NAME

       shmat, shmdt — attach or detach shared memory

LIBRARY

       Standard C Library (libc, -lc)

SYNOPSIS

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

       void *
       shmat(int shmid, const void *addr, int flag);

       int
       shmdt(const void *addr);

DESCRIPTION

       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 kernel.

       •   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.

       If the SHM_REMAP flag is specified and the passed addr is not NULL, any existing mappings in the  virtual
       addresses  range  are  cleared before the segment is attached.  If the flag is not specified, addr is not
       NULL, and the virtual address range contains some pre-existing mappings, the shmat() call fails.

       The shmdt() system call detaches the shared memory segment at the address  specified  by  addr  from  the
       calling process's address space.

RETURN VALUES

       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 error.

ERRORS

       The shmat() system call will fail if:

       [EINVAL]           No shared memory segment was found corresponding to shmid.

       [EINVAL]           The addr argument was not an acceptable address.

       [ENOMEM]           The specified addr cannot be used for mapping, for  instance  due  to  the  amount  of
                          available  space being smaller than the segment size, or because pre-existing mappings
                          are in the range and no SHM_REMAP flag was provided.

       [EMFILE]           Failed to attach the shared memory segment  because  the  per-process  kern.ipc.shmseg
                          sysctl(3) limit was reached.

       The shmdt() system call will fail if:

       [EINVAL]           The addr argument does not point to a shared memory segment.

SEE ALSO

       shmctl(2), shmget(2)

Debian                                          January 14, 2019                                        SHMAT(2)