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: [email protected]
Reporter: [email protected]
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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