Ubuntu Manpage: shmget - allocates a System V shared memory segment

NAME

       shmget - allocates a System V shared memory segment

LIBRARY

       Standard C library (libc, -lc)

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 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:

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 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 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).

STANDARDS

       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 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 /proc/sys/kernel/shmmni.

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

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

EXAMPLES

       See shmop(2).