shmop(2)                  SYSTEM CALLS                   shmop(2)



NAME
     shmop: shmat, shmdt - shared memory operations

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

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

     int shmdt (void *shmaddr);

DESCRIPTION
     shmat attaches the shared memory segment associated with the
     shared memory identifier specified by shmid to the data seg-
     ment of the calling process.  The segment is attached at the
     address specified by one of the following criteria:

          If shmaddr is equal to  (void  *)  0,  the  segment  is
          attached  at the first available address as selected by
          the system.

          If  shmaddr  is  not  equal   to   (void   *)   0   and
          (shmflg&SHMRND)  is  true,  the segment is attached at
          the  address  given  by  (shmaddr  -  (shmaddr  modulus
          SHMLBA)).

          If  shmaddr  is  not  equal   to   (void   *)   0   and
          (shmflg&SHMRND)  is  false, the segment is attached at
          the address given by shmaddr.

     shmdt detaches from the calling process's data  segment  the
     shared  memory  segment  located at the address specified by
     shmaddr.

     The segment is attached for reading  if  (shmflg&SHMRDONLY)
     is  true  {READ},  otherwise  it is attached for reading and
     writing {READ/WRITE}.

     shmat fails and does not attach the shared memory segment if
     one or more of the following are true:

     EINVAL         shmid is not a valid shared  memory  identif-
                    ier.

     EACCES         Operation permission is denied to the calling
                    process [see intro(2)].

     ENOMEM         The available data space is not large  enough
                    to accommodate the shared memory segment.

     EINVAL         shmaddr is not equal to zero, and  the  value



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shmop(2)                  SYSTEM CALLS                   shmop(2)



                    of  (shmaddr - (shmaddr modulus SHMLBA)).  is
                    an illegal address.

     EINVAL         shmaddr    is    not    equal    to     zero,
                    (shmflg&SHMRND)  is  false, and the value of
                    shmaddr is an illegal address.

     EMFILE         The number of shared memory segments attached
                    to  the  calling  process  would  exceed  the
                    system-imposed limit.

     EINVAL         shmdt fails and does not  detach  the  shared
                    memory  segment  if  shmaddr  is not the data
                    segment start address of a shared memory seg-
                    ment.

SEE ALSO
     intro(2), exec(2), exit(2), fork(2), shmctl(2), shmget(2).

DIAGNOSTICS
     Upon successful completion, the return value is as follows:

          shmat returns the data segment  start  address  of  the
          attached shared memory segment.

          shmdt returns a value of 0.

     Otherwise, a value of -1 is returned and  errno  is  set  to
     indicate the error.

NOTES
     The user must explicitly remove shared memory segments after
     the last reference to them has been removed.






















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