Question

Given this code:

char * pathname; // some string 
int returnValue = shm_open(pathname, O_CREAT | O_TRUNC | O_RDWR, mode);  // create a shared memory object 

If I do this: note – same pathname!)

int returnValue2 = shm_open(pathname, O_CREAT | O_TRUNC | O_RDWR, mode);  // create a shared memory object 

I get no error, and the return value is okay (0).

Does it mean that I’m recreating a shared memory segment/object?

Answer 1

You’re just re-opening the existing shared memory segment.

O_CREAT

Create the shared memory object if it does not exist. (…)

O_EXCL

If O_CREAT was also specified, and a shared memory object with the given name already exists, return an error.

(From the Linux man shm_open.)

Additionally, from POSIX (a bit more explicit):

O_CREAT

If the shared memory object exists, this flag has no effect, except as noted under O_EXCL below. Otherwise, the shared memory object is created. (…)