关于shm_open和shm_unlink的使用问题referencefunctionobjectsystembehaviorlinuxC programming in the UNIX environment的编程手册，一般都会为进程间用共享内存的方法通信提供两组方法：1.      POSIX定义的：int shm_open(const char *name, int oflag, mode_t mode);int shm_unlink(const char *name);int ftruncate(int fd, off_t length);2.      SYSTEM V定义的int shmget(key_t key, int size, int shmflg);void *shmat(int shmid, const void *shmaddr, int shmflg);int shmdt(const void *shmaddr);int shmctl(int shmid, int cmd, struct shmid_ds *buf); 由于POSIX标准比较通用，一般建议使用该标准定义的方法集。但是在使用shm_open和shm_unlink两个函数时，你可能遇到和我同样的问题,见如下代码。该代码旨在测试你的系统是否支持POSIX定义的共享内存函数集。 /* This is just to test if the function is found in the libs. */#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <fcntl.h>#include <sys/mman.h>#include <sys/stat.h> intmain (void){     int i;      i = shm_open ("/tmp/shared", O_CREAT | O_EXCL, S_IRUSR | S_IWUSR);     printf ("shm_open rc = %d/n", i);         shm_unlink ("/tmp/shared");      return (0);} 假设它所在的文件为＂test.c＂我这么编译：gcc -o test test.c结果为：/tmp/ccaGhdRt.o(.text+0x23): In function `main‘:: undefined reference to `shm_open‘/tmp/ccaGhdRt.o(.text+0x49): In function `main‘:: undefined reference to `shm_unlink‘collect2: ld returned 1 exit status 编译结果实际上是说，没include相应的头文件，或是头文件不存在（即系统不支持该库函数）但我man shm_open是可以找到帮助文件的（说明系统支持），原因何在？？？ 请注意一下man shm_open的帮助文件的最后几行：NOTES       These functions are provided in glibc 2.2 and  later.   Programs  using       these  functions  must  specify  the  -lrt  flag to cc in order to link       against the required ("realtime") library.        POSIX leaves the behavior of the combination of  O_RDONLY  and  O_TRUNC       unspecified.   On  Linux,  this  will successfully truncate an existing       shared memory object - this may not be so on other Unices.        The POSIX shared memory object implementation on Linux 2.4 makes use ofa dedicated file system, which is normally mounted under /dev/shm. 如果你注意到的话，这样编译就能通过了：gcc -lrt -o test test.c
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