shmat(2)shmat(2)NAMEshmat - Attach a shared memory region
SYNOPSIS
#include <sys/shm.h>
void *shmat(
int shmid,
const void *shmaddr,
int shmflgs );
If programs are being developed for multiple platforms, application
developers may want to specify #include statements for <sys/types.h>
and <sys/ipc.h> before the one for <sys/shm.h>. The additional #include
statements are not required on Tru64 UNIX systems or by ISO or XSH
standards, but may be required on other vendors' systems that conform
to these standards.
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards as follows:
shmat(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Specifies the ID for the shared memory region. The ID is typically
returned by a previous shmget() function. Specifies the virtual
address at which the process wants to attach the shared memory region.
The process can also specify 0 (zero) to have the kernel select an
appropriate address. Specifies the attach options. Possible values
are: If the addr parameter is not 0 (zero), the kernel rounds off the
address, if necessary. If the calling process has read permission, the
kernel attaches the region for reading only.
DESCRIPTION
The shmat() function attaches the shared memory region identified by
the shmid parameter to the virtual address space of the calling
process. For the addr parameter, the process can specify either an
explicit address or 0 (zero), to have the kernel select the address. If
an explicit address is used, the process can set the SHM_RND flag to
have the kernel round off the address, if necessary.
Access to the shared memory region is determined by the operation per‐
missions in the shm_perm.mode member in the region's shmid_ds struc‐
ture. The low-order bits in shm_perm.mode are interpreted as follows:
Read by user Write by user Read by group Write by group Read by others
Write by others
The calling process is granted read and write permissions on the
attached region if at least one of the following is true: The effective
user ID of the process is superuser. The effective user ID of the
process is equal to shm_perm.cuid or shm_perm.uid and bit 0600 in
shm_perm.mode is set. The effective group ID of the process is equal
to shm_perm.cgid or shm_perm.gid and bit 0060 in shm_perm.mode is set.
Bit 0006 in shm_perm.mode is set.
If the process has read permission, it can attach the region as read
only by setting the SHM_RDONLY option.
NOTES
The librt library contains alternative interfaces for interprocess com‐
munication. The names of these routines adhere to the format shm_* and
their reference pages are listed in SEE ALSO.
RETURN VALUES
Upon successful completion, shmat() increments the value of shm_nattach
in the data structure associated with the shared memory ID of the
attached shared memory segment and returns the segment's start address.
If the shmat() function fails, the shared memory region is not
attached, a value of -1 is returned, and errno is set to indicate the
error.
ERRORS
The shmat() function sets errno to the specified values for the follow‐
ing conditions: The calling process does not have the appropriate priv‐
ilege. The shmid parameter does not specify a valid shared memory
region ID; the addr parameter is not 0 (zero) and not a valid address;
or the addr parameter is not 0 (zero) and not a valid address, and
SHM_RND is not set.
[Tru64 UNIX] The shmid refers to a segmented shared memory
region and the addr parameter is not aligned at SSM_SIZE (that
is, (addr % SSM_SIZE) != 0). An attempt to attach a shared mem‐
ory region exceeded the maximum number of attached regions
allowed for any one process. There was not enough data space
available to attach the shared memory region.
[Tru64 UNIX] The shmid refers to a segmented shared memory
region whose size is not a multiple of SSM_SIZE and where not
enough data space is available for the rounded-up size.
SEE ALSO
Commands: sysconfig(8)
Functions: exec(2), exit(2), fork(2), shmctl(2), shmdt(2), shmget(2),
shm_open(3), shm_unlink(3)
Data structures: shmid_ds(4)
Standards: standards(5)shmat(2)