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System V 共享内存

分类： LINUX

2012-04-01 13:19:11

    为了在多个进程间交换数据，内核专门留出了一块共享内存区域，所有需要访问该共享区域的进程都要把这共享区域映射到本进程的地址空间中。每个内存区域都有一个标示符(shmid)，进程可以通过这个标示符访问内存区域。
    共享内存通过shmget()获得或创建一个IPC共享内存区域，并返回相应的标示符。然后调用系统调用shmat()完成将该共享内存区域映射到进程地址空间。
1、函数接口

参考：http://blog.chinaunix.net/uid-25272011-id-3157708.html

2、给出实例以及运行结果：
write.c

点击(此处)折叠或打开

  1. #include <sys/types.h>
  2. #include <sys/ipc.h>
  3. #include <stdio.h>
  4. #include <errno.h>
  5. #include <stdlib.h>
  6. #include <sys/shm.h>
  7. #include <string.h>

  9. #define MEM_SIZE 4096

  11. //首先创建共享内存，并写入数据：（将struct People data[4]中的数据写入创建的共享内存）
  12. //编译运行后，可用Shell命令ipcs -m查看，删除可用ipcrm -m (shmid号)。

  14. struct People{
  15.     char name[20];
  16.     char age;
  17. };

  19. int main()
  20. {
  21.     int i;
  22.     key_t key;
  23.     char *pathname="./ftok.txt";
  24.     int proj_id=5;
  25.     int shm_id;
  26.     struct People *shm_addr;
  27.     struct People data[4]={
  28.         {"name1",21},{"name2",22},{"name3",23},{"name4",24}
  29.     };

  31.     if ((key=ftok(pathname,proj_id))==-1){
  32.         perror("ftok");
  33.         exit(-1);
  34.     }
  35.     else{
  36.         printf("ftok success! key=%d\n",key);
  37.     }

  39.     if((shm_id=shmget(key,MEM_SIZE,IPC_EXCL|IPC_CREAT|0600))==-1){
  40.         perror("shmget");
  41.         exit(-1);
  42.     }
  43.     else{
  44.         printf("shmget success! shm_id=%d\n",shm_id);
  45.     }

  47.     if((shm_addr=shmat(shm_id,0,0))==(void *)-1){
  48.         perror("shmat");
  49.         exit(-1);
  50.     }
  51.     else{
  52.         printf("shmat success!shm_addr=%p\n",shm_addr);
  53.     }

  55.     printf("start copy data to memory\n");
  56.     memcpy((void *)shm_addr,(void *)data,sizeof(data));
  57.     printf("copy over\n");

  59.     for(i=0;i<4;i++){
  60.         printf("name:%s\t",shm_addr[i].name);
  61.         printf("age=%d\n",shm_addr[i].age);
  62.     }
  63.     if(shmdt(shm_addr)==-1){
  64.         perror("shmdt");
  65.         exit(-1);
  66.     }
  67.     else{
  68.         printf("shmdt success!\n");
  69.     }

  71.     return 0;
  72. }

read.c

点击(此处)折叠或打开

  1. #include <sys/types.h>
  2. #include <sys/ipc.h>
  3. #include <stdio.h>
  4. #include <errno.h>
  5. #include <stdlib.h>
  6. #include <sys/shm.h>
  7. #include <string.h>

  9. #define MEM_SIZE 4096

  11. //读出共享内存中的数据，并接删除共享内存：
  12. //编译运行后，再次用Shell命令ipcs -m查看，就没有创建的共享内存了。

  14. struct People{
  15.     char name[20];
  16.     char age;
  17. };

  19. int main()
  20. {
  21.     int i;
  22.     key_t key;
  23.     char *pathname="./ftok.txt";
  24.     int proj_id=5;
  25.     int shm_id;
  26.     struct People *shm_addr;

  28.     if ((key=ftok(pathname,proj_id))==-1){
  29.         perror("ftok");
  30.         exit(-1);
  31.     }
  32.     else{
  33.         printf("ftok success! key=%d\n",key);
  34.     }

  36.     if((shm_id=shmget(key,MEM_SIZE,0600))==-1){
  37.         perror("shmget");
  38.         exit(-1);
  39.     }
  40.     else{
  41.         printf("shmget success! shm_id=%d\n",shm_id);
  42.     }

  44.     if((shm_addr=shmat(shm_id,0,0))==(void *)-1){
  45.         perror("shmat");
  46.         exit(-1);
  47.     }
  48.     else{
  49.         printf("shmat success!shm_addr=%p\n",shm_addr);
  50.     }

  52.     for(i=0;i<4;i++){
  53.         printf("name:%s\t",shm_addr[i].name);
  54.         printf("age=%d\n",shm_addr[i].age);
  55.     }
  56.     if(shmdt(shm_addr)==-1){
  57.         perror("shmdt");
  58.         exit(-1);
  59.     }
  60.     else{
  61.         printf("shmdt success!\n");
  62.     }

