【Linux线程同步专题】五、进程间同步

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进程间同步

1. 互斥量mutex
1.1 互斥量属性对象的创建与销毁
1.2 属性的设置与获取

2. 文件锁

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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>

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;
}