高级Linux程序设计第五章:进程间通信
五种进程间通信的方式:
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共享内存(shared memory):其允许多个进程通过读写同一块内存地址来相互通信。
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内存映射(Mapped memory):其和共享内存相似,然而它是和文件系统上的一个文件相关联的。
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管道(Pipe):其允许一个进程到另一个相关进程的顺序通信。
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先入先出队列(FIFO):和管道类似,然而因为其对应于文件系统上的文件名,可以在两个不相关的进程间通信。
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Socket:其允许在不同的计算机上的不同进程间通信。
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1、共享内存(Shared Memory)
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共享内存时进程间通信方式中最快的一种,因为进程是共享同一块内存。
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内核并不提供对共享内存访问的同步机制,因而必须自己提供同步方式。
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要用共享内存块,需要一个进程首先分配此内存块。
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欲访问共享内存块的进程必须要连接到此内存块。
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在使用完共享内存块的时候,进程必须要卸载此内存块。
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需要有一个进程释放此内存块。
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所有的共享内存块都是以4KB的整数倍分配。
1.1、分配
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进程用函数shmget分配一个共享内存块。
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第一个参数是共享内存块的key
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不同的进程可以根据此key来访问同一个共享内存块。
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使用IPC_PRIVATE作为key会保证创建一个新的共享内存块。
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如果多个进程访问同一个共享内存块,则必须用同一个key。
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第二个参数表示内存块的大小。
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第三个参数是一系列标志位:
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IPC_CREAT创建一个新的内存块。
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IPC_EXCL此标志位和IPC_CREAT一起使用。如果key已经存在,则此标志位使得shmget失败。
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1.2、连接(Attachment )和卸载(Detachment)
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一个进程需要调用shmat来连接一个共享内存。
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第一个参数是共享内存块的id,由shmget返回。
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第二个参数是一个指针,其指向共享内存块映射的内存地址,如果是NULL,则系统会自动选择一个可用的内存地址。
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第三个参数是标志位:
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SHM_RND表示第二个参数所指定的地址必须同页的大小对齐。
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SHM_RDONLY表示此内存块只读。
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此函数返回值是连接的共享内存的起始地址。
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共享内存块可用函数shmdt卸载,应传给它共享内存块的起始地址。
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调用exit及exec函数自动卸载共享内存块。
1.3、控制和释放共享内存块
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shmctl函数可用返回和修改共享内存块的信息。
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第一个参数是共享内存块id
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欲得到一个共享内存块的信息,第二个参数设为IPC_STAT,第三个参数是指向shmid_ds结构体的指针。
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欲删除一个共享内存块,第二个参数设为IPC_RMID,第三个参数设为NULL。
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一个共享内存块在使用结束后,必须用shmctl显式的释放。
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调用exit和exec自动卸载共享内存块,但是不释放。
#include <stdio.h> #include <sys/shm.h> #include <sys/stat.h> int main () { int segment_id; char* shared_memory; struct shmid_ds shmbuffer; int segment_size; const int shared_segment_size = 0x6400; /* Allocate a shared memory segment. */ segment_id = shmget (IPC_PRIVATE, shared_segment_size, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR); /* Attach the shared memory segment. */ shared_memory = (char*) shmat (segment_id, 0, 0); printf (“shared memory attached at address %p\n”, shared_memory); /* Determine the segment’s size. */ shmctl (segment_id, IPC_STAT, &shmbuffer); segment_size = shmbuffer.shm_segsz; printf (“segment size: %d\n”, segment_size); /* Write a string to the shared memory segment. */ sprintf (shared_memory, “Hello, world.”); /* Detach the shared memory segment. */ shmdt (shared_memory); /* Reattach the shared memory segment, at a different address. */ shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0); printf (“shared memory reattached at address %p\n”, shared_memory); /* Print out the string from shared memory. */ printf (“%s\n”, shared_memory); /* Detach the shared memory segment. */ shmdt (shared_memory); /* Deallocate the shared memory segment. */ shmctl (segment_id, IPC_RMID, 0); return 0; }