  64.     if(shmctl(shm_id,IPC_RMID,0)==-1){        //这边才是删除那块共享缓冲区
  65.         perror("shmctl");
  66.         exit(-1);
  67.     }
  68.     else{
  69.         printf("shmctl success!\n");
  70.     }

  72.     return 0;
  73. }

详细的运行结果：

点击(此处)折叠或打开

  1. root@jgf:~/JGF/study/shm# ipcs -h
  2. ipcs provides information on ipc facilities for which you have read access.
  3. Resource Specification:
  4. -m : shared_mem
  5. -q : messages
  6. -s : semaphores
  7. -a : all (default)
  8. Output Format:
  9. -t : time
  10. -p : pid
  11. -c : creator
  12. -l : limits
  13. -u : summary
  14. -i id [-s -q -m] : details on resource identified by id
  15. usage : ipcs -asmq -tclup
  16. ipcs [-s -m -q] -i id
  17. ipcs -h for help.
  18. root@jgf:~/JGF/study/shm#
  19. root@jgf:~/JGF/study/shm# ipcs -m           //先打出系统当前的IPC

  21. ------ Shared Memory Segments --------
  22. key shmid owner perms bytes nattch status
  23. 0x00000000 65536 root 600 393216 2 dest
  24. 0x00000000 98305 root 600 393216 2 dest
  25. 0x00000000 131074 root 600 393216 2 dest
  26. 0x00000000 5046275 root 600 393216 2 dest
  27. 0x00000000 196612 root 600 393216 2 dest
  28. 0x00000000 229381 root 600 393216 2 dest
  29. 0x00000000 262150 root 600 393216 2 dest
  30. 0x00000000 294919 root 600 393216 2 dest
  31. 0x00000000 5079048 root 600 393216 2 dest
  32. 0x00000000 9568267 root 600 393216 2 dest
  33. 0x00000000 4292620 root 600 393216 2 dest
  34. 0x0056a4d5 10846221 root 660 488 0
  35. 0x0056a4d6 10878990 root 660 131072 0

  37. root@jgf:~/JGF/study/shm# ipcs -q

  39. ------ Message Queues --------
  40. key msqid owner perms used-bytes messages

  42. root@jgf:~/JGF/study/shm#
  43. root@jgf:~/JGF/study/shm#
  44. root@jgf:~/JGF/study/shm#
  45. root@jgf:~/JGF/study/shm# ipcs -s

  47. ------ Semaphore Arrays --------
  48. key semid owner perms nsems
  49. 0x0056a4d5 589824 root 660 1

  51. root@jgf:~/JGF/study/shm# ./write                //运行
  52. ftok: No such file or directory
  53. root@jgf:~/JGF/study/shm#
  54. root@jgf:~/JGF/study/shm#
  55. root@jgf:~/JGF/study/shm#
  56. root@jgf:~/JGF/study/shm# touch ftok.txt
  57. root@jgf:~/JGF/study/shm#
  58. root@jgf:~/JGF/study/shm#
  59. root@jgf:~/JGF/study/shm#
  60. root@jgf:~/JGF/study/shm# ./write
  61. ftok success! key=83989182
  62. shmget success! shm_id=13369353                  //创建了一块共享内存，返回ID号
  63. shmat success!shm_addr=0xb7769000
  64. start copy data to memory
  65. copy over
  66. name:name1 age=21
  67. name:name2 age=22
  68. name:name3 age=23
  69. name:name4 age=24
  70. shmdt success!
  71. root@jgf:~/JGF/study/shm# ipcs -m

  73. ------ Shared Memory Segments --------
  74. key shmid owner perms bytes nattch status
  75. 0x00000000 65536 root 600 393216 2 dest
  76. 0x00000000 98305 root 600 393216 2 dest
  77. 0x00000000 131074 root 600 393216 2 dest
  78. 0x00000000 5046275 root 600 393216 2 dest
  79. 0x00000000 196612 root 600 393216 2 dest
  80. 0x00000000 229381 root 600 393216 2 dest
  81. 0x00000000 262150 root 600 393216 2 dest
  82. 0x00000000 294919 root 600 393216 2 dest
  83. 0x00000000 5079048 root 600 393216 2 dest
  84. 0x050192be 13369353 root 600 4096 0              //在这里
  85. 0x00000000 9568267 root 600 393216 2 dest
  86. 0x00000000 4292620 root 600 393216 2 dest
  87. 0x0056a4d5 10846221 root 660 488 0
  88. 0x0056a4d6 10878990 root 660 131072 0

  90. root@jgf:~/JGF/study/shm# ./read
  91. ftok success! key=83989182
  92. shmget success! shm_id=13369353            //获取到了上面那块映射好的共享内存ID
  93. shmat success!shm_addr=0xb771b000
  94. name:name1 age=21
  95. name:name2 age=22
  96. name:name3 age=23
  97. name:name4 age=24
  98. shmdt success!
  99. shmctl success!
  100. root@jgf:~/JGF/study/shm# ipcs -m

