SHMGET(2) System Calls Manual SHMGET(2)

shmgetget shared memory area identifier

#include <sys/shm.h>

shmget(key_t key, size_t size, int shmflg);

() returns the shared memory identifier associated with the key key.

A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated with it, and the IPC_CREAT bit is set in shmflg.

If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:

Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

The shmget() system call will fail if:

A shared memory segment is already associated with key and the caller has no permission to access it.
Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associated with it is less than size, which is non-zero.
IPC_CREAT was not set in shmflg and no shared memory segment associated with key was found.
There is not enough memory left to created a shared memory segment of the requested size.
A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.

#include <sys/types.h> #include <sys/ipc.h> #include <sys/shm.h>

(key_t key, int size, int shmflg);;

All of these include files are necessary. The type of size has changed.

ftok(3), shmat(2), shmctl(2), shmdt(2), compat(5)

August 17, 1995 macOS 14.4