PHP / C++: shm_open() error when sharing memory

Question

I've been all over the internet looking for an answer to my problem. Here is the setup, I am running embedded Linux (created with Yocto) which is running the Lighttpd web server with PHP5. In my C++ code I have the following:

shared = shm_open(SHARED_FILE_NAME, O_RDWR | O_CREAT | O_TRUNC, 0666);
ftruncate(shared, FILE_SIZE);
map = mmap(...);
// shm_unlink() isn't called until my C++ thread ends.

Everything works well and I do not get any errors and other C++ processes and threads are also able to access the shared memory and map without any problems (I have one writer thread and all other threads and processes do a read only on the memory). The memory is used as a ring buffer where the writing thread is updating data very quickly. The problems start to occur when trying to access that same memory in PHP. In PHP I do (need read only):

<?php
$shm_key = ftok("/dev/shm/shared_file.shm", 'c');
$shm_id = shm_open($shm_key, "a", 0, 0);
...
?>

When looking at the value from ftok() it returns a non -1 number which means it did not fail. I do get a fail on the PHP's shm_open() call which reads:

Warning: shmop_open(): unable to attach or create shared memory segment in /www/pages/shared.php on line 9

I've changed the permission of the file with chmod 777 /dev/shm/shared.shm just to rule out any file permission issues. Also when I run ipcs -m I do not get any listings for shared memory segments, yet my C++ code is running just fine. I've also looked for SELinux and tried entering setenforce 0 but I get a response of -sh: setenforce: command not found so I figure this isn't an issue. I've also tried running wget <local ip address>/shared.php to see if running locally would return the correct data but when looking at the file which was returned it had the same error messages.

I am looking to be able to have a web page on my embedded system read this shared memory and stream back chunks of binary to feed a graph when a request comes in (not interested in web sockets at the time). I am able to get named pipes to work across PHP and C++ just fine but I need shared memory for this application and the shared memory access seems to be troublesome. Any help is appreciated.

Answer 1

I'm developing PHP functions that need to use C Shared Memory. As your code, my C functions use shm_open, mmap, etc.. and I guess to use PHP ftok(), shmop_open() to access the C's shared memory but this PHP functions don't work.
The two area are not compatible. I found different properties of the two areas in this documents http://menehune.opt.wfu.edu/Kokua/More_SGI/007-2478-008/sgi_html/ch03.html :

• C (with shm_open, mmap, like the Straton source code) use “POSIX Shared Memory”
• PHP (with shmop_* functions) use “System V Shared Memory”

I suggest you to try with Sync http://php.net/manual/en/book.sync.php : you need the PECL sync extension.