since Linux 2.6.15)
                   This flag serves the same purpose as the mmap(2)  MAP_NORE-
                   SERVE  flag.   Do  not reserve swap space for this segment.
                   When swap space is reserved, one has the guarantee that  it
                   is  possible to modify the segment.  When swap space is not
                   reserved one might get SIGSEGV upon a write if no  physical
                   memory  is  available.  See also the discussion of the file
                   /proc/sys/vm/overcommit_memory in proc(5).

       In addition to the above flags, the least significant 9 bits of  shmflg
       specify the permissions granted to the owner, group, and others.  These
       bits have the same format, and the same meaning, as the  mode  argument
       of open(2).  Presently, execute permissions are not used by the system.

       When  a new shared memory segment is created, its contents are initial-
       ized to zero values, and its associated data structure,  shmid_ds  (see
       shmctl(2)), is initialized as follows:

              shm_perm.cuid  and shm_perm.uid are set to the effective user ID
              of the calling process.

              shm_perm.cgid and shm_perm.gid are set to the effective group ID
              of the calling process.

              The  least  significant  9  bits of shm_perm.mode are set to the
              least significant 9 bit of shmflg.

              shm_segsz is set to the value of size.

              shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0.

              shm_ctime is set to the current time.

       If the shared memory segment already exists, the permissions are  veri-
       fied, and a check is made to see if it is marked for destruction.

       On success, a valid shared memory identifier is returned.  On error, -1
       is returned, and errno is set to indicate the error.

       On failure, errno is set to one of the following:

       EACCES The user does not have permission to access  the  shared  memory
              segment,  and  does not have the CAP_IPC_OWNER capability in the
              user namespace that governs its IPC namespace.

       EEXIST IPC_CREAT and IPC_EXCL were specified in shmflg,  but  a  shared
              memory segment already exists for key.

       EINVAL A  new segment was to be created and size is less than SHMMIN or
              greater than SHMMAX.

       EINVAL A segment for the given key exists, but size is greater than the
              size of that segment.

       ENFILE The system-wide limit on the total number of open files has been

       ENOENT No segment exists for the given key, and IPC_CREAT was not spec-

       ENOMEM No memory could be allocated for segment overhead.

       ENOSPC All  possible shared memory IDs have been taken (SHMMNI), or al-
              locating a segment of the requested size would cause the  system
              to exceed the system-wide limit on shared memory (SHMALL).

       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
              ileged (did not have the CAP_IPC_LOCK capability).

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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

       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.

       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
       is  used for key, the system call ignores all but the least significant
       9 bits of shmflg and creates a new shared memory segment.

   Shared memory limits
       The following limits on shared  memory  segment  resources  affect  the
       shmget() call:

       SHMALL System-wide limit on the total amount of shared memory, measured
              in units of the system page size.

              On Linux, this limit can be read and modified via /proc/sys/ker-
              nel/shmall.   Since Linux 3.16, the default value for this limit

                  ULONG_MAX - 2^24

              The effect of this value (which is suitable for both 32-bit  and
              64-bit systems) is to impose no limitation on allocations.  This
              value, rather than ULONG_MAX, was chosen as the default to  pre-
              vent  some cases where historical applications simply raised the
              existing limit without first checking its current  value.   Such
              applications  would cause the value to overflow if the limit was
              set at ULONG_MAX.

              From Linux 2.4 up to Linux 3.15,  the  default  value  for  this
              limit was:

                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)

              If SHMMAX and SHMMNI were not modified, then multiplying the re-
              sult of this formula by the page size (to get a value in  bytes)
              yielded a value of 8 GB as the limit on the total memory used by
              all shared memory segments.

       SHMMAX Maximum size in bytes for a shared memory segment.

              On Linux, this limit can be read and modified via /proc/sys/ker-
              nel/shmmax.   Since Linux 3.16, the default value for this limit

                  ULONG_MAX - 2^24

              The effect of this value (which is suitable for both 32-bit  and
              64-bit  systems) is to impose no limitation on allocations.  See
              the description of SHMALL for a discussion of why  this  default
              value (rather than ULONG_MAX) is used.

              From Linux 2.2 up to Linux 3.15, the default value of this limit
              was 0x2000000 (32 MB).

              Because it is not possible to map just part of a  shared  memory
              segment,  the  amount  of virtual memory places another limit on
              the maximum size of a usable segment: for example, on  i386  the
              largest  segments  that  can  be  mapped  have  a size of around
              2.8 GB, and on x86-64 the limit is around 127 TB.

       SHMMIN Minimum size in bytes for a shared memory  segment:  implementa-
              tion dependent (currently 1 byte, though PAGE_SIZE is the effec-
              tive minimum size).

       SHMMNI System-wide limit on the number of shared memory  segments.   In
              Linux 2.2, the default value for this limit was 128; since Linux
              2.4, the default value is 4096.

              On Linux, this limit can be read and modified via /proc/sys/ker-

       The  implementation  has no specific limits for the per-process maximum
       number of shared memory segments (SHMSEG).

   Linux notes
       Until version 2.3.30, Linux would return EIDRM  for  a  shmget()  on  a
       shared memory segment scheduled for deletion.

       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.

       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili-
       ties(7), shm_overview(7), sysvipc(7)

       This  page  is  part of release 5.05 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

Linux                             2019-08-02                         SHMGET(2)
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