ipcs命令可用查看进程间通信机制的信息 使用-m可查看共享内存的信息 % ipcs -m ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00000000 1627649 user 640 25600 0 ipcrm命令可删除进程间通信对象. % ipcrm shm 1627649 [liuchao@localhost ~]$ ipcs ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status 0x00000000 196608 liuchao 600 393216 2 dest 0x764867bd 65537 liuchao 600 1 0 0x2c0056d5 98306 liuchao 600 1 0 0x500e7827 131075 liuchao 600 1 0 0x20e0f21d 163844 liuchao 600 1 0 0x00000000 229381 liuchao 600 393216 2 dest 0x00000000 262150 liuchao 600 393216 2 dest 0x00000000 294919 liuchao 600 393216 2 dest 0x00000000 327688 liuchao 600 393216 2 dest 0x00000000 360457 liuchao 600 393216 2 dest 0x00000000 393226 liuchao 600 393216 2 dest 0x00000000 425995 liuchao 600 393216 2 dest 0x00000000 458764 liuchao 600 393216 2 dest 0x00000000 491533 liuchao 600 393216 2 dest 0x00000000 557070 liuchao 600 393216 2 dest 0x00000000 589839 liuchao 600 393216 2 dest ------ Semaphore Arrays -------- key semid owner perms nsems 0x59d9bc4a 0 liuchao 600 1 0x3bd464f2 32769 liuchao 600 1 ------ Message Queues -------- key msqid owner perms used-bytes messages |
2、进程信号量
2.1、分配(Allocation)和释放(Deallocation)
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调用semget分配一个信号量,调用semctl来释放一个信号量。
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semget的参数为一个信号量集的key,信号量集中的信号量的个数,权限标志位,返回值为信号量集id。
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semctl的参数为信号量集的id,信号量集中的信号量的个数,IPC_RMID。
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当所有的使用信号量的进程结束后,信号量仍然存在。
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最后一个使用信号量集的进程必须显式的删除它。
#include <sys/ipc.h> #include <sys/sem.h> #include <sys/types.h> /* We must define union semun ourselves. */ union semun { int val; struct semid_ds *buf; unsigned short int *array; struct seminfo *__buf; }; /* Obtain a binary semaphore’s ID, allocating if necessary. */ int binary_semaphore_allocation (key_t key, int sem_flags) { return semget (key, 1, sem_flags); } /* Deallocate a binary semaphore. All users must have finished their use. Returns -1 on failure. */ int binary_semaphore_deallocate (int semid) { union semun ignored_argument; return semctl (semid, 1, IPC_RMID, ignored_argument); } |
2.2、初始化信号量
#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> /* We must define union semun ourselves. */ union semun { int val; struct semid_ds *buf; unsigned short int *array; struct seminfo *__buf; }; /* Initialize a binary semaphore with a value of 1. */ int binary_semaphore_initialize (int semid) { union semun argument; unsigned short values[1]; values[0] = 1; argument.array = values; return semctl (semid, 0, SETALL, argument); } |
2.3、Wait和Post操作
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semop函数支持wait和post操作。
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第一个参数是信号量集id。
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第二个参数是一个sembuf结构体的数组。
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第三个参数是数组的长度。
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sembuf结构体:
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sem_num是信号量集中作为操作对象的信号量的号。
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sem_op表示对信号量的操作。如果sem_op是正数,则其将被加到信号量的值上。如果sem_op是负数,则得到其绝对值,如果此值能够使得信号量的值为负,则阻塞当前线程,直到此信号量的值等于sem_op的绝对值。如果sem_op为零,阻塞当前线程,直到信号量的值为零。
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sem_flg是标志位,IPC_NOWAIT使得此操作不会被阻塞,SEM_UNDO表示当进程结束的时候,系统自动取消此次操作。
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#include <sys/types.h> #include <sys/ipc.h> #include <sys/sem.h> /* Wait on a binary semaphore. Block until the semaphore value is positive, then decrement it by 1. */ int binary_semaphore_wait (int semid) { struct sembuf operations[1]; /* Use the first (and only) semaphore. */ operations[0].sem_num = 0; /* Decrement by 1. */ operations[0].sem_op = -1; /* Permit undo’ing. */ operations[0].sem_flg = SEM_UNDO; return semop (semid, operations, 1); } /* Post to a binary semaphore: increment its value by 1. This returns immediately. */ int binary_semaphore_post (int semid) { struct sembuf operations[1]; /* Use the first (and only) semaphore. */ operations[0].sem_num = 0; /* Increment by 1. */ operations[0].sem_op = 1; /* Permit undo’ing. */ operations[0].sem_flg = SEM_UNDO; return semop (semid, operations, 1); } |
