UNIX 共享内存应用中的问题及解决方法( 六 ) _云知道

if (isAttached) {
　// Attention: the following line should be "shmdt((char
*)mem_addr);", if on Solaris
shmdt((void *)mem_addr);
printf("%s, -.Successfully detached, times [d]n", argv[0], i);
}
// purposely remove the shared memory at times [5]
if (i==5) {
shmctl(mem_id, IPC_RMID, NULL);
printf("%s, *.Remove executed, times [d], errno=%dn",
argv[0], i, errno);
}
}
return 0;
}
上述程序均可在AIX、HP-UX、Linux平台上编译通过；在Solaris平台上只需按注释提示的要求，将shmdt的参数强制为char *类型也可编译通过（第5节中已介绍过）。
将test03.P1.c、test03.P2.c各自编译为可执行程序test03.P1、test03.P2，并通过下面的shell脚本：runtest，运行它们：
#!/bin/sh
./test03.P1&
sleep 2
./test03.P2
在Linux平台（Red Hat 8.0）上的运行结果如下：
[root@localhost tmp]# ./runtest
./test03.P1,.Successfully attached shared memory
./test03.P2,.Successfully attached shared memory, times [01]
./test03.P2, -.Successfully detached, times [01]
./test03.P2,.Successfully attached shared memory, times [02]
./test03.P2, -.Successfully detached, times [02]
./test03.P2,.Successfully attached shared memory, times [03]
./test03.P2, -.Successfully detached, times [03]
./test03.P2,.Successfully attached shared memory, times [04]
./test03.P2, -.Successfully detached, times [04]
./test03.P2,.Successfully attached shared memory, times [05]
./test03.P2, -.Successfully detached, times [05]
./test03.P2, *.Remove executed, times [05], errno=0
./test03.P2,.Successfully attached shared memory, times [06]
./test03.P2, -.Successfully detached, times [06]
./test03.P2,.Successfully attached shared memory, times [07]
./test03.P2, -.Successfully detached, times [07]
./test03.P2,.Successfully attached shared memory, times [08]
./test03.P2, -.Successfully detached, times [08]
./test03.P2,.Successfully attached shared memory, times [09]
./test03.P2, -.Successfully detached, times [09]
[root@localhost tmp]# ./test03.P1, -.Successfully detached shared memory
./test03.P1, Failed to attach the removed shared memory, errno:22
根据运行结果，我们可以看到，在Linux平台上，即便对共享内存执行了删除操作（在第5次连接之后，test03.P2进程调用了shmctl的IPC_RMID删除操作），只要该共享内存依然存在（test03.P1进程保持着连接，因此共享内存不会被立即删除），则它仍然是可连接的（test03.P2进程的第6到第9次连接均是成功的）。
然而，在AIX、HP-UX、Solaris平台上的运行结果却不同于Linux：
# ./runtest
./test03.P1,.Successfully attached shared memory
./test03.P2,.Successfully attached shared memory, times [01]
./test03.P2, -.Successfully detached, times [01]
./test03.P2,.Successfully attached shared memory, times [02]
./test03.P2, -.Successfully detached, times [02]
./test03.P2,.Successfully attached shared memory, times [03]
./test03.P2, -.Successfully detached, times [03]
./test03.P2,.Successfully attached shared memory, times [04]
./test03.P2, -.Successfully detached, times [04]
./test03.P2,.Successfully attached shared memory, times [05]
./test03.P2, -.Successfully detached, times [05]
./test03.P2, *.Remove executed, times [05], errno=0
./test03.P2, Failed to attach shared memory, times [06], errno:22
./test03.P2, Failed to attach shared memory, times [07], errno:22
./test03.P2, Failed to attach shared memory, times [08], errno:22
./test03.P2, Failed to attach shared memory, times [09], errno:22
# ./test03.P1, -.Successfully detached shared memory
./test03.P1, Failed to attach the removed shared memory, errno:22
根据结果，可以发现，test03.P2进程的第6到第9次连接都是失败的，也就说明，在AIX、HP-UX、Solaris平台上一旦通过shmctl对共享内存进行了删除操作，则该共享内存将不能再接受任何新的连接，即使它依然存在于系统中！

UNIX 共享内存应用中的问题及解决方法( 六 )

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