Solve Linux system v shared memory problem
- 2021-07-03 01:14:07
- OfStack
system v Shared Memory
#include <sys/types.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);Establish: The relationship between process and shared memory
key_t key: A non-0 digit in hexadecimal.
1. There are two ways to set it.
Type 1: Call the fotk function
Chapter 2: Using IPC_PRIVATE directly
size: Size of shared memory
shmflg:
IPC_CREAT IPC_EXCL The permissions of users, group users and other users on this memory are represented by 9 bit, such as 664Return value: Successfully returned the identification number of this shared memory; Failure returns-1 and errno is set.
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(int shmid, const void *shmaddr, int shmflg);
shmid: Created by the shmget function, which is the return value of the shmget function
shmaddr:
NULL: Let the kernel request memory space
Non-NULL: Use malloc to create a space for shared memory shmid to be associated with this address. But if it is not an integer multiple of 4K, the kernel will adjust up or down.
shmflg:
SHM_RND: Read and write
SHM_RDONLY: Read-only
Return value:
Success: Return memory address
Failure: Return (void *)-1
#include <sys/types.h>
#include <sys/shm.h>
int shmdt(const void *shmaddr);Unassociate process with shared memory
shmaddr: Return value of shmat
Return value: Success 0; Failed-1 and set errno.
#include <sys/types.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);For shared memory operation, more cmd different, shared memory for different operations.
shmid: Created by the shmget function, which is the return value of the shmget function
cmd:
IPC_STAT: Get the status of shared memory
IPC_RMID: Mark deletes shared memory (deletes when the reference count of shared memory becomes 0)
IPC_SET: Set shared memory properties (modify permissions, modify shmid, and so on)
Wait
buf: shmid_ds structure
Return value: When cmd is IPC_RMID: Success 0; Failed-1 and set errno
Use the command "ipcs" to 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 3Summarize