[Bug 215612] shm_open.2 man page does not accurately describe support for read(2)/write(2) on POSIX shared-memory objects

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215612

            Bug ID: 215612
           Summary: shm_open.2 man page does not accurately describe
                    support for read(2)/write(2) on POSIX shared-memory
                    objects
           Product: Documentation
           Version: Latest
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: Documentation
          Assignee: freebsd-doc at FreeBSD.org
          Reporter: rwatson at FreeBSD.org

The shm_open(2) man page states the following about POSIX shared-memory
objects:

     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.

     In FreeBSD, read(2) and write(2) on a shared memory object will fail with
     EOPNOTSUPP and neither shared memory objects nor their contents persist
     across reboots.

However, this is no longer up-to-date: the kernel implementation explicitly
includes support for read(2)/write(2).  This documentation also does not
mention the confusing property that ftruncate(2) must be called to extend a
shared-memory object before it can be written to. I.e., the following does
require a call to ftruncate(2):

        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__);

If the truncate is missing, then pwrite(2) will return 0 bytes written (EOF).
It is not clear that this is a bug, but it probably deserves mention.

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