shmat - Attach a shared memory region
#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.
Interfaces documented on this reference page conform to
industry standards 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.
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.
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 permissions in the shm_perm.mode member in the
region's shmid_ds structure. 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.
The librt library contains alternative interfaces for
interprocess communication. The names of these routines
adhere to the format shm_* and their reference pages are
listed in SEE ALSO.
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.
The shmat() function sets errno to the specified values
for the following conditions: The calling process does not
have the appropriate privilege. 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 memory 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.
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)
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