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> int pthread_mutexattr_destroy(pthread_mutexattr_t *attr); int pthread_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> int pthread_mutexattr_getpshared(const pthread_mutexattr_t *restrict attr, int *restrict pshared); int pthread_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> int fcntl(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
struct flock { ... 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" int main(int argc, char* argv[]) { int fd = open(_FILE_PATH_, O_RDWR | O_CREAT, 0644); if(fd < 0) { perror("open err"); return -1; } struct flock 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); } return 0; }
编译运行
此时我们再开一个终端运行上面的程序