3、内存映射(Mapped Memory)
3.1、映射一个普通文件
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使用mmap函数可将一个普通文件映射到进程内存中。
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第一个参数是文件将映射到的内存地址,NULL使得Linux自动选择一个可用的地址。
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第二个参数是映射的长度。
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第三个参数是映射的内存的保护模式:PROT_READ,PROT_WRITE,PROT_EXEC。
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第四个参数是一个标志位:
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MAP_FIXED表示映射的内存地址必须和页对齐。
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MAP_PRIVATE表示写入映射的内存的数据不会写入关联的文件,而是写入另一个文件副本,对其他线程不可见。
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MAP_SHARED表示写入映射的内存的数据会立即写入关联的文件,不会有缓存。
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第五个参数是关联文件的文件描述符。
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第六个参数是映射的文件的偏移量。
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(mmap-write.c) Write a Random Number to a Memory-Mapped File #include <stdlib.h> #include <stdio.h> #include <fcntl.h> #include <sys/mman.h> #include <sys/stat.h> #include <time.h> #include <unistd.h> #define FILE_LENGTH 0x100 /* Return a uniformly random number in the range [low,high]. */ int random_range (unsigned const low, unsigned const high) { unsigned const range = high - low + 1; return low + (int) (((double) range) * rand () / (RAND_MAX + 1.0)); } int main (int argc, char* const argv[]) { int fd; void* file_memory; /* Seed the random number generator. */ srand (time (NULL)); /* Prepare a file large enough to hold an unsigned integer. */ fd = open (argv[1], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR); lseek (fd, FILE_LENGTH+1, SEEK_SET); write (fd, “”, 1); lseek (fd, 0, SEEK_SET); /* Create the memory mapping. */ file_memory = mmap (0, FILE_LENGTH, PROT_WRITE, MAP_SHARED, fd, 0); close (fd); /* Write a random integer to memory-mapped area. */ sprintf((char*) file_memory, “%d\n”, random_range (-100, 100)); /* Release the memory (unnecessary because the program exits). */ munmap (file_memory, FILE_LENGTH); return 0; } (mmap-read.c) Read an Integer from a Memory-Mapped File, and Double It #include <stdlib.h> #include <stdio.h> #include <fcntl.h> #include <sys/mman.h> #include <sys/stat.h> #include <unistd.h> #define FILE_LENGTH 0x100 int main (int argc, char* const argv[]) { int fd; void* file_memory; int integer; /* Open the file. */ fd = open (argv[1], O_RDWR, S_IRUSR | S_IWUSR); /* Create the memory mapping. */ file_memory = mmap (0, FILE_LENGTH, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); close (fd); /* Read the integer, print it out, and double it. */ sscanf (file_memory, “%d”, &integer); printf (“value: %d\n”, integer); sprintf ((char*) file_memory, “%d\n”, 2 * integer); /* Release the memory (unnecessary because the program exits). */ munmap (file_memory, FILE_LENGTH); return 0; } |
3.2、共同访问一个文件
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不同的进程可以通过将同一个文件映射到内存来通信。
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设置MAP_SHARD使得写入到映射的内存的数据会立即写入关联的文件,并对另一个文件可见。
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如果不做以上设定,则Linux会对数据进行缓存,可以用函数msync将缓存写入文件。
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前两个参数表示映射的内存块。
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第三个参数是标志位:
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MS_ASTYNC:写缓存并不立即执行。
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MS_SYNC:写缓存立即执行。
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MS_INVALIDATE:所有的文件映射都被刷新,可以看到最新的更新。
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msync (mem_addr, mem_length, MS_SYNC | MS_INVALIDATE);
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设置MAP_PRIVATE将创建一个写即复制的映射区。写入这些映射区的数据仅仅在当前进程可见,对其他进程不可见。
4、管道(Pipes)
4.1、创建管道
int pipe_fds[2]; int read_fd; int write_fd; pipe (pipe_fds); read_fd = pipe_fds[0]; write_fd = pipe_fds[1]; |
4.2、用管道来进行子进程和父进程之间的通信
