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

NAME
       shmget - allocates a System V shared memory segment

SYNOPSIS
       #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 identi‐
       fier 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 anal‐
       ogous 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 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 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 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  ini‐
       tialized 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.

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
       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 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).

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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

NOTES
       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  cur‐
              rent 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 /proc/sys/kernel/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), capabilities(7), shm_overview(7), sysvipc(7)

Linux                                                                          2021-03-22                                                                      SHMGET(2)