shmop_open() can create or open a shared memory block.
shmop_open() takes 4 parameters: key, which is the system's id for the shared memory block, this parameter can be passed as a decimal or hex. The second parameter are the flags that you can use:
"a" for access (sets SHM_RDONLY for shmat) use this flag when you need to open an existing shared memory segment for read only
"c" for create (sets IPC_CREATE) use this flag when you need to create a new shared memory segment or if a segment with the same key exists, try to open it for read and write
"w" for read & write access use this flag when you need to read and write to a shared memory segment, use this flag in most cases.
"n" create a new memory segment (sets IPC_CREATE|IPC_EXCL) use this flag when you want to create a new shared memory segment but if one already exists with the same flag, fail. This is useful for security purposes, using this you can prevent race condition exploits.
Note: Note: the 3rd and 4th should be entered as 0 if you are opening an existing memory segment. On success shmop_open() will return an id that you can use to access the shared memory segment you've created.
This example opened a shared memory block with a system id returned by ftok().
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