Movatterモバイル変換

SHMOP(2)                   Linux Programmer's ManualSHMOP(2)NAME       shmat, shmdt - System V shared memory operationsSYNOPSIS       #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  at-          tach 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 re-              place 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.       Thebrk(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       (seeshmctl(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 spec-       ified 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 fol-       lows:              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  seg-              ment 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  indi-       cate 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 ta-              bles.       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  afork(2),  the  child inherits the attached shared memory seg-       ments.       After anexecve(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 specify-              ing 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).SEE ALSObrk(2),mmap(2),shmctl(2),shmget(2),capabilities(7),   shm_over-view(7),sysvipc(7)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2019-08-02SHMOP(2)