/*共享内存允许两个或多个进程进程共享同一块内存(这块内存会映射到各个进程自己独立的地址空间)从而使得这些进程可以相互通信。在GNU/Linux中所有的进程都有唯一的虚拟地址空间，而共享内存应用编程接口API允许一个进程使用公共内存区段。但是对内存的共享访问其复杂度也相应增加。共享内存的优点是简易性。使用消息队列时，一个进程要向队列中写入消息，这要引起从用户地址空间向内核地址空间的一次复制，同样一个进程进行消息读取时也要进行一次复制。共享内存的优点是完全省去了这些操作。共享内存会映射到进程的虚拟地址空间，进程对其可以直接访问，避免了数据的复制过程。因此，共享内存是GNU/Linux现在可用的最快速的IPC机制。进程退出时会自动和已经挂接的共享内存区段分离，但是仍建议当进程不再使用共享区段时调用shmdt来卸载区段。注意，当一个进程分支出父进程和子进程时，父进程先前创建的所有共享内存区段都会被子进程继承。如果区段已经做了删除标记(在前面以IPC——RMID指令调用shmctl)，而当前挂接数已经变为0，这个区段就会被移除。*/
/*shmget(  )  创建一个新的共享内存区段取得一个共享内存区段的描述符shmctl(  )  取得一个共享内存区段的信息为一个共享内存区段设置特定的信息移除一个共享内存区段shmat(  )   挂接一个共享内存区段shmdt(  )   于一个共享内存区段的分离*/
//创建一个共享内存区段，并显示其相关信息，然后删除该内存共享区
#include <stdio.h>
#include <unistd.h>  //getpagesize(  )
#include <sys/ipc.h>
#include <sys/shm.h>
#define MY_SHM_ID 67483
int main(  ){//获得系统中页面的大小printf( "page size=%d/n",getpagesize(  ) );//创建一个共享内存区段int shmid,ret;shmid=shmget( MY_SHM_ID,4096,0666|IPC_CREAT );//创建了一个4KB大小共享内存区段。指定的大小必须是当前系统架构//中页面大小的整数倍if( shmid>0 )printf( "Create a shared memory segment %d/n",shmid );//获得一个内存区段的信息struct shmid_ds shmds;//shmid=shmget( MY_SHM_ID,0,0 );//示例怎样获得一个共享内存的标识符ret=shmctl( shmid,IPC_STAT,&shmds );if( ret==0 ){printf( "Size of memory segment is %d/n",shmds.shm_segsz );printf( "Numbre of attaches %d/n",( int )shmds.shm_nattch );}else{printf( "shmctl(  ) call failed/n" );}//删除该共享内存区ret=shmctl( shmid,IPC_RMID,0 );if( ret==0 )printf( "Shared memory removed /n" );elseprintf( "Shared memory remove failed /n" );return 0;}//共享内存区段的挂载,脱离和使用
//理解共享内存区段就是一块大内存
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <errno.h>
#define MY_SHM_ID 67483
int main(  ){//共享内存区段的挂载和脱离int shmid,ret;void* mem;shmid=shmget( MY_SHM_ID,0,0 );if( shmid>=0 ){mem=shmat( shmid,( const void* )0,0 );//shmat()返回进程地址空间中指向区段的指针if( ( int )mem!=-1 ){printf( "Shared memory was attached in our address space at %p/n",mem );//向共享区段内存写入数据strcpy( ( char* )mem,"This is a test string./n" );printf( "%s/n",(char*)mem );//脱离共享内存区段ret=shmdt( mem );if( ret==0 )printf( "Successfully detached memory /n" );elseprintf( "Memory detached failed %d/n",errno );}elseprintf( "shmat(  ) failed/n" );}elseprintf( "shared memory segment not found/n" );return 0;}
/*内存共享区段与旗语和消息队列不同，一个区段可以被锁定。被锁定的区段不允许被交换出内存。这样做的优势在于，与其把内存区段交换到文件系统，在某个应用程序调用时再交换回内存，不如让它一直处于内存中，且对多个应用程序可见。从提升性能的角度来看，很重要的。*/
int shmid;
//...
