HP 3000 Manuals HP 3000 Manuals
shmat
Attaches the calling process to a shared memory area.
Syntax
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
char *shmat (int shmid, char *shmaddr, int shmflg);
Parameters
shmid Passes a shared memory identifier returned by a shmget() call.
shmaddr Passes either 0 or a valid memory address. Set shmaddr to
zero to attach a shared memory area to the address space of
the calling process. Otherwise, set shmaddr to the data start
address of a shared memory area that is already attached to
another process.
shmflg Passes a value specifying that the calling process has
read/write access to the attached shared memory area. It is
not possible to attach for write-only access or read-only
access.
Return Values
addr Success. shmat() returns the data area start address of the
attached shared memory area.
-1 An error occurred, and errno is set to indicate the error
condition.
Description
The shmat() function attaches the shared memory area associated with the
shared memory identifier specified by shmid to the data area of the
calling process.
If the shared memory area is already attached to another process, a
non-zero value of shmaddr is accepted, provided the specified address is
identical to the current attach address of the area. The area is
attached for both reading and writing.
Implementation Considerations
The MPE/iX implementation of SVID IPC shared memory emulates the
equivalent functionality on a Series 800 HP9000 computer system. [REV
BEG]A process cannot attach to the same shmid multiple times.[REV END]
The address must be the same in all processes. Specifying a different
address results in an error.
When attaching to a shared memory area for the first time, shmaddr must
be set to zero.
The SHM_RND and SHMLBA flags are not supported. Specifying SHM_RND
results in an error, and errno is set to EACCES.
The maximum number of shared memory areas a process can attach to is 256.
When fork() is called, the child process inherits all shared memory areas
to which the parent process is attached. When exec() is called, the
shared memory attached to the calling process is not attached to the new
process.
Errors
If an error occurs, errno is set to one of the following values.
EACCES CAUSE The calling process does not have permission, or
SHM_RND was specified.
ACTION Ensure that the calling process has permission to
access the area as requested, or do not specify the
SHM_RND and SHMLBA flags.
EINVAL CAUSE shmid is not a valid shared memory identifier, or
shmaddr is not zero and not equal to the current
attach location for the shared memory area, or the
calling process is already attached to the shared
memory area.
ACTION Check that shmid is valid and has not been removed
from the system. If there is no current attach
location for the shared memory area, make sure
shmaddr is zero. A process cannot attach more than
once (concurrently) to the same shared memory area.
EMFILE CAUSE The number of shared memory areas attached to the
calling process would exceed the system-defined
limit.
ACTION None. The operation can be retried if the process
detaches from another shared memory area to which it
is currently attached.
ENOMEM CAUSE The available data space is not large enough to
accommodate the shared memory area.
ACTION None. The operation can be retried later.
ESYSERR CAUSE An operating system error occurred that does not map
directly to any of the above errors.
ACTION Examine the MPE/iX process error stack for the type
of system error.
See Also
shmctl(), shmdt(), shmget(), SVID2 (Section 12)