shmget.2 (2011 03)

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shmget(2) shmget(2)

If IPC_MEM_FIRST_TOUCH

is set in shmﬂg, physical memory will come from the locality domain of the

ﬁrst processor to read or write that memory. If the shared memory is paged out and then back in again,

physical memory will come from the locality domain of the processor bringing in that memory.

System V Shared Memory Under Adaptive Address Space

Itanium-based platforms have the capability to create two types of processes: MGAS (Mostly Global

Address Space), which is the default, and MPAS (Mostly Private Address Space). See ld(1) and chatr (1).

If 64-bit MPAS and 64-bit MGAS processes want to share a mapping among themselves without incurring

the cost of aliasing, then (shmﬂg &

IPC_GLOBAL) should be "true", but (shmﬂg &

IPC_SHARE32)

should be "false". 64-bit MPAS processes should also specify the

IPC_GLOBAL ﬂag to the shmat (2) call.

Such a mapping cannot be shared with 32-bit MGAS processes.

See the Adaptive Address Space Whitepaper for details.

EXAMPLES

The following call to

shmget() returns a unique shmid for the newly created shared memory segment of

4096 bytes:

int myshmid;

myshmid = shmget (IPC_PRIVATE, 4096, 0600);

RETURN VALUE

Upon successful completion, a non-negative integer, namely a shared memory identiﬁer, is returned.

Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

shmget() fails if any of the following conditions are encountered:

[EINVAL] size is less than the system-imposed minimum or greater than the system-imposed

maximum.

[EINVAL] A shared memory identiﬁer exists for key but is in 64-bit address space and the pro-

cess performing the request has been compiled as a 32-bit executable. In order to

avoid receiving this error, both

IPC_SHARE32 and IPC_CREAT must be set in

shmﬂg by the 64-bit processes upon segment creation.

[EACCES] A shared memory identiﬁer exists for key but operation permission (see glossary (9))

as speciﬁed by the low-order 9 bits of shmﬂg would not be granted.

[EINVAL] A shared memory identiﬁer exists for key but the size of the segment associated

with it is less than size and size is not equal to zero.

[ENOENT] A shared memory identiﬁer does not exist for key and (shmﬂg &

IPC_CREAT)is

"false".

[ENOSPC] A shared memory identiﬁer is to be created but the system-imposed limit on the

maximum number of allowed shared memory identiﬁers system wide would be

exceeded.

[ENOMEM] A shared memory identiﬁer and associated shared memory segment are to be

created, but the amount of available physical memory is not sufﬁcient to ﬁll the

request.

[ENOMEM] A shared memory segment with

IPC_CREAT or IPC_GLOBAL speciﬁed was to be

created; however, there is not enough space in the global quadrant.

[EEXIST] A shared memory identiﬁer exists for key but ( (shmﬂg &

IPC_CREAT)&&(shmﬂg

& IPC_EXCL) ) is "true".