Linux 读写锁
线程的读写锁函数：
1，读写锁的初始化与销毁，静态初始化的话，可以直接使用PTHREAD_RWLOCK_INITIALIZER。
#include <pthread.h>
int pthread_rwlock_destroy(pthread_rwlock_t *rwlock);
int pthread_rwlock_init(pthread_rwlock_t *restrict rwlock,
const pthread_rwlockattr_t *restrict attr);
pthread_rwlock_t rwlock = PTHREAD_RWLOCK_INITIALIZER;
2，用读的方式加锁和尝试（没锁上就立即返回）加锁。
#include <pthread.h>
int pthread_rwlock_rdlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_tryrdlock(pthread_rwlock_t *rwlock);
3，用写的方式加锁和尝试（没锁上就立即返回）加锁。
#include <pthread.h>
int pthread_rwlock_wrlock(pthread_rwlock_t *rwlock);
int pthread_rwlock_trywrlock(pthread_rwlock_t *rwlock);
4，解锁
#include <pthread.h>
int pthread_rwlock_unlock(pthread_rwlock_t *rwlock);
多个进程在同时读写同一个文件，会发生什么？
例子1：用下面的例子的执行结果，观察多个进程在同时读写同一个文件，会发生什么。
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define MAXLINE 100
#define FN "num1"
void my_lock(int fd){
return;
}
void my_unlock(int fd){
return;
}
int main(int args, char** argv){
int fd;
long i,seqno;
pid_t pid;
ssize_t n;
char line[MAXLINE + 1];
pid = getpid();
fd = open(FN, O_RDWR, 0664);
for(i = 0; i < 20; ++i){
my_lock(fd);
lseek(fd, 0L, SEEK_SET);
n = read(fd, line, MAXLINE);
line[n] = '\0';
seqno = atol(line);
printf("%s:pid = %ld, seq = %ld\n", argv[0], (long)pid, seqno);
seqno++;
snprintf(line, sizeof(line), "%ld\n", seqno);
lseek(fd, 0L, SEEK_SET);
write(fd, line, strlen(line));
my_unlock(fd);
}
return 0;
}
执行方法：同时执行上面例子的程序2次，也就是2个进程同时读写同一个文件。
ubuntu$ ./flockmain1 & ./flockmain1 &
执行结果如下，发现2个进程同时读写，在①处开始，内核切换进程时，数字乱套了。
ubuntu$ ./flockmain1 & ./flockmain1 &
[1] 4760
[2] 4761
ubuntu$ ./flockmain1:pid = 4761, seq = 1
./flockmain1:pid = 4761, seq = 2
./flockmain1:pid = 4761, seq = 3
./flockmain1:pid = 4761, seq = 4
./flockmain1:pid = 4761, seq = 5
./flockmain1:pid = 4761, seq = 6
./flockmain1:pid = 4761, seq = 7
./flockmain1:pid = 4761, seq = 8
./flockmain1:pid = 4761, seq = 9
./flockmain1:pid = 4761, seq = 10 ------------①
./flockmain1:pid = 4760, seq = 10
./flockmain1:pid = 4761, seq = 11
./flockmain1:pid = 4761, seq = 12
./flockmain1:pid = 4761, seq = 13
./flockmain1:pid = 4761, seq = 14
./flockmain1:pid = 4761, seq = 15
./flockmain1:pid = 4761, seq = 16
./flockmain1:pid = 4761, seq = 17
./flockmain1:pid = 4761, seq = 18
./flockmain1:pid = 4761, seq = 19
./flockmain1:pid = 4761, seq = 20
./flockmain1:pid = 4760, seq = 11
./flockmain1:pid = 4760, seq = 12
./flockmain1:pid = 4760, seq = 13
./flockmain1:pid = 4760, seq = 14
./flockmain1:pid = 4760, seq = 15
./flockmain1:pid = 4760, seq = 16
./flockmain1:pid = 4760, seq = 17
./flockmain1:pid = 4760, seq = 18
./flockmain1:pid = 4760, seq = 19
./flockmain1:pid = 4760, seq = 20
./flockmain1:pid = 4760, seq = 21
./flockmain1:pid = 4760, seq = 22
./flockmain1:pid = 4760, seq = 23
./flockmain1:pid = 4760, seq = 24
./flockmain1:pid = 4760, seq = 25
./flockmain1:pid = 4760, seq = 26
./flockmain1:pid = 4760, seq = 27
./flockmain1:pid = 4760, seq = 28
./flockmain1:pid = 4760, seq = 29
为了解决上面的问题，必须对文件的内容进行加锁。
如何对文件内容加锁？
使用fcntl函数，它既可以锁整文件，也可以锁文件里的某段内容。通过结构体flock来指定要锁的范围。如果 whence = SEEK_SET;l_start = 0;l_len = 0;就是锁定整个文件。
struct flock {
...