#include <stdlib.h> #include <stdio.h> #include <unistd.h> /* Write COUNT copies of MESSAGE to STREAM, pausing for a second between each. */ void writer (const char* message, int count, FILE* stream) { for (; count > 0; --count) { /* Write the message to the stream, and send it off immediately. */ fprintf (stream, “%s\n”, message); fflush (stream); /* Snooze a while. */ sleep (1); } } /* Read random strings from the stream as long as possible. */ void reader (FILE* stream) { char buffer[1024]; /* Read until we hit the end of the stream. fgets reads until either a newline or the end-of-file. */ while (!feof (stream) && !ferror (stream) && fgets (buffer, sizeof (buffer), stream) != NULL) fputs (buffer, stdout); } int main () { int fds[2]; pid_t pid; /* Create a pipe. File descriptors for the two ends of the pipe are placed in fds. */ pipe (fds); /* Fork a child process. */ pid = fork (); if (pid == (pid_t) 0) { FILE* stream; /* This is the child process. Close our copy of the write end of the file descriptor. */ close (fds[1]); /* Convert the read file descriptor to a FILE object, and read from it. */ stream = fdopen (fds[0], “r”); reader (stream); close (fds[0]); } else { /* This is the parent process. */ FILE* stream; /* Close our copy of the read end of the file descriptor. */ close (fds[0]); /* Convert the write file descriptor to a FILE object, and write to it. */ stream = fdopen (fds[1], “w”); writer (“Hello, world.”, 5, stream); close (fds[1]); } return 0; } |
4.3、用管道重定向标准输入,标准输出,错误流。
#include <stdio.h> #include <sys/types.h> #include <sys/wait.h> #include <unistd.h> int main () { int fds[2]; pid_t pid; /* Create a pipe. File descriptors for the two ends of the pipe are placed in fds. */ pipe (fds); /* Fork a child process. */ pid = fork (); if (pid == (pid_t) 0) { /* This is the child process. Close our copy of the write end of the file descriptor. */ close (fds[1]); /* Connect the read end of the pipe to standard input. */ dup2 (fds[0], STDIN_FILENO); /* Replace the child process with the “sort” program. */ execlp (“sort”, “sort”, 0); } else { /* This is the parent process. */ FILE* stream; /* Close our copy of the read end of the file descriptor. */ close (fds[0]); /* Convert the write file descriptor to a FILE object, and write to it. */ stream = fdopen (fds[1], “w”); fprintf (stream, “This is a test.\n”); fprintf (stream, “Hello, world.\n”); fprintf (stream, “My dog has fleas.\n”); fprintf (stream, “This program is great.\n”); fprintf (stream, “One fish, two fish.\n”); fflush (stream); close (fds[1]); /* Wait for the child process to finish. */ waitpid (pid, NULL, 0); } return 0; } |
4.4、打开(popen)和关闭(pclose)管道
#include <stdio.h> #include <unistd.h> int main () { FILE* stream = popen (“sort”, “w”); fprintf (stream, “This is a test.\n”); fprintf (stream, “Hello, world.\n”); fprintf (stream, “My dog has fleas.\n”); fprintf (stream, “This program is great.\n”); fprintf (stream, “One fish, two fish.\n”); return pclose (stream); } |
4.5、先进先出队列(FIFOs)
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一个先进先出队列是一个管道,只不过在文件系统中有文件名与之对应。
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FIFOs又被称为命名管道。
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mkfifo命令可以创建一个FIFO
% mkfifo /tmp/fifo % ls -l /tmp/fifo prw-rw-rw- 1 samuel users 0 Jan 16 14:04 /tmp/fifo |
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mkfifo函数可以创建一个FIFO
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第一个参数是文件系统中的路径。
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第二个参数是权限。
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访问FIFO和访问一个普通文件相同。
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如果两个进程通过FIFO进行通信,则需要一个进程打开一个FIFO用于写,另一个进程打开同一个FIFO用于读。
5、套接字(Sockets)
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创建一个套接字:
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命名空间:PF_LOCAL和PF_UNIX表示本地命名空间,PF_INET表示互联网命名空间。
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通信方式:SOCK_STREAM表示面向连接的套接字,SOCK_DGRAM表示面向数据报的套接字。
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关闭套接字:close
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连接套接字:欲在客户端和服务器段建立连接,客户端调用connect,指向服务器的地址,服务器端等待accept连接。
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绑定套接字:bind,将套接字绑定到一个地址。
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监听套接字:listen,使得服务器监听一个端口,等待accept一个连接。
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接受套接字:accept,接受一个来自客户端的连接。
5.1、本地命名空间套接字
访问同一台机器的套接字可以使用本地命名空间:PF_LOCAL和PF_UNIX
