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SHMGET(2)		    BSD System Calls Manual		     SHMGET(2)

     shmget — obtain a shared memory identifier

     Standard C Library (libc, -lc)

     #include <machine/param.h>
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     shmget(key_t key, size_t size, int flag);

     Based on the values of key and flag, shmget() returns the identifier of a
     newly created or previously existing shared memory segment.  The key is
     analogous to a filename: it provides a handle that names an IPC object.
     There are three ways to specify a key:

     ·	 IPC_PRIVATE may be specified, in which case a new IPC object will be

     ·	 An integer constant may be specified.	If no IPC object corresponding
	 to key is specified and the IPC_CREAT bit is set in flag, a new one
	 will be created.

     ·	 The ftok(3) may be used to generate a key from a pathname.

     The mode of a newly created IPC object is determined by OR'ing the fol‐
     lowing constants into the flag argument:

     SHM_R	   Read access for user.

     SHM_W	   Write access for user.

     (SHM_R>>3)	   Read access for group.

     (SHM_W>>3)	   Write access for group.

     (SHM_R>>6)	   Read access for other.

     (SHM_W>>6)	   Write access for other.

     When creating a new shared memory segment, size indicates the desired
     size of the new segment in bytes.	The size of the segment may be rounded
     up to a multiple convenient to the kernel (i.e., the page size).

     Upon successful completion, shmget() returns the positive integer identi‐
     fier of a shared memory segment.  Otherwise, -1 is returned and errno set
     to indicate the error.

     The shmget() system call will fail if:

     [EINVAL]		Size specified is greater than the size of the previ‐
			ously existing segment.	 Size specified is less than
			the system imposed minimum, or greater than the system
			imposed maximum.

     [ENOENT]		No shared memory segment was found matching key, and
			IPC_CREAT was not specified.

     [ENOSPC]		The kernel was unable to allocate enough memory to
			satisfy the request.

     [EEXIST]		IPC_CREAT and IPC_EXCL were specified, and a shared
			memory segment corresponding to key already exists.

     shmat(2), shmctl(2), shmdt(2), ftok(3)

BSD				 July 3, 1995				   BSD

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