short l_type; /* Type of lock: F_RDLCK,
F_WRLCK, F_UNLCK */
short l_whence; /* How to interpret l_start:
SEEK_SET, SEEK_CUR, SEEK_END */
off_t l_start; /* Starting offset for lock */
off_t l_len; /* Number of bytes to lock */
pid_t l_pid; /* PID of process blocking our lock
(set by F_GETLK and F_OFD_GETLK) */
...
};
- F_SETLK：上锁。如果发现已经被别的进程上锁了，就直接返回-1，errno被设置成EACCES或者EAGAIN，不阻塞。
- F_SETLKW：上锁。阻塞等待。
- F_GETLK：得到锁的状态。
修改上面的函数my_lock，my_unlock。main函数不变。
例子2：
void my_lock(int fd){
struct flock lock;
lock.l_type = F_WRLCK;
wlock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLKW, lock);
}
void my_unlock(int fd){
struct flock lock;
lock.l_type = F_UNLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, lock);
}
执行结果如下，发现数字不乱套了。
ubuntu$ ./flockmain & ./flockmain &
[1] 4882
[2] 4883
ubuntu$ ./flockmain:pid = 4883, seq = 1
./flockmain:pid = 4883, seq = 2
./flockmain:pid = 4883, seq = 3
./flockmain:pid = 4883, seq = 4
./flockmain:pid = 4883, seq = 5
./flockmain:pid = 4883, seq = 6
./flockmain:pid = 4883, seq = 7
./flockmain:pid = 4883, seq = 8
./flockmain:pid = 4883, seq = 9
./flockmain:pid = 4883, seq = 10
./flockmain:pid = 4883, seq = 11
./flockmain:pid = 4883, seq = 12
./flockmain:pid = 4883, seq = 13
./flockmain:pid = 4883, seq = 14
./flockmain:pid = 4883, seq = 15
./flockmain:pid = 4883, seq = 16
./flockmain:pid = 4883, seq = 17
./flockmain:pid = 4883, seq = 18
./flockmain:pid = 4883, seq = 19
./flockmain:pid = 4883, seq = 20
./flockmain:pid = 4882, seq = 21
./flockmain:pid = 4882, seq = 22
./flockmain:pid = 4882, seq = 23
./flockmain:pid = 4882, seq = 24
./flockmain:pid = 4882, seq = 25
./flockmain:pid = 4882, seq = 26
./flockmain:pid = 4882, seq = 27
./flockmain:pid = 4882, seq = 28
./flockmain:pid = 4882, seq = 29
./flockmain:pid = 4882, seq = 30
./flockmain:pid = 4882, seq = 31
./flockmain:pid = 4882, seq = 32
./flockmain:pid = 4882, seq = 33
./flockmain:pid = 4882, seq = 34
./flockmain:pid = 4882, seq = 35
./flockmain:pid = 4882, seq = 36
./flockmain:pid = 4882, seq = 37
./flockmain:pid = 4882, seq = 38
./flockmain:pid = 4882, seq = 39
./flockmain:pid = 4882, seq = 40
到此为止，貌似解决了问题，但是如果同时执行例子1和例子2，结果如下，发现还是乱的。
也就是说在协作线程（cooperating processes）间，文件锁（也叫劝告性上锁）也起作用的。但是不完全不相关的进程中，文件锁也不起作用的。如何解决呢？使用强制性上锁。
ys@ys-VirtualBox:~/IPC$ ./flockmain1 & ./flockmain &
[1] 3602
[2] 3603
ys@ys-VirtualBox:~/IPC$ ./flockmain1:pid = 3602, seq = 1
./flockmain:pid = 3603, seq = 1
./flockmain:pid = 3603, seq = 2
./flockmain:pid = 3603, seq = 3
./flockmain:pid = 3603, seq = 4
./flockmain:pid = 3603, seq = 5
./flockmain:pid = 3603, seq = 6
./flockmain:pid = 3603, seq = 7
./flockmain:pid = 3603, seq = 8
./flockmain:pid = 3603, seq = 9
./flockmain:pid = 3603, seq = 10
./flockmain1:pid = 3602, seq = 2
./flockmain1:pid = 3602, seq = 3
./flockmain1:pid = 3602, seq = 4
./flockmain:pid = 3603, seq = 11
