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(), or moved with a call to shm_rename(). 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_rename() system call atomically removes a shared memory object named path_from and relinks it at path_to. If another object is already linked at path_to, that object will be unlinked, unless one of the following flags are provided:

SHM_RENAME_EXCHANGE	Atomically exchange the shms at path_from and path_to.
SHM_RENAME_NOREPLACE	Return an error if an shm exists at path_to, rather than unlinking it.

shm_rename() is also a FreeBSD extension.

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

The memfd_create() function creates an anonymous shared memory object, identical to that created by shm_open() when SHM_ANON is specified. Newly created objects start off with a size of zero. The size of the new object must be adjusted via ftruncate(2).

The name argument must not be NULL, but it may be an empty string. The length of the name argument may not exceed NAME_MAX minus six characters for the prefix "memfd:", which will be prepended. The name argument is intended solely for debugging purposes and will never be used by the kernel to identify a memfd. Names are therefore not required to be unique.

The following flags may be specified to memfd_create():

MFD_CLOEXEC	Set FD_CLOEXEC on the resulting file descriptor.
MFD_ALLOW_SEALING
	Allow adding seals to the resulting file descriptor using the F_ADD_SEALS fcntl(2) command.
MFD_HUGETLB	This flag is currently unsupported.

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__);