shmat(2) [netbsd man page]

SHMAT(2)						      BSD System Calls Manual							  SHMAT(2)

NAME

     shmat, shmdt -- map/unmap shared memory

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/shm.h>

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

     int
     shmdt(const void *shmaddr);

DESCRIPTION

     shmat() maps the shared memory segment associated with the shared memory identifier shmid into the address space of the calling process.  The
     address at which the segment is mapped is determined by the shmaddr parameter.  If it is equal to 0, the system will pick an address itself.
     Otherwise, an attempt is made to map the shared memory segment at the address shmaddr specifies.  If SHM_RND is set in shmflg, the system
     will round the address down to a multiple of SHMLBA bytes (SHMLBA is defined in <sys/shm.h>).

     A shared memory segment can be mapped read-only by specifying the SHM_RDONLY flag in shmflg.

     shmdt() unmaps the shared memory segment that is currently mapped at shmaddr from the calling process' address space.  shmaddr must be a
     value returned by a prior shmat() call.  A shared memory segment will remain in existence until it is removed by a call to shmctl(2) with the
     IPC_RMID command.

RETURN VALUES

     shmat() returns the address at which the shared memory segment has been mapped into the calling process' address space when successful,
     shmdt() returns 0 on successful completion.  Otherwise, a value of -1 is returned, and the global variable errno is set to indicate the
     error.

ERRORS

     shmat() will fail if:

     [EACCES]		The calling process has no permission to access this shared memory segment.

     [ENOMEM]		There is not enough available data space for the calling process to map the shared memory segment.

     [EINVAL]		shmid is not a valid shared memory identifier.

			shmaddr specifies an illegal address.

     [EMFILE]		The number of shared memory segments has reached the system-wide limit.

     shmdt() will fail if:

     [EINVAL]		shmaddr is not the start address of a mapped shared memory segment.

SEE ALSO

     ipcrm(1), ipcs(1), mmap(2), shmctl(2), shmget(2)

STANDARDS

     The shmat and shmdt system calls conform to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

HISTORY

     Shared memory segments appeared in the first release of AT&T System V UNIX.

BSD
								   June 17, 2002							       BSD

shmop(2)							System Calls Manual							  shmop(2)

Name
       shmop, shmat, shmdt - shared memory operations

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

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

       int shmdt (shmaddr)
       char *shmaddr;

Description
       The  system  call attaches the shared memory segment associated with the shared memory identifier specified by shmid to the data segment of
       the calling process.  The segment is attached at the address specified by one of the following criteria:

       If shmaddr is equal to zero, the segment is attached at the first available address as selected by the system.

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

       If shmaddr is not equal to zero and (shmflg & SHM_RND ) is false, the segment is attached at the address given by shmaddr.

       The segment is attached for reading if (shmflg & SHM_RDONLY ) is true. Otherwise, it is attached for reading and writing.

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

Return Values
       Upon successful completion, the return values are as follows:

       o   The system call returns the data segment start address of the attached shared memory segment.

       o   The system call returns a value of zero (0).

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

Diagnostics
       The system call fails and not attach the shared memory segment, if any of the following is true:

       [EINVAL]       The shmid is not a valid shared memory identifier.

       [EACCES]       Operation permission is denied to the calling process.  For further information, see

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

       [EINVAL]       The shmaddr is not equal to zero, and the value of (shmaddr- (shmaddr modulus SHMLBA )) is an illegal address.

       [EINVAL]       The shmaddr is not equal to zero, (shmflg & SHM_RND ) 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.

       The fails and does not detach the shared memory segment if:

       [EINVAL]       The shmaddr is not the data segment start address of a shared memory segment.

See Also
       execve(2), exit(2), fork(2), shmctl(2), shmget(2)

																	  shmop(2)