NAME

shmget - allocates a System V shared memory segment

LIBRARY

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.

EXAMPLES

See shmop(2).

ERRORS

STANDARDS

HISTORY

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.