UNIX环境高级编程：记录上锁（fcntl函数）以及死锁检测

七、死锁检测程序

//《APUE》程序14－2：加锁和解锁一个文件区域  
//《APUE》程序14－4：死锁检测实例  
#include <stdio.h>  
#include <stdlib.h>  
#include <unistd.h>  
#include <fcntl.h>  
#include <errno.h>  
#include <signal.h>  
#include <sys/stat.h>   
      
#define read_lock(fd, offset, whence, len)   
    lock_reg((fd), F_SETLK, F_RDLCK, (offset), (whence), (len))  
#define readw_lock(fd, offset, whence, len)   
    lock_reg((fd), F_SETLKW, F_RDLCK, (offset), (whence), (len))  
#define write_lock(fd, offset, whence, len) 
    lock_reg((fd), F_SETLK, F_WRLCK, (offset), (whence), (len))  
#define writew_lock(fd, offset, whence, len) 
    lock_reg((fd), F_SETLKW, F_WRLCK, (offset), (whence), (len))  
#define un_lock(fd, offset, whence, len) 
    lock_reg((fd), F_SETLK, F_UNLCK, (offset), (whence), (len))  
      
#define FILE_MODE   (S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH)  
      
sig_atomic_t sigflag; /* set nonzero by sig handler */
sigset_t newmask, oldmask, zeromask;  
      
//输出错误信息并退出      
void error_quit(const char *str)      
{      
    fprintf(stderr, "%sn", str);      
    exit(1);      
}  
      
static void sig_usr(int signo)  /* one signal handler for SIGUSR1 and SIGUSR2 */
{  
    sigflag = 1;  
}  
      
void TELL_WAIT(void)  
{  
    if (signal(SIGUSR1, sig_usr) == SIG_ERR)  
        error_quit("signal(SIGUSR1) error");  
    if (signal(SIGUSR2, sig_usr) == SIG_ERR)  
        error_quit("signal(SIGUSR2) error");  
    sigemptyset(&zeromask);  
    sigemptyset(&newmask);  
    sigaddset(&newmask, SIGUSR1);  
    sigaddset(&newmask, SIGUSR2);  
      
    /* 
    * Block SIGUSR1 and SIGUSR2, and save current signal mask. 
    */
    if (sigprocmask(SIG_BLOCK, &newmask, &oldmask) < 0)  
        error_quit("SIG_BLOCK error");  
}  
      
void TELL_PARENT(pid_t pid)  
{  
    kill(pid, SIGUSR2);     /* tell parent we're done */
}  
      
void WAIT_PARENT(void)  
{  
    while (sigflag == 0)  
        sigsuspend(&zeromask);  /* and wait for parent */
    sigflag = 0;  
      
    /* 
    * Reset signal mask to original value. 
    */
    int temp = sigprocmask(SIG_SETMASK, &oldmask, NULL);  
    if (temp < 0)  
        error_quit("SIG_SETMASK error");  
}  
      
void TELL_CHILD(pid_t pid)  
{  
    kill(pid, SIGUSR1);         /* tell child we're done */
}  
      
void WAIT_CHILD(void)  
{  
    while (sigflag == 0)  
        sigsuspend(&zeromask);  /* and wait for child */
    sigflag = 0;  
      
    /* 
    * Reset signal mask to original value. 
    */
    int temp = sigprocmask(SIG_SETMASK, &oldmask, NULL);  
    if (temp < 0)  
        error_quit("SIG_SETMASK error");  
}  
      
//加锁或解锁某个文件区域  
int lock_reg(int fd, int cmd, int type, off_t offset,  
             int whence, off_t len)  
{  
    struct flock lock;  
    lock.l_type = type;  
    lock.l_start = offset;  
    lock.l_whence = whence;  
    lock.l_len = len;  
    return fcntl(fd, cmd, &lock);  
}  
      
//锁住文件中的一个字节  
void lockabyte(const char *name, int fd, off_t offset)  
{  
    //在我的系统上（Ubuntu10.04），发生死锁时writew_lock并不会返回-1  
    if( writew_lock(fd, offset, SEEK_SET, 1) < 0 )  
        error_quit("writew_lock error");  
    printf("%s: got the lock, byte %ldn", name, offset);  
}  
      
int main(void)  
{  
    int fd;  
    pid_t pid;  
      
    fd = creat("templock", FILE_MODE);  
    if( fd < 0 )  
        error_quit("create error");  
    if( write(fd, "ab", 2) != 2 )  
        error_quit("write error");  
      
    TELL_WAIT();  
    pid = fork();  
    if( pid < 0 )  
        error_quit("fork error");  
    else if( pid == 0 )  
    {  
        lockabyte("child", fd, 0);  
        TELL_PARENT( getpid() );  
        WAIT_PARENT();  
        lockabyte("child", fd, 1);  
    }  
    else
    {  
        lockabyte("parent", fd, 1);  
        TELL_CHILD(pid);  
        WAIT_CHILD();  
        lockabyte("parent", fd, 0);  
    }  
    return 0;  
}

运行结果：

huangcheng@ubuntu:~$ ./a.out
parent: got the lock, byte 1  
child: got the lock, byte 0  
^C  
huangcheng@ubuntu:~$

注解：

1：在该程序中，子进程锁住字节0，父进程锁住字节1，然后，它们又都试图锁住对方已经加锁的字节，这样就造成了死锁。

2：《APUE》上说：检测到死锁时，内核必须选择一个进程出错返回。但在我的系统（ubuntu 10.04）中，父子进程都被卡住，只有当你强制中断时（Ctrl+C）时，程序才会结束。

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