Provided by: freebsd-manpages_8.0-1_all
shmat, shmdt - attach or detach shared memory
Standard C Library (libc, -lc)
shmat(int shmid, const void *addr, int flag);
shmdt(const void *addr);
The shmat() system call attaches the shared memory segment identified by
shmid to the calling process’s address space. The address where the
segment is attached is determined as follows:
· If addr is 0, the segment is attached at an address selected by the
· If addr is nonzero and SHM_RND is not specified in flag, the segment
is attached the specified address.
· If addr is specified and SHM_RND is specified, addr is rounded down
to the nearest multiple of SHMLBA.
The shmdt() system call detaches the shared memory segment at the address
specified by addr from the calling process’s address space.
Upon success, shmat() returns the address where the segment is attached;
otherwise, -1 is returned and errno is set to indicate the error.
The shmdt() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate the
The shmat() system call will fail if:
[EINVAL] No shared memory segment was found corresponding to
[EINVAL] The addr argument was not an acceptable address.
The shmdt() system call will fail if:
[EINVAL] The addr argument does not point to a shared memory