Date: Tue, 27 Dec 2016 13:58:57 +0000 From: bugzilla-noreply@freebsd.org To: freebsd-doc@FreeBSD.org Subject: [Bug 215612] shm_open.2 man page does not accurately describe support for read(2)/write(2) on POSIX shared-memory objects Message-ID: <bug-215612-9@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D215612 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@FreeBSD.org Reporter: rwatson@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 content= s, persist across reboots. In FreeBSD, read(2) and write(2) on a shared memory object will fail w= ith 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 =3D 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 =3D pwrite(fd, buffer, getpagesize(), 0); if (len < 0) err(EX_IOERR, "%s: pwrite", __func__); if (len !=3D 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. --=20 You are receiving this mail because: You are the assignee for the bug.=
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