Shared Memory application example

1. Example of parent-child process communication. The source code of SHM. C is as follows:

# Include <stdio. h>

# Include <unistd. h>

# Include <string. h>

# Include <sys/IPC. h>

# Include <sys/SHM. h>

# Include <error. h>

# Define size 1024

Int main ()

{

Int shmid;

Char * shmaddr;

Struct shmid_ds Buf;

Int flag = 0;

Int PID;

Shmid = shmget (ipc_private, size, ipc_creat | 0600 );

If (shmid <0)

{

Perror ("Get SHM ipc_id error ");

Return-1;

}

PID = fork ();

If (pid = 0)

{

Shmaddr = (char *) shmat (shmid, null, 0 );

If (INT) shmaddr =-1)

{

Perror ("shmat ADDR error ");

Return-1;

}

Strcpy (shmaddr, "Hi, I am child process! \ N ");

Shmdt (shmaddr );

Return 0;

} Else if (pid> 0 ){

Sleep (3 );

Flag = shmctl (shmid, ipc_stat, & BUF );

If (flag =-1)

{

Perror ("shmctl SHM error ");

Return-1;

}

Printf ("shm_segsz = % d bytes \ n", Buf. shm_segsz );

Printf ("parent pid = % d, shm_cpid = % d \ n", getpid (), Buf. shm_cpid );

Printf ("chlid pid = % d, shm_lpid = % d \ n", PID, Buf. shm_lpid );

Shmaddr = (char *) shmat (shmid, null, 0 );

If (INT) shmaddr =-1)

{

Perror ("shmat ADDR error ");

Return-1;

}

Printf ("% s", shmaddr );

Shmdt (shmaddr );

Shmctl (shmid, ipc_rmid, null );

} Else {

Perror ("fork error ");

Shmctl (shmid, ipc_rmid, null );

}

Return 0;

}

Compile gcc shm. C-o SHM.

Run./SHM. The execution result is as follows:

Shm_segsz = 1024 bytes

Shm_cpid = 9503

Shm_lpid = 1, 9504

Hi, I am child process!

2. multi-process read/write example

Multi-process read/write refers to one process write shared memory, and one or more processes read shared memory. The following example implements a process write shared memory and a process read shared memory.

(1) The following program creates shared memory and writes messages.

The source code of shmwrite. C is as follows:

# Include <stdio. h>

# Include <sys/IPC. h>

# Include <sys/SHM. h>

# Include <sys/types. h>

# Include <unistd. h>

# Include <string. h>

Typedef struct {

Char name [8];

Int age;

} People;

Int main (INT argc, char ** argv)

{

Int shm_id, I;

Key_t key;

Char temp [8];

People * p_map;

Char pathname [30];

Strcpy (pathname, "/tmp ");

Key = ftok (pathname, 0x03 );

If (Key =-1)

{

Perror ("ftok error ");

Return-1;

}

Printf ("Key = % d \ n", key );

Shm_id = shmget (Key, 4096, ipc_creat | ipc_excl | 0600 );

If (shm_id =-1)

{

Perror ("shmget error ");

Return-1;

}

Printf ("shm_id = % d \ n", shm_id );

P_map = (People *) shmat (shm_id, null, 0 );

Memset (temp, 0x00, sizeof (temp ));

Strcpy (temp, "test ");

Temp [4] = '0 ';

For (I = 0; I <3; I ++)

{

Temp [4] + = 1;

Strncpy (p_map + I)-> name, temp, 5 );

(P_map + I)-> age = 0 + I;

}

Shmdt (p_map );

Return 0;

}

(2) The following program reads messages from the shared memory.

The source code of shmread. C is as follows:

# Include <stdio. h>

# Include <string. h>

# Include <sys/IPC. h>

# Include <sys/SHM. h>

# Include <sys/types. h>

# Include <unistd. h>

Typedef struct {

Char name [8];

Int age;

} People;

Int main (INT argc, char ** argv)

{

Int shm_id, I;

Key_t key;

People * p_map;

Char pathname [30];

Strcpy (pathname, "/tmp ");

Key = ftok (pathname, 0x03 );

If (Key =-1)

{

Perror ("ftok error ");

Return-1;

}

Printf ("Key = % d \ n", key );

Shm_id = shmget (Key, 0, 0 );

If (shm_id =-1)

{

Perror ("shmget error ");

Return-1;

}

Printf ("shm_id = % d \ n", shm_id );

P_map = (People *) shmat (shm_id, null, 0 );

For (I = 0; I <3; I ++)

{

Printf ("Name: % s \ n", (* (p_map + I). Name );

Printf ("Age % d \ n", (* (p_map + I). Age );

}

If (shmdt (p_map) =-1)

{

Perror ("detach error ");

Return-1;

}

Return 0;

}

(3) Compilation and execution

① Compile GCC shmwrite. C-o shmwrite.

② Execute./shmwrite. The execution result is as follows:

Key = 1, 50453281

Shm_id = 688137

③ Compile GCC shmread. C-o shmread.

④ Run./shmread. The execution result is as follows:

Key = 1, 50453281

Shm_id = 688137

Name: test1

Age 0

Name: Test2

Age 1

Name: test3

Age 2

⑤ Run./shmwrite again. The execution result is as follows:

Key = 1, 50453281

Shmget error: file exists

⑥ Use ipcrm-M 688137 to delete the shared memory.

Shared Memory application example