POSIX shared memory and semaphores permissions set incorrectly by open calls
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);}
From what I understand, POSIX semaphores are created in shared memory. So you need to make sure that users have
rw permissions to /dev/shm for the semaphores to be created.
Then, as a handy option, put the following line in your /etc/fstab file to mount tmpfs:
none /dev/shm tmpfs defaults 0 0
So that when your machine is rebooted, the permissions are set right from the start.
Two of the three had /dev/shm set to drwxrwxrwx and the machine that would not allow creation of semaphores had it set to drwxr_xr_x.
You can also look at shared memory limits:
------ Shared Memory Limits --------max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18446744073642442748
min seg size (bytes) = 1