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       shmget - allocates a System V shared memory segment


       Standard C library (libc, -lc)


       #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 corresponding 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:

              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 segment.

              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/hugetlbpage.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 configured 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 similarly named constants in

       SHM_NORESERVE (since Linux 2.6.15)
              This flag serves the same purpose  as  the  mmap(2)  MAP_NORESERVE  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 initialized 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

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

       •  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 verified, 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.


       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

       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

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

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

       ENOENT No segment exists for the given key, and IPC_CREAT was not specified.

       ENOMEM No memory could be allocated for segment overhead.

       ENOSPC All possible shared memory IDs have been taken (SHMMNI), or allocating 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 privileged (did not have
              the CAP_IPC_LOCK capability) and is not a member  of  the  sysctl_hugetlb_shm_group
              group; see the description of /proc/sys/vm/sysctl_hugetlb_shm_group in proc(5).


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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.


       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/kernel/shmall.   Since
              Linux 3.16, the default value for this limit is:

                  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 prevent 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 result 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/kernel/shmmax.  Since
              Linux 3.16, the default value for this limit is:

                  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 MiB).

              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:  implementation  dependent
              (currently 1 byte, though PAGE_SIZE is the effective 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/kernel/shmmni.

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

   Linux notes
       Until  Linux  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


       See shmop(2).


       memfd_create(2), shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), shm_overview(7),