Ubuntu Manpage: shmat, shmdt - System V shared memory operations

NAME

       shmat, shmdt - System V shared memory operations

SYNOPSIS

       #include <sys/types.h>
       #include <sys/shm.h>

       void *shmat(int shmid, const void *shmaddr, int shmflg);

       int shmdt(const void *shmaddr);

DESCRIPTION

shmat()
shmat() attaches the System V shared memory segment identified by shmid to the address space of the
calling process. The attaching address is specified by shmaddr with one of the following criteria:

* If shmaddr is NULL, the system chooses a suitable (unused) page-aligned address to attach the segment.

* If shmaddr isn't NULL and SHM_RND is specified in shmflg, the attach occurs at the address equal to
shmaddr rounded down to the nearest multiple of SHMLBA.

* Otherwise, shmaddr must be a page-aligned address at which the attach occurs.

In addition to SHM_RND, the following flags may be specified in the shmflg bit-mask argument:

SHM_EXEC (Linux-specific; since Linux 2.6.9)
Allow the contents of the segment to be executed. The caller must have execute permission on the
segment.

SHM_RDONLY
Attach the segment for read-only access. The process must have read permission for the segment.
If this flag is not specified, the segment is attached for read and write access, and the process
must have read and write permission for the segment. There is no notion of a write-only shared
memory segment.

SHM_REMAP (Linux-specific)
This flag specifies that the mapping of the segment should replace any existing mapping in the
range starting at shmaddr and continuing for the size of the segment. (Normally, an EINVAL error
would result if a mapping already exists in this address range.) In this case, shmaddr must not
be NULL.

The brk(2) value of the calling process is not altered by the attach. The segment will automatically be
detached at process exit. The same segment may be attached as a read and as a read-write one, and more
than once, in the process's address space.

A successful shmat() call updates the members of the shmid_ds structure (see shmctl(2)) associated with
the shared memory segment as follows:

shm_atime is set to the current time.

shm_lpid is set to the process-ID of the calling process.

shm_nattch is incremented by one.

shmdt()
shmdt() detaches the shared memory segment located at the address specified by shmaddr from the address
space of the calling process. The to-be-detached segment must be currently attached with shmaddr equal
to the value returned by the attaching shmat() call.

On a successful shmdt() call, the system updates the members of the shmid_ds structure associated with
the shared memory segment as follows:

shm_dtime is set to the current time.

shm_lpid is set to the process-ID of the calling process.

shm_nattch is decremented by one. If it becomes 0 and the segment is marked for deletion, the
segment is deleted.

RETURN VALUE

       On  success,  shmat() returns the address of the attached shared memory segment; on error, (void *) -1 is
       returned, and errno is set to indicate the cause of the error.

       On success, shmdt() returns 0; on error -1 is returned, and errno is set to indicate  the  cause  of  the
       error.

ERRORS

       When shmat() fails, errno is set to one of the following:

       EACCES The calling process does not have the required permissions for the requested attach type, and does
              not have the CAP_IPC_OWNER capability in the user namespace that governs its IPC namespace.

       EIDRM  shmid points to a removed identifier.

       EINVAL Invalid shmid value, unaligned (i.e., not page-aligned and SHM_RND was not specified)  or  invalid
              shmaddr  value,  or  can't  attach  segment at shmaddr, or SHM_REMAP was specified and shmaddr was
              NULL.

       ENOMEM Could not allocate memory for the descriptor or for the page tables.

       When shmdt() fails, errno is set as follows:

       EINVAL There is no shared memory segment attached at shmaddr; or,  shmaddr  is  not  aligned  on  a  page
              boundary.

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.

       In  SVID  3  (or  perhaps  earlier),  the type of the shmaddr argument was changed from char * into const
       void *, and the returned type of shmat() from char * into void *.

NOTES

After a fork(2), the child inherits the attached shared memory segments.

After an execve(2), all attached shared memory segments are detached from the process.

Upon _exit(2), all attached shared memory segments are detached from the process.

Using shmat() with shmaddr equal to NULL is the preferred, portable way of attaching a shared memory
segment. Be aware that the shared memory segment attached in this way may be attached at different
addresses in different processes. Therefore, any pointers maintained within the shared memory must be
made relative (typically to the starting address of the segment), rather than absolute.

On Linux, it is possible to attach a shared memory segment even if it is already marked to be deleted.
However, POSIX.1 does not specify this behavior and many other implementations do not support it.

The following system parameter affects shmat():

SHMLBA Segment low boundary address multiple. When explicitly specifying an attach address in a call to
shmat(), the caller should ensure that the address is a multiple of this value. This is necessary
on some architectures, in order either to ensure good CPU cache performance or to ensure that
different attaches of the same segment have consistent views within the CPU cache. SHMLBA is
normally some multiple of the system page size. (On many Linux architectures, SHMLBA is the same
as the system page size.)

The implementation places no intrinsic per-process limit on the number of shared memory segments
(SHMSEG).

COLOPHON

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