Manpage of SHMOP

SHMOP

Section: Linux Programmer's Manual (2)
Updated: 2015-08-08
Index
 

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 shmidto the address space of the calling process. The attaching address is specified by shmaddrwith one of the following criteria:
*
If shmaddris NULL, the system chooses a suitable (unused) address at which to attach the segment.
*
If shmaddrisn't NULL and SHM_RNDis specified in shmflg, the attach occurs at the address equal to shmaddrrounded down to the nearest multiple of SHMLBA.
*
Otherwise, shmaddrmust be a page-aligned address at which the attach occurs.

In addition to SHM_RND, the following flags may be specified in the shmflgbit-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 shmaddrand continuing for the size of the segment. (Normally, an EINVALerror would result if a mapping already exists in this address range.) In this case, shmaddrmust 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_dsstructure (see shmctl(2)) associated with the shared memory segment as follows:

shm_atimeis set to the current time.
shm_lpidis set to the process-ID of the calling process.
shm_nattchis incremented by one.
 

shmdt()

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

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

shm_dtimeis set to the current time.
shm_lpidis set to the process-ID of the calling process.
shm_nattchis 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 *) -1is returned, and errnois set to indicate the cause of the error.

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

ERRORS

When shmat() fails, errnois 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_OWNERcapability.
EIDRM
shmid points to a removed identifier.
EINVAL
Invalid shmidvalue, unaligned (i.e., not page-aligned and SHM_RND was not specified) or invalid shmaddrvalue, or can't attach segment at shmaddr, or SHM_REMAPwas specified and shmaddrwas NULL.
ENOMEM
Could not allocate memory for the descriptor or for the page tables.

When shmdt() fails, errnois set as follows:

EINVAL
There is no shared memory segment attached at shmaddr; or, shmaddris 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 shmaddrequal 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. SHMLBAis normally some multiple of the system page size (on many Linux architectures, it is the same as the system page size).

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

SEE ALSO

brk(2), mmap(2), shmctl(2), shmget(2), capabilities(7), shm_overview(7), svipc(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
shmat()
shmdt()
RETURN VALUE
ERRORS
CONFORMING TO
NOTES
SEE ALSO

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Time: 22:27:42 GMT, June 20, 2016