Mastering Linux Kernel Development

Attaching a shared memory

The shared memory region must be attached to its address space for a process to access it. shmat() is invoked to attach the shared memory to the address space of the calling process:

void *shmat(int shmid, const void *shmaddr, int shmflg);

The segment indicated by shmid is attached by this function. shmaddr specifies a pointer indicating the location in the process's address space where the segment is to be mapped. The third argument shmflg is a flag, which can be one of the following:

• SHM_RND: This is specified when shmaddr isn't a NULL value, indicating the function to attach the segment at the address, computed by rounding off the shmaddr value to the nearest multiple of page size; otherwise, the user must take ...