Linux network Programming system V shared memory and series functions

Like Message Queuing, shared memory has its own data structure, as follows:

struct Shmid_ds {

struct Ipc_perm shm_perm; /* Ownership and Permissions * *

size_t Shm_segsz; /* Size of Segment (bytes) * *

time_t Shm_atime; /* Last Attach time * *

time_t Shm_dtime; /* Last Detach time * *

time_t Shm_ctime; /* Last Change time * *

pid_t Shm_cpid; /* PID of Creator * *

pid_t Shm_lpid; /* PID of Last Shmat (2)/SHMDT (2) * *

shmatt_t Shm_nattch; /* No. of current Attaches * *

...

};

Similarly, the first member is a shared IPC kernel data structure, and the remainder is private.

Here are a few shared memory functions:

#include <sys/ipc.h>

#include <sys/shm.h>

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

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

int SHMDT (const void *SHMADDR);

int shmctl (int shmid, int cmd, struct shmid_ds *buf);

Features: Used to create shared memory

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

Parameters

Key: This shared memory segment name

Size: Shared memory size

SHMFLG: Consists of nine permission flags, and their usage is the same as the mode mode flag used to create the file

Return value: Successfully returns a non-negative integer, that is, the identity code of the shared memory segment; failed return-1

Function: Connect shared memory segments to process address space

Prototype

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

Parameters

Shmid: Shared Memory identity

SHMADDR: Specify the address of the connection

SHMFLG: Its two possible values are shm_rnd and shm_rdonly

Return value: Successfully returns a pointer to the first byte of shared memory; failed return-1

SHMADDR is NULL, core automatically selects an address

SHMADDR is not null and SHMFLG has no shm_rnd tag, the SHMADDR is the connection address.

SHMADDR is not null and the SHM_RND tag is set SHMFLG, the address of the connection is automatically adjusted downward to the integer multiple of Shmlba.

Formula: shmaddr-(shmaddr% Shmlba)

Shmflg=shm_rdonly, which indicates that the connection operation is used to read-only shared memory

Function: Detach the shared memory segment from the current process

Prototype int SHMDT (const void *SHMADDR);

Parameters

SHMADDR: Pointers returned by Shmat

Return value: successful return 0; failure return-1

Note: Breaking the shared memory segment from the current process is not equal to deleting the shared memory segment

Features: Used to control shared memory

Prototype int shmctl (int shmid, int cmd, struct shmid_ds *buf);

Parameters

Shmid: Shared memory identity code returned by Shmget

CMD: Action to be taken (three desirable values)

BUF: Point to a data structure that holds the mode state and access rights of shared memory

Return value: successful return 0; failure return-1

The value of CMD is the following, similar to Message Queuing:

Ipc_stat sets the data in the SHMID_DS structure to the current associated value of shared memory

Ipc_set Sets the current association value of shared memory to the value given in the SHMID_DS data structure, provided that the process has sufficient permissions

Ipc_rmid Delete Shared memory segment