shmget(2) — Linux manual page


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

NAME         top

       shmget - allocates a System V shared memory segment

SYNOPSIS         top

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

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

       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 simi‐
              larly 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 avail‐
              able.  See also the discussion of the file /proc/sys/vm/over‐
              commit_memory in proc(5).

       In addition to the above flags, the least significant 9 bits of shm‐
       flg 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 ini‐
       tialized 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 ver‐
       ified, and a check is made to see if it is marked for destruction.

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

       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

       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).

CONFORMING TO         top

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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

NOTES         top

       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 (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:
              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.

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

       This page is part of release 5.08 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                            2020-04-11                        SHMGET(2)

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