shmid=shmget( MY_SHM_ID,0,0 );
ret=shmctl( shmid,SHM_LOCK,0 );
if( ret==0 )printf( "Locked!/n" );/*使用旗语协调共享内存的例子使用和编译命令gcc -Wall test.c -o test./test create./test use a &./test use b &./test read &./test remove */
#include <stdio.h>
#include <sys/shm.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#define MY_SHM_ID 34325
#define MY_SEM_ID 23234
#define MAX_STRING 200
typedef struct
{int semID;int counter;char string[ MAX_STRING+1 ];
}MY_BLOCK_T;
int main(int argc,char** argv){int shmid,ret,i;MY_BLOCK_T* block;struct sembuf sb;char user;//make sure there is a commandif( argc>=2 ){//create the shared memory segment and init it//with the semaphoreif( !strncmp(argv[ 1 ],"create",6) ){//create the shared memory segment and semaphoreprintf( "Creating the shared memory/n" );shmid=shmget( MY_SHM_ID,sizeof( MY_BLOCK_T ),( IPC_CREAT|0666 ) );block=( MY_BLOCK_T* )shmat( shmid,( const void* )0,0 );block->counter=0;//create the semaphore and initblock->semID=semget(MY_SEM_ID,1,( IPC_CREAT|0666 ));sb.sem_num=0;sb.sem_op=1;sb.sem_flg=0;semop( block->semID,&sb,1 );//now detach the segmentshmdt( ( void* )block );printf( "Create the shared memory and semaphore successuflly/n" );}else if( !strncmp(argv[ 1 ],"use",3) ){/*use the segment*///must specify  also a letter to write to the bufferif( argc<3 ) exit( -1 );user=( char )argv[ 2 ][ 0 ];//grab the segmentshmid=shmget( MY_SHM_ID,0,0 );block=( MY_BLOCK_T* )shmat( shmid,( const void* )0,0 );/*##########重点就是使用旗语对共享区的访问###########*/for( i=0;i<100;++i ){sleep( 1 ); //设置成1s就会看到 a/b交替出现，为0则a和b连续出现//grab the semaphoresb.sem_num=0;sb.sem_op=-1;sb.sem_flg=0;if( semop( block->semID,&sb,1 )!=-1 ){//write the letter to the segment buffer//this is our CRITICAL SECTIONblock->string[ block->counter++ ]=user;sb.sem_num=0;sb.sem_op=1;sb.sem_flg=0;if( semop( block->semID,&sb,1 )==-1 )printf( "Failed to release the semaphore/n" );}elseprintf( "Failed to acquire the semaphore/n" );}//do some clear workret=shmdt(( void*)block);}else if( !strncmp(argv[ 1 ],"read",4) ){//here we will read the buffer in the shared segmentshmid=shmget( MY_SHM_ID,0,0 );if( shmid!=-1 ){block=( MY_BLOCK_T* )shmat( shmid,( const void* )0,0 );block->string[ block->counter+1 ]=0;printf( "%s/n",block->string );printf( "Length=%d/n",block->counter );ret=shmdt( ( void*)block );}elseprintf( "Unable to read segment/n" );}else if( !strncmp(argv[ 1 ],"remove",6) ){shmid=shmget( MY_SHM_ID,0,0 );if( shmid>=0 ){block=( MY_BLOCK_T* )shmat( shmid,( const void* )0,0 );//remove the semaphoreret=semctl( block->semID,0,IPC_RMID );if( ret==0 )printf( "Successfully remove the semaphore /n" );//remove the shared segmentret=shmctl( shmid,IPC_RMID,0 );if( ret==0 )printf( "Successfully remove the segment /n" );}}elseprintf( "Unkonw command/n" );}return 0;}