Standards
| Standards / Extensions |
C or C++ |
Dependencies |
|---|
XPG4
XPG4.2
Single UNIX Specification, Version 3
|
both |
|
Format
#define _XOPEN_SOURCE
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
General description
The shmat()
function attaches the shared memory segment associated with the shared
memory identifier,
shmid, to the address
space of the calling process. The segment is attached at the address
specified by one of the following criteria:
- If shmaddr is a NULL pointer, the segment
is attached at the first available address as selected by the system.
- If shmaddr is not a NULL pointer, and
the flag, SHM_RND was
specified, the segment is attached at the address given by (shmaddr-((prtdiff_t)shmaddr%SHMLBA))
where % is the 'C' language remainder operator.
- If shmaddr is not a NULL pointer, and
the flag, SHM_RND was
not specified, the segment is attached at the address given by shmaddr.
- The segment is attached for reading if the flag, SHM_RDONLY, is specified with shmflg and
the calling process has read permission. If the flag is not set and
the process has both read and write permission, the segment is attached
for reading and writing.
The first attach of newly created __IPC_MEGA
segment, as well as subsequent attaches, will have write access to
the segment, regardless of the SHM_RDONLY option.
- All attaches to an __IPC_MEGA shared memory segment have
the same Write or Read access authority. If a segment is enabled
for writes then all attaches have the ability to read and write to
the segment. If the segment is disabled for writes, then all attaches
have the ability to read from the segment and cannot write to the
segment
The first attach of newly created __IPC_MEGA segment,
as well as subsequent attaches, will have write access to the segment,
regardless of the SHM_RDONLY option. Write/Read access can be changed
by the shmctl() function, Shared Memory Control Operations.
An __IPC_MEGA shared memory segment is attached as follows:
- If shmaddr is zero and __IPC_MEGA segment, then the segment will
be attached at the first available address selected by the system
on a segment boundary.
- If shmaddr is not zero and SHM_RND is specified and __IPC_MEGA
segment, the segment address will be truncated to the segment boundary
(last 20 bits zero).
- If shmaddr is not zero and SHM_RND is not specified and __IPC_MEGA
segment, the segment address must be a megabyte multiple (segment
boundary).
Returned value
If successful, 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 starting address.
If unsuccessful, shmat()
returns -1 and sets errno to one of the following values:
- Error Code
- Description
- EACCES
- Operation permission is denied to the calling process.
- EINVAL
- The value of shmid is not a valid shared
memory identifier; the shmaddr is not a
NULL pointer and the value of (shmaddr-((ptrdiff_t)shmaddr%SHMLBA))
is an illegal address for attaching shared memory segments; or the shmaddr is
not a NULL pointer, SHM_RND was specified, and the value of shmaddr is
an illegal address for attaching shared memory segments.
The shared
memory address, *shmaddr, is not zero, is not on a megabyte boundary,
and SHM_RND was not specified.
- EMFILE
- The number of shared memory segments attached to the calling process
would exceed the system-imposed limit.
- ENOMEM
- The available data space is not large enough to accommodate the
shared memory segment.