【Linux线程同步专题】五、进程间同步

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进程间同步

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1. 互斥量mutex

进程间也可以通过互斥锁来达到同步的目的。在pthread_mutex_init初始化之前需要修改属性为进程间共享。

1.1 互斥量属性对象的创建与销毁

头文件及函数原型

#include<pthread.h>intpthread_mutexattr_destroy(pthread_mutexattr_t*attr);intpthread_mutexattr_init(pthread_mutexattr_t*attr);

函数描述- The pthread_mutexattr_destroy() function shall destroy a mutex attributes object; the object becomes, in effect, uninitialized.- The pthread_mutexattr_init() function shall initialize a mutex attributes object attr with the default value for all of the attributes defined by the implementation.
函数参数- attr：互斥量属性对象
函数返回值Upon successful completion, pthread_mutexattr_destroy() and pthread_mutexattr_init() shall return zero; otherwise, an error number shall be returned to indicate the error.

1.2 属性的设置与获取

头文件及函数原型

#include<pthread.h>intpthread_mutexattr_getpshared(constpthread_mutexattr_t*restrict attr,int*restrict pshared);intpthread_mutexattr_setpshared(pthread_mutexattr_t*attr,int pshared);

函数描述- The pthread_mutexattr_getpshared() function shall obtain the value of the process-shared attribute from the attributes object referenced by attr.- The pthread_mutexattr_setpshared() function shall set the process-shared attribute in an initialized attributes object referenced by attr.
函数参数- attr- pshared
函数返回值Upon successful completion, pthread_mutexattr_setpshared() shall return zero; otherwise, an error number shall be returned to indicate the error. Upon successful completion, pthread_mutexattr_getpshared() shall return zero and store the value of the process-shared attribute of attr into the object referenced by the pshared parameter. Otherwise, an error number shall be returned to indicate the error.

2. 文件锁

借助fcntl()函数来实现锁机制，操作文件的进程没有获得锁时，可以打开，但无法执行read和write操作。文件锁具有读写锁的特点，写独占，读共享，写优先级高。

头文件及函数原型

#include<unistd.h>#include<fcntl.h>intfcntl(int fd,int cmd,.../* arg */);

函数描述fcntl() performs one of the operations described below on the open file descriptor fd. The operation is determined by cmd. 获取、设置文件访问控制属性。
函数参数- fd：文件描述符- cmd： - F_SETLK (struct flock *)：设置文件锁trylock- F_SETLKW (struct flock *)：设置文件锁lock- F_GETLK (struct flock *)：获取文件锁structflock{...short l_type;/* Type of lock: F_RDLCK, F_WRLCK, F_UNLCK 锁的类型 */short l_whence;/* How to interpret l_start: SEEK_SET, SEEK_CUR, SEEK_END 偏移位置 */off_t l_start;/* Starting offset for lock 起始偏移 */off_t l_len;/* Number of bytes to lock 长度，0表示整个文件加锁 */pid_t l_pid;/* PID of process blocking our lock (F_GETLK only) 持有该锁的进程ID */...};
函数返回值- For a successful call, the return value depends on the operation:- F_DUPFD The new descriptor.- F_GETFD Value of flags.- F_GETFL Value of flags.- F_GETLEASE Type of lease held on file descriptor.- F_GETOWN Value of descriptor owner.- F_GETSIG Value of signal sent when read or write becomes possible, or zero for traditional SIGIO behavior.All other commands Zero.- On error, -1 is returned, and errno is set appropriately.

用法示例：

#include<stdio.h>#include<unistd.h>#include<sys/types.h>#include<sys/stat.h>#include<fcntl.h>#include<stdlib.h>#define_FILE_PATH_"/home/qq/dm/daemon/test.lock"intmain(int argc,char* argv[]){int fd =open(_FILE_PATH_, O_RDWR | O_CREAT,0644);if(fd <0){perror("open err");return-1;}structflock lk;
    lk.l_type = F_WRLCK;
    lk.l_whence =SEEK_SET;
    lk.l_start =0;
    lk.l_len =0;if(fcntl(fd, F_SETLK,&lk)<0){perror("lock err");exit(1);}while(1){printf("pid: %d\n",getpid());sleep(1);}return0;}

编译运行

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