./flockmain:pid = 3603, seq = 12
./flockmain1:pid = 3602, seq = 5
./flockmain:pid = 3603, seq = 13
./flockmain1:pid = 3602, seq = 6
./flockmain1:pid = 3602, seq = 7
./flockmain1:pid = 3602, seq = 8
./flockmain:pid = 3603, seq = 14
./flockmain:pid = 3603, seq = 15
./flockmain1:pid = 3602, seq = 9
./flockmain1:pid = 3602, seq = 10
./flockmain:pid = 3603, seq = 16
./flockmain:pid = 3603, seq = 17
./flockmain1:pid = 3602, seq = 11
./flockmain:pid = 3603, seq = 18
./flockmain1:pid = 3602, seq = 12
./flockmain1:pid = 3602, seq = 13
./flockmain1:pid = 3602, seq = 14
./flockmain:pid = 3603, seq = 19
./flockmain:pid = 3603, seq = 20
./flockmain1:pid = 3602, seq = 15
./flockmain1:pid = 3602, seq = 16
./flockmain1:pid = 3602, seq = 17
./flockmain1:pid = 3602, seq = 18
./flockmain1:pid = 3602, seq = 19
./flockmain1:pid = 3602, seq = 20
第一个问题：假如一个文件被一个进程以读的方式锁定，并有另一个进程在等待读锁定解锁后，用写入的方式锁定，这时是否允许另一个进程的还以读的方式取得锁定？
用例子3来观察：
#include <time.h>
#include <sys/time.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
void gftime(char* buf){
struct timeval tv;
gettimeofday(&tv, NULL);
long usec = tv.tv_usec;
struct tm* tm = localtime(&tv.tv_sec);
sprintf(buf, "%d:%d:%d.%ld",tm->tm_hour, tm->tm_min, tm->tm_sec,usec);
}
int main(){
char buff[100] = {0};
int fd = open("test.dat", O_RDWR | O_CREAT, 0664);
struct flock lock;
lock.l_type = F_RDLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, &lock);
gftime(buff);
printf("%s: parent has read lock\n", buff);
//first child
if(fork() == 0){
char buf2[100] = {0};
sleep(1);
gftime(buf2);
printf("%s: first child tries to obtain write lock\n", buf2);
struct flock lock2;
lock2.l_type = F_WRLCK;
lock2.l_whence = SEEK_SET;
lock2.l_start = 0;
lock2.l_len = 0;
fcntl(fd, F_SETLKW, &lock2);
gftime(buf2);
printf("%s: first child obtains write lock\n", buf2);
sleep(2);
lock2.l_type = F_UNLCK;
lock2.l_whence = SEEK_SET;
lock2.l_start = 0;
lock2.l_len = 0;
fcntl(fd, F_SETLK, &lock2);
gftime(buf2);
printf("%s: first child releases write lock\n", buf2);
exit(0);
}
//secodn child
if(fork() == 0){
char buf1[100] = {0};
sleep(3);
gftime(buf1);
printf("%s: second child tries to obtain read lock\n", buf1);
struct flock lock1;
lock1.l_type = F_RDLCK;
lock1.l_whence = SEEK_SET;
lock1.l_start = 0;
lock1.l_len = 0;
fcntl(fd, F_SETLKW, &lock1);
gftime(buf1);
printf("%s: second child obtains read lock\n", buf1);
sleep(4);
lock1.l_type = F_UNLCK;
lock1.l_whence = SEEK_SET;
lock1.l_start = 0;
lock1.l_len = 0;
fcntl(fd, F_SETLK, &lock1);
gftime(buf1);
printf("%s: second child release read lock\n", buf1);
exit(0);
}
//parent process
sleep(5);
lock.l_type = F_UNLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, &lock);
gftime(buff);
printf("%s: parent releases read lock\n", buff);
wait(NULL);
wait(NULL);
exit(0);
}
在ubuntu上执行结果：
17:49:44.348946: parent has read lock
17:49:45.350191: first child tries to obtain write lock
