shmget(2) — Linux manual page


shmget(2)                  System Calls Manual                 shmget(2)

NAME         top

       shmget - allocates a System V shared memory segment

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/shm.h>

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

DESCRIPTION         top

       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

              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

       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

RETURN VALUE         top

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

ERRORS         top

       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 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

       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).

STANDARDS         top


       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

HISTORY         top

       POSIX.1-2001, SVr4.

NOTES         top

       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

   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

       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.

BUGS         top

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

EXAMPLES         top

       See shmop(2).

SEE ALSO         top

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                      shmget(2)

Pages that refer to this page: ipcrm(1)ipcs(1)lsipc(1)pcp-ipcs(1)getrlimit(2)ipc(2)mbind(2)memfd_create(2)shmctl(2)shmop(2)syscalls(2)umask(2)ftok(3)sem_init(3)proc_sys_kernel(5)proc_sys_vm(5)tmpfs(5)sem_overview(7)shm_overview(7)sysvipc(7)