Manual Pages  — SHM_OPEN

If O_CREAT is specified, then a new shared memory object named path 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).

As a FreeBSD extension, the constant SHM_ANON may be used for the path argument to shm_open(). In this case, an anonymous, unnamed shared memory object is created. Since the object has no name, it cannot be removed via a subsequent call to shm_unlink(). Instead, the shared memory object will be garbage collected when the last reference to the shared memory object is removed. The shared memory object may be shared with other processes by sharing the file descriptor via fork(2) or sendmsg(2). Attempting to open an anonymous shared memory object with O_RDONLY will fail with EINVAL. All other flags are ignored.

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

POSIX specifications state that 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. However, the FreeBSD kernel implementation explicitly includes support for read(2) and write(2).

FreeBSD also supports zero-copy transmission of data from shared memory objects with sendfile(2).

Neither shared memory objects nor their contents persist across reboots.

Writes do not extend shared memory objects, so ftruncate(2) must be called before any data can be written. See EXAMPLES.

        uint8_t buffer[getpagesize()];
        ssize_t len;
        int fd;

        fd = shm_open(SHM_ANON, O_RDWR | O_CREAT, 0600);
        if (fd < 0)
                err(EX_OSERR, "%s: shm_open", __func__);
        if (ftruncate(fd, getpagesize()) < 0)
                err(EX_IOERR, "%s: ftruncate", __func__);
        len = pwrite(fd, buffer, getpagesize(), 0);
        if (len < 0)
                err(EX_IOERR, "%s: pwrite", __func__);
        if (len != getpagesize())
                errx(EX_IOERR, "%s: pwrite length mismatch", __func__);