17:49:47.350155: second child tries to obtain read lock
17:49:47.350409: second child obtains read lock
17:49:49.349442: parent releases read lock
17:49:51.351197: second child release read lock
17:49:51.351582: first child obtains write lock
17:49:53.351689: first child releases write lock
第一个问题的答案：允许另一个进程的还以读的方式取得锁定
第二个问题：假如一个文件被一个进程以写的方式锁定，这时又有2个进程在等待这个锁的释放，其中一个进程是以写锁的方式等待，其中另一个进程是以读锁的方式等待，哪一个会优先取得锁？
用例子4来观察：
#include <time.h>
#include <sys/time.h>
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/wait.h>
void gftime(char* buf){
struct timeval tv;
gettimeofday(&tv, NULL);
long usec = tv.tv_usec;
struct tm* tm = localtime(&tv.tv_sec);
sprintf(buf, "%d:%d:%d.%ld",tm->tm_hour, tm->tm_min, tm->tm_sec,usec);
}
int main(){
char buff[100] = {0};
int fd = open("test.dat", O_RDWR | O_CREAT, 0664);
struct flock lock;
lock.l_type = F_WRLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, &lock);
gftime(buff);
printf("%s: parent has write lock\n", buff);
//first child
if(fork() == 0){
char buf2[100] = {0};
sleep(1);
gftime(buf2);
printf("%s: first child tries to obtain write lock\n", buf2);
struct flock lock2;
lock2.l_type = F_WRLCK;
lock2.l_whence = SEEK_SET;
lock2.l_start = 0;
lock2.l_len = 0;
fcntl(fd, F_SETLKW, &lock2);
gftime(buf2);
printf("%s: first child obtains write lock\n", buf2);
sleep(2);
lock2.l_type = F_UNLCK;
lock2.l_whence = SEEK_SET;
lock2.l_start = 0;
lock2.l_len = 0;
fcntl(fd, F_SETLK, &lock2);
gftime(buf2);
printf("%s: first child releases write lock\n", buf2);
exit(0);
}
//secodn child
if(fork() == 0){
char buf1[100] = {0};
sleep(3);
gftime(buf1);
printf("%s: second child tries to obtain read lock\n", buf1);
struct flock lock1;
lock1.l_type = F_RDLCK;
lock1.l_whence = SEEK_SET;
lock1.l_start = 0;
lock1.l_len = 0;
fcntl(fd, F_SETLKW, &lock1);
gftime(buf1);
printf("%s: second child obtains read lock\n", buf1);
sleep(4);
lock1.l_type = F_UNLCK;
lock1.l_whence = SEEK_SET;
lock1.l_start = 0;
lock1.l_len = 0;
fcntl(fd, F_SETLK, &lock1);
gftime(buf1);
printf("%s: second child release read lock\n", buf1);
exit(0);
}
//parent process
sleep(5);
lock.l_type = F_UNLCK;
lock.l_whence = SEEK_SET;
lock.l_start = 0;
lock.l_len = 0;
fcntl(fd, F_SETLK, &lock);
gftime(buff);
printf("%s: parent releases write lock\n", buff);
wait(NULL);
wait(NULL);
exit(0);
}
在ubuntu上执行结果：
17:49:29.796599: parent has write lock
17:49:30.797099: first child tries to obtain write lock
17:49:32.796885: second child tries to obtain read lock
17:49:34.796868: parent releases write lock
17:49:34.796987: second child obtains read lock
17:49:38.797148: second child release read lock
17:49:38.797297: first child obtains write lock
17:49:40.797727: first child releases write lock
第二个问题的答案：没有准确答案。在Ubuntu上的执行结果上看，读锁优先了，但是，可能在别的环境上又是写锁优先。按道理来说应该写锁优先吧？
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