fcntl的文件锁导致写锁饿死的情况

gothicane

#include <stdio.h> #include <string.h> #include <unistd.h> #include <fcntl.h> #include <stdlib.h> #include <errno.h> #include <sys/file.h> #include <pthread.h> int fdd = -1; void lockk() {    &nbsp;&nbsp;&nbsp;&nbsp;if ( fdd == -1 ) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return; &nbsp;&nbsp;&nbsp;&nbsp;printf("fd %d\n", fdd); &nbsp;&nbsp;&nbsp;&nbsp;struct flock lock; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_type = F_WRLCK; &nbsp;&nbsp;&nbsp;&nbsp;/*lock.l_type = F_RDLCK;*/ &nbsp;&nbsp;&nbsp;&nbsp;lock.l_whence = SEEK_SET; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_start = 0; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_len = 0; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_pid = -1; &nbsp;&nbsp;&nbsp;&nbsp;if ( fcntl(fdd, F_SETLKW, &lock) == -1) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("Lock1 err\n", strerror(errno)); &nbsp;&nbsp;&nbsp;&nbsp;} } void unlockk() { &nbsp;&nbsp;&nbsp;&nbsp;if( fdd == -1 ) &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return; &nbsp;&nbsp;&nbsp;&nbsp;printf("fd %d\n", fdd); &nbsp;&nbsp;&nbsp;&nbsp;struct flock lock; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_type = F_UNLCK; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_whence = SEEK_SET; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_start = 0; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_len = 0; &nbsp;&nbsp;&nbsp;&nbsp;lock.l_pid = -1; &nbsp;&nbsp;&nbsp;&nbsp;if ( fcntl(fdd, F_SETLK, &lock) == -1) &nbsp;&nbsp;&nbsp;&nbsp;{ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;printf("Lock1 err\n", strerror(errno)); &nbsp;&nbsp;&nbsp;&nbsp;} } int main(int argc, char * argv[]) { &nbsp;&nbsp;&nbsp;&nbsp;fdd = open("/root/lltest", O_CREAT | O_RDWR); &nbsp;&nbsp;&nbsp;&nbsp;lockk(); &nbsp;&nbsp;&nbsp;&nbsp;printf("locked\n"); &nbsp;&nbsp;&nbsp;&nbsp;printf("lock %d\n", getpid()); &nbsp;&nbsp;&nbsp;&nbsp;sleep(40); &nbsp;&nbsp;&nbsp;&nbsp;unlockk(); &nbsp;&nbsp;&nbsp;&nbsp;printf("unlocked\n"); &nbsp;&nbsp;&nbsp;&nbsp;return -1; }

上面的代码编译成两个程序lockrd,lockw.区别在lock.l_type，一个加读锁，一个加写锁。
按下面顺序执行程序
lockrd
lockw
lockrd
lockrd
lockrd
lockrd
lockrd

结果显示lockw一直阻塞，直到最后一个lockrd进程解锁后lockw才得以加锁，这样岂不是会饿死写锁？
有没有人遇到同样的问题啊，请多指教！！

fxpbupt

我的打印顺序是这样的
$ ./lockrd& ./lockw& ./lockrd&
[1] 25252
lock read  25252
[2] 25253
lock write  25253
lock read  25254
[3] 25254
unlock read 25252
unlock write 25253
unlock read 25254
不存在你说的情况阿。。。

[ 本帖最后由 fxpbupt 于 2009-7-10 23:53 编辑 ]

gothicane

我用的是 ubuntu 9.04 Linux version 2.6.28-13-generic ,我在控制台下分别执行的那两个程序