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.   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:

   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

   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/vm/hugetlbpage.txt  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 mmap(2).

   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

   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 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
   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.


   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

   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

   EPERM  The  SHM_HUGETLB  flag  was  specified,  but  the caller was not
          privileged (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/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 (32MB).

          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

   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),
   capabilities(7), shm_overview(7), svipc(7)


   This  page  is  part of release 4.09 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


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