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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 seg-
                 ment.

     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  config-
                 ured  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 sim-
                 ilarly named constants in mmap(2).

     SHM_NORESERVE (since Linux 2.6.15)
                 This  flag serves the same purpose as the mmap(2) MAP_NORE-
                 SERVE 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 initial-
     ized 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  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
            allocating a segment of the requested size would cause the  sys-
            tem 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
            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/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.