SHMGET(2)



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

NAME
       shmget - allocates a System V shared memory segment

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

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

DESCRIPTION
       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 segment.

       IPC_EXCL
              This  flag  is used with IPC_CREAT to ensure that this call cre-
              ates 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 con-
              stants 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_mem-
              ory 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:

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

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

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

       o shm_segsz is set to the value of size.

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

       o 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.

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

ERRORS
       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
              reached.

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

       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).

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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

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.

       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
              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 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
              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 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-
              nel/shmmni.

       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.

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

EXAMPLES
       See shmop(2).

SEE ALSO
       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili-
       ties(7), shm_overview(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                         SHMGET(2)

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