(socket-server.c) Local Namespace Socket Server #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/socket.h> #include <sys/un.h> #include <unistd.h> /* Read text from the socket and print it out. Continue until the socket closes. Return nonzero if the client sent a “quit” message, zero otherwise. */ int server (int client_socket) { while (1) { int length; char* text; /* First, read the length of the text message from the socket. If read returns zero, the client closed the connection. */ if (read (client_socket, &length, sizeof (length)) == 0) return 0; /* Allocate a buffer to hold the text. */ text = (char*) malloc (length); /* Read the text itself, and print it. */ read (client_socket, text, length); printf (“%s\n”, text); /* Free the buffer. */ free (text); /* If the client sent the message “quit,” we’re all done. */ if (!strcmp (text, “quit”)) return 1; } } int main (int argc, char* const argv[]) { const char* const socket_name = argv[1]; int socket_fd; struct sockaddr_un name; int client_sent_quit_message; /* Create the socket. */ socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0); /* Indicate that this is a server. */ name.sun_family = AF_LOCAL; strcpy (name.sun_path, socket_name); bind (socket_fd, &name, SUN_LEN (&name)); /* Listen for connections. */ listen (socket_fd, 5); /* Repeatedly accept connections, spinning off one server() to deal with each client. Continue until a client sends a “quit” message. */ do { struct sockaddr_un client_name; socklen_t client_name_len; int client_socket_fd; /* Accept a connection. */ client_socket_fd = accept (socket_fd, &client_name, &client_name_len); /* Handle the connection. */ client_sent_quit_message = server (client_socket_fd); /* Close our end of the connection. */ close (client_socket_fd); } while (!client_sent_quit_message); /* Remove the socket file. */ close (socket_fd); unlink (socket_name); return 0; } (socket-client.c) Local Namespace Socket Client #include <stdio.h> #include <string.h> #include <sys/socket.h> #include <sys/un.h> #include <unistd.h> /* Write TEXT to the socket given by file descriptor SOCKET_FD. */ void write_text (int socket_fd, const char* text) { /* Write the number of bytes in the string, including NUL-termination. */ int length = strlen (text) + 1; write (socket_fd, &length, sizeof (length)); /* Write the string. */ write (socket_fd, text, length); } int main (int argc, char* const argv[]) { const char* const socket_name = argv[1]; const char* const message = argv[2]; int socket_fd; struct sockaddr_un name; /* Create the socket. */ socket_fd = socket (PF_LOCAL, SOCK_STREAM, 0); /* Store the server’s name in the socket address. */ name.sun_family = AF_LOCAL; strcpy (name.sun_path, socket_name); /* Connect the socket. */ connect (socket_fd, &name, SUN_LEN (&name)); /* Write the text on the command line to the socket. */ write_text (socket_fd, message); close (socket_fd); return 0; } |
5.2、互联网套接字
(socket-inet.c) Read from a WWW Server #include <stdlib.h> #include <stdio.h> #include <netinet/in.h> #include <netdb.h> #include <sys/socket.h> #include <unistd.h> #include <string.h> /* Print the contents of the home page for the server’s socket. Return an indication of success. */ void get_home_page (int socket_fd) { char buffer[10000]; ssize_t number_characters_read; /* Send the HTTP GET command for the home page. */ sprintf (buffer, “GET /\n”); write (socket_fd, buffer, strlen (buffer)); /* Read from the socket. The call to read may not return all the data at one time, so keep trying until we run out. */ while (1) { number_characters_read = read (socket_fd, buffer, 10000); if (number_characters_read == 0) return; /* Write the data to standard output. */ fwrite (buffer, sizeof (char), number_characters_read, stdout); } } int main (int argc, char* const argv[]) { int socket_fd; struct sockaddr_in name; struct hostent* hostinfo; /* Create the socket. */ socket_fd = socket (PF_INET, SOCK_STREAM, 0); /* Store the server’s name in the socket address. */ name.sin_family = AF_INET; /* Convert from strings to numbers. */ hostinfo = gethostbyname (argv[1]); if (hostinfo == NULL) return 1; else name.sin_addr = *((struct in_addr *) hostinfo->h_addr); /* Web servers use port 80. */ name.sin_port = htons (80); /* Connect to the Web server */ if (connect (socket_fd, &name, sizeof (struct sockaddr_in)) == -1) { perror (“connect”); return 1; } /* Retrieve the server’s home page. */ get_home_page (socket_fd); return 0; } |