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System v shared memory

# include # include int shmget (key_t key, size_t size, int shmflg)

Establish: the relationship between processes and shared memory

A non-zero number in key_t key:16.

There are generally two ways to set it.

The first one: call the fotk function

Chapter 2: using IPC_PRIVATE directly

Size: size of shared memory

Shmflg:

The permissions of IPC_CREATIPC_EXCL users, group users, and other users to this memory can be represented by 9 bit, such as 664

Return value: successfully returns the identification number of this piece of shared memory; failure returns-1 errno is set.

# include # include 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 apply for memory space

Non-NULL: create a space with malloc to associate the shared memory shmid with this address. But if it is not an integral multiple of 4K, the kernel will be adjusted up or down.

Shmflg:

SHM_RND: read and write

SHM_RDONLY: read-only

Return value:

Success: return memory address

Failed: return (void *)-1

# include # include int shmdt (const void * shmaddr)

Disassociate a process from shared memory

The return value of shmaddr:shmat

Return value: success 0; failure-1, and set errno.

# include # include int shmctl (int shmid, int cmd, struct shmid_ds * buf)

For shared memory operations, it is more cmd-like, and different operations are performed on shared memory.

Shmid: created by the shmget function, which is the return value of the shmget function

Cmd:

IPC_STAT: get the status of the shared memory

IPC_RMID: Mark to delete shared memory (when the reference count of shared memory becomes 0)

IPC_SET: set the properties of shared memory (modify permissions, modify shmid, etc.)

Wait

Buf:shmid_ds structure

Return value: when cmd is IPC_RMID: success 0; failure-1, and set errno

You can view the status of shared memory with the command [ipcs]

-Shared Memory Segments-key shmid owner perms bytes nattch status0x00007fff 65536 ys 664256 00x00007ffe 98305 ys 664256 00x0000555e 131074 ys 664256 00x00000011 229379 ys 664256 3key: the first parameter specified by function shmget shmid: return value of function shmget owner: perms created by which user: access to this shared memory bytes: size nattch The number of processes using this shared memory status: the status of the shared memory

Summary

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