  102. ------ Shared Memory Segments --------
  103. key shmid owner perms bytes nattch status
  104. 0x00000000 65536 root 600 393216 2 dest
  105. 0x00000000 98305 root 600 393216 2 dest
  106. 0x00000000 131074 root 600 393216 2 dest
  107. 0x00000000 5046275 root 600 393216 2 dest
  108. 0x00000000 196612 root 600 393216 2 dest
  109. 0x00000000 229381 root 600 393216 2 dest
  110. 0x00000000 262150 root 600 393216 2 dest
  111. 0x00000000 294919 root 600 393216 2 dest
  112. 0x00000000 5079048 root 600 393216 2 dest
  113. 0x00000000 9568267 root 600 393216 2 dest
  114. 0x00000000 4292620 root 600 393216 2 dest
  115. 0x0056a4d5 10846221 root 660 488 0
  116. 0x0056a4d6 10878990 root 660 131072 0
  117. --------------------------------------------------------------------------------
  118. --------------------------------------------------------------------------------
  119. root@jgf:~/JGF/study/shm# ./write       //重新运行
  120. ftok success! key=83989182              //键值相同
  121. shmget success! shm_id=13402121         //但ID不同了，应该是换了一块内存区域了
  122. shmat success!shm_addr=0xb77a3000
  123. start copy data to memory
  124. copy over
  125. name:name1 age=21
  126. name:name2 age=22
  127. name:name3 age=23
  128. name:name4 age=24
  129. shmdt success!
  130. root@jgf:~/JGF/study/shm#
  131. root@jgf:~/JGF/study/shm# ipcs -m

  133. ------ Shared Memory Segments --------
  134. key shmid owner perms bytes nattch status
  135. 0x00000000 65536 root 600 393216 2 dest
  136. 0x00000000 98305 root 600 393216 2 dest
  137. 0x00000000 131074 root 600 393216 2 dest
  138. 0x00000000 5046275 root 600 393216 2 dest
  139. 0x00000000 196612 root 600 393216 2 dest
  140. 0x00000000 229381 root 600 393216 2 dest
  141. 0x00000000 262150 root 600 393216 2 dest
  142. 0x00000000 294919 root 600 393216 2 dest
  143. 0x00000000 5079048 root 600 393216 2 dest
  144. 0x050192be 13402121 root 600 4096 0
  145. 0x00000000 9568267 root 600 393216 2 dest
  146. 0x00000000 4292620 root 600 393216 2 dest
  147. 0x0056a4d5 10846221 root 660 488 0
  148. 0x0056a4d6 10878990 root 660 131072 0

  150. root@jgf:~/JGF/study/shm#
  151. root@jgf:~/JGF/study/shm# ipcrm -h
  152. ipcrm: invalid option -- 'h'
  153. ipcrm: illegal option -- ?
  154. usage: ipcrm [ [-q msqid] [-m shmid] [-s semid]
  155. [-Q msgkey] [-M shmkey] [-S semkey] ... ]
  156. root@jgf:~/JGF/study/shm# ipcrm --h
  157. ipcrm: invalid option -- '-'
  158. ipcrm: illegal option -- ?
  159. usage: ipcrm [ [-q msqid] [-m shmid] [-s semid]
  160. [-Q msgkey] [-M shmkey] [-S semkey] ... ]
  161. root@jgf:~/JGF/study/shm# ipcrm -m 13402121    //删除该共享内存区
  162. root@jgf:~/JGF/study/shm#
  163. root@jgf:~/JGF/study/shm#
  164. root@jgf:~/JGF/study/shm#
  165. root@jgf:~/JGF/study/shm# ipcs -m

  167. ------ Shared Memory Segments --------
  168. key shmid owner perms bytes nattch status
  169. 0x00000000 65536 root 600 393216 2 dest
  170. 0x00000000 98305 root 600 393216 2 dest
  171. 0x00000000 131074 root 600 393216 2 dest
  172. 0x00000000 5046275 root 600 393216 2 dest
  173. 0x00000000 196612 root 600 393216 2 dest
  174. 0x00000000 229381 root 600 393216 2 dest
  175. 0x00000000 262150 root 600 393216 2 dest
  176. 0x00000000 294919 root 600 393216 2 dest
  177. 0x00000000 5079048 root 600 393216 2 dest
  178. 0x00000000 9568267 root 600 393216 2 dest
  179. 0x00000000 4292620 root 600 393216 2 dest
  180. 0x0056a4d5 10846221 root 660 488 0
  181. 0x0056a4d6 10878990 root 660 131072 0

  183. root@jgf:~/JGF/study/shm# ./read        //此时那块区域被删除掉了
  184. ftok success! key=83989182              //键值相同
  185. shmget: No such file or directory       //但是get不到那块区域了
  186. root@jgf:~/JGF/study/shm#
  187. root@jgf:~/JGF/study/shm#
  188. root@jgf:~/JGF/study/shm#

本文的源代码参考自：http://waiter94.blog.163.com/blog/static/5277376920100213456475/?fromdm&fromSearch&isFromSearchEngine=yes

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