I'm trying to create a shared memory which will be used by several processes, which will not necessarily be started by the same user, so I create the segment with the following line:
fd = shm_open(SHARE_MEM_NAME,O_RDWR | O_CREAT,0606);
however, when I check out the permissions of the file created in /dev/shm they are:
-rw----r-- 1 lmccauslin lmccauslin 1784 2012-08-10 17:11 /dev/shm/CubeConfigShare
not -rw----rw-
as I'd expected.
the permissions for /dev/shm are lrwxrwxrwx.
The exact same thing happens with the semaphore created similarly.
kernel version: 3.0.0-23-generic
glibc version: EGLIBC 2.13-20ubuntu5.1
Anyone got any ideas?
It's probably umask
.
Citing the manpage of shm_open
:
O_CREAT Create the shared memory object if it does not exist. The user and
group ownership of the object are taken from the corresponding effec‐
tive IDs of the calling process, and the object's permission bits are
set according to the low-order 9 bits of mode, except that those bits
set in the process file mode creation mask (see umask(2)) are cleared
for the new object. A set of macro constants which can be used to
define mode is listed in open(2). (Symbolic definitions of these
constants can be obtained by including <sys/stat.h>.)
So, in order to allow creating files which are world-writable, you'd need to set an umask permitting it, for example:
umask(0);
Set like this, umask
won't affect any permissions on created files anymore. However, you should note that if you will then create another file without specifying permissions explicitly, it will be world-writable as well.
Thus, you may want to clear the umask only temporarily, and then restore it:
#include <sys/types.h>
#include <sys/stat.h>
...
void yourfunc()
{
// store old
mode_t old_umask = umask(0);
int fd = shm_open(SHARE_MEM_NAME,O_RDWR | O_CREAT,0606);
// restore old
umask(old_umask);
}