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SHM_OPEN(3)                Library Functions Manual                SHM_OPEN(3)

     shm_open, shm_unlink - shared memory object operations

     POSIX Real-time Library (librt, -lrt)

     #include <sys/types.h>
     #include <sys/mman.h>
     #include <fcntl.h>

     shm_open(const char *name, int flags, mode_t mode);

     shm_unlink(const char *name);

     The shm_open() function creates or opens a POSIX shared memory object
     named name.  The flags argument contains a subset of the flags used by
     open(2).  An access mode of either O_RDONLY or O_RDWR must be included in
     flags.  The optional flags O_CREAT, O_EXCL, and O_TRUNC may also be

     If O_CREAT is specified, then a new shared memory object named name will
     be created if it does not exist.  In this case, the shared memory object
     is created with mode mode subject to the process' umask value.  If both
     the O_CREAT and O_EXCL flags are specified and a shared memory object
     named path already exists, then shm_open() will fail with EEXIST.

     Newly created objects start off with a size of zero.  If an existing
     shared memory object is opened with O_RDWR and the O_TRUNC flag is
     specified, then the shared memory object will be truncated to a size of
     zero.  The size of the object can be adjusted via ftruncate(2) and
     queried via fstat(2).

     The new descriptor is set to close during execve(2) system calls; see
     close(2) and fcntl(2).

     The shm_unlink() system call removes a shared memory object named path.

     If successful, shm_open() returns a non-negative integer, and
     shm_unlink() returns zero.  Both functions return -1 on failure, and set
     errno to indicate the error.

     The path argument does not necessarily represent a pathname (although it
     does in most other implementations).  Two processes opening the same path
     are guaranteed to access the same shared memory object if and only if
     path begins with a slash (`/') character.

     Only the O_RDONLY, O_RDWR, O_CREAT, O_EXCL, and O_TRUNC flags may be used
     in portable programs.

     The result of using open(2), read(2), or write(2) on a shared memory
     object, or on the descriptor returned by shm_open(), is undefined.  It is
     also undefined whether the shared memory object itself, or its contents,
     persist across reboots.

     The following errors are defined for shm_open():

     [EACCES]           The required permissions (for reading or reading and
                        writing) are denied.

     [EEXIST]           O_CREAT and O_EXCL are specified and the named shared
                        memory object does exist.

     [EFAULT]           The path argument points outside the process'
                        allocated address space.

     [EINVAL]           A flag other than O_RDONLY, O_RDWR, O_CREAT, O_EXCL,
                        or O_TRUNC was included in flags; or the path does not
                        begin with a slash (`/') character.

     [EMFILE]           The process has already reached its limit for open
                        file descriptors.

     [ENAMETOOLONG]     The entire pathname exceeded {PATH_MAX} characters.

     [ENFILE]           The system file table is full.

     [ENOENT]           O_CREAT is specified and the named shared memory
                        object does not exist.

     [ENOTSUP]          Not supported, most likely due to missing or incorrect
                        /var/shm mount.

     The following errors are defined for shm_unlink():

     [EACCES]           The required permissions are denied.  shm_unlink()
                        requires write permission to the shared memory object.

     [EFAULT]           The path argument points outside the process'
                        allocated address space.

     [ENAMETOOLONG]     The entire pathname exceeded {PATH_MAX} characters.

     [ENOENT]           The named shared memory object does not exist.

     close(2), fstat(2), ftruncate(2), mmap(2), munmap(2)

     The shm_open() and shm_unlink() functions are expected to conform to IEEE
     Std 1003.1b-1993 ("POSIX.1b").

     These functions first appeared in NetBSD 7.0.

NetBSD 8.99.34                 December 19, 2013                NetBSD 8.99.34