Mode of shared memory between Linux interprocess communication

Shared memory: A portion of the physical memory is shared as multiple processes.

Shared memory is one of the fastest ways to share data between processes, a process writes data to a shared memory area, and all processes that share that memory can immediately see the contents.

Shared Memory Implementation steps:

First, create shared memory, use the Shmget function.

mapping shared memory, mapping the shared memory created to a specific process space, using the Shmat function.

Create shared memory Shmget:

Intshmget (key_t key, size_t size, int shmflg)

Function: Get a shared memory identifier or create a shared memory object and return the shared memory identifier.

Key:0 (ipc_private) creates a shared memory object

Size: An integer greater than 0 that is the size, in bytes, of the new shared memory. When only shared memory is acquired, specify 0.

SHMFLG:

0 indicates that the shared memory identifier is taken, and the function will complain if it does not exist;

Ipc_creat creates a shared memory if there is no shared memory with key values equal to key in the kernel, and returns the identifier of the shared memory if such shared memory exists;

Ipc_creat| IPC_EXCL: If there is no shared memory with key values equal to key in the kernel, a new message queue is created, and an error occurs if there is such shared memory;

function return value: Returns the identifier of the memory in case of success, and returns 1 for error.

Map shared memory to the address space of the calling process Shmat:

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

MSQID: Shared Memory identifier

SHMADDR: Specifies where the shared memory appears in the process memory address, specifying NULL directly to allow the kernel to determine a suitable address location for itself.

Shmflg:shm_rdonly read-only mode, others for read-write mode

function return value: Returns the attached shared memory address for success; error return-1, the cause of errors is in error

Disconnect the shared memory connection SHMDT:

INTSHMDT (const void *SHMADDR)

Function: Incoming shmaddr, connect the shared memory start address, return 0 if the fracture succeeds, and return 1 for error.

Parent-Child interprocess communication instance:

#include

#include

#include

#include

#include

#include

int main (int argc, char **argv) {

if (argc< 2) {//need to enter shared data

printf ("Pleaseinput the Shared DATA.N");

Exit (-1);

}

Intshmid;

Shmid= Shmget (0,1024,ipc_creat);

if (shmid==-1) {//Request shared memory failed

printf ("Createshare memory FAILED.N");

Exit (-1);

}

if (fork ()) {//parent process

CHAR*P_SHMADDR;

P_shmaddr= Shmat (shmid, NULL, 0); Map to an address in the parent process

memset (p_shmaddr,0, 1024); Initializing shared memory

strcpy (p_shmaddr,argv[1]); Copy shared data to shared memory

Wait (NULL); Wait for child process to end

Exit (0);

}

else{

Sleep (2); Waiting for the parent process to write data

CHAR*C_SHMADDR;

C_shmaddr= Shmat (shmid,null,0); Map to a child process an address, specifically by kernel

printf ("Theshare data is:%SN", c_shmaddr); Child process output shared data

Exit (0);

}

}