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 {
               ...
               shor