Linux system v Shared Memory

shared memory system v

#include <sys/types.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);

Created: association process with the shared memory

• key_t key: non-zero digit hexadecimal.

  There are two general ways to set it up.

  The first: the function call fotk

  Chapter II: Direct use IPC_PRIVATE

• size: shared memory size

• shmflg：

  • IPC_CREAT
  • IPC_EXCL
  • User, group of users, the other users permission to this memory sheet, nine bit represented, such as 664

• Return Value: returns the success of this identification number of the shared memory; failure returns -1, errno is set.

#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);

• shmid: created by shmget function, which is the function's return value shmget
• shmaddr:
  • NULL: to apply the kernel memory space
  • Non-NULL: own use malloc to open up a space for the shared memory shmid and associate this address. However, if not a multiple of 4K, the kernel will adjust up or down.
• shmflg：
  • SHM_RND: reading and writing
  • SHM_RDONLY: Read-only
• return value:
  • Success: Returns memory address
  • Failure: return (void *) -1

#include <sys/types.h>
#include <sys/shm.h>
int shmdt(const void *shmaddr);

Association to cancel the process and the shared memory

• shmaddr: shmat return value
• Return Value: 0 success; failure -1, and set errno.

#include <sys/types.h>
#include <sys/shm.h> 
int shmctl(int shmid, int cmd, struct shmid_ds *buf);

Shared memory operations, more cmd different shared memory for different operations.

• shmid: created by shmget function, which is the function's return value shmget
• cmd：
  • IPC_STAT: get the state shared memory
  • IPC_RMID: Remove shared memory tag (memory when the shared reference count becomes 0, deleted)
  • IPC_SET: set the properties of shared memory (permission to modify, amend shmid, etc.)
  • and many more
• buf: shmid_ds structure
• Return value: when cmd is IPC_RMID time: 0 success; failure -1, and errno

[] Command ipcs can view the status of shared memory

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x00007fff 65536      ys         664        256        0
0x00007ffe 98305      ys         664        256        0
0x0000555e 131074     ys         664        256        0
0x00000011 229379     ys         664        256        3

• key: the first argument specified shmget
• shmid: shmget return value of the function
• owner: belongs to which user-created
• perms: The shared memory access
• bytes: size
• The number of shared memory using this process: nattch
• status: status of shared memory