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 {
...
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