The definitive answer can be found in the source code for the Darwin kernel, xnu (links are below):

1. The shmmax, shmmin, shmmni, shmseg, and shmall settings cannot be changed after the shared memory system is initialized

2. The shared memory system is initialized immediately after all 5 settings have been configured

3. The shmall setting must be a multiple of the page size (on both 10.3 and 10.4)

Here are excerpts from the sysctl_shminfo() function found in sysv_shm.c:

    /* Set the values only if shared memory is not initialised */
    if (!shm_inited) {
        if ((error = SYSCTL_IN(req, arg1, sizeof(user_ssize_t)))
            != 0) {
            sysctl_shminfo_ret = error;
            goto sysctl_shminfo_out;

        /* Initialize only when all values are set */
        if ((shminfo.shmmax != (user_ssize_t)-1) &&
            (shminfo.shmmin != (user_ssize_t)-1) &&
            (shminfo.shmmni != (user_ssize_t)-1) &&
            (shminfo.shmseg != (user_ssize_t)-1) &&
            (shminfo.shmall != (user_ssize_t)-1)) {

        if (arg1 == &shminfo.shmmax) {
            if (shminfo.shmmax & PAGE_MASK) {
                shminfo.shmmax = -1;

        if (arg1 == &shminfo.shmmax) {
            if (shminfo.shmmax & PAGE_MASK_64) {
                shminfo.shmmax = -1;

PAGE_MASK is "(PAGE_SIZE - 1)", and PAGE_MASK_64 is simply "(unsigned long long)PAGE_MASK"

/etc/rc contains commands to set all 5 of the parameters. In 10.3.9 and later, you can place your own customized settings inside /etc/ sysctl.conf that will be executed before /etc/rc's commands. But if you don't set them all, the shared memory system isn't initialized, and /etc/rc will overwrite them.

Does that make sense?

In a nutshell, if you want to customize the shared memory settings, you must:

(On 10.3.9 and later)
Set ALL 5 settings inside /etc/sysctl.conf, and make sure that shmmax is a multiple of the page size (4096?). The page size is a runtime variable that's set by kernel bootstrap mechanisms calling vm_set_page_size() (I haven't tracked those down yet).

(On earlier systems)
Modify the desired settings inside /etc/rc, and be aware that OS updates will overwrite your changes. If you upgrade to 10.3.9, migrate to using /etc/sysctl.conf.

In my own OS X products that use a PostgreSQL server, I install an / etc/sysctl.conf file on all servers. If installing on anything earlier than 10.3, my installer modifies /etc/rc. That way, when the customer upgrades their server to 10.3.9 and the /etc/rc file is overwritten, the /etc/sysctl.conf file is already in place.


- Chris

Referenced source code:

Mac OS X 10.4.4: xnu-792.6.56/bsd/kern/sysv_shm.c

Mac OS X 10.3.9: bsd/kern/sysv_shm.c

Mac OS X 10.3: kern/sysv_shm.c

Attachment: smime.p7s
Description: S/MIME cryptographic signature

Reply via email to