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seek接口
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seek接口用來對設備的讀寫位置進行定位。file_operations結構中對應的seek接口如下：
loff_t?(*llseek)?(struct?file?*filp,?loff_t?off,?int?whence)?
其中off是偏移量，whence參數指起點位置。
例1.6? 字符設備seek實例
代碼見光盤\src\1drivermodel\1-6lseek。核心代碼如下所示：
ssize_t?simple_read(struct?file?*filp,?char?__user?*buf,?size_t?count,loff_t?*f_pos) ?{ ?????loff_t?pos=?*f_pos;//獲取文件指針 ?????if(pos>=256) ?????{ ?????????count=0; ?????????goto?out; ?????} ?????if(count>(256-pos)) ?????{ ?????????count=256-pos; ?????} ?????pos?+=?count; ?????//復制數據到指定的地址 ?????if?(copy_to_user(buf,demoBuffer+*f_pos,count)) ?????{ ????????count=-EFAULT;? ????????goto?out; ?????} ?????*f_pos?=?pos; ??out: ?????return?count; ?} ?loff_t?simple_llseek(struct?file?*filp,?loff_t?off,?int?whence) ?{ ?????loff_t?pos; ?????pos?=?filp->f_pos; ?????switch?(whence)? ?????{ ?????case?0: ?????????pos?=?off; ?????????break; ?????case?1: ?????????pos?+=?off; ?????????break; ?????case?2: ?????????pos?=255+off; ?????????break; ?????default: ?????????return?-EINVAL; ?????} ?????if?((pos>=256)?||?(pos<0))? ?????{ ?????????return?-EINVAL; ?????} ?????return?filp->f_pos=pos; ?} ?struct?file_operations?simple_fops?=?{ ?????.owner?=????THIS_MODULE, ?????.llseek?=???simple_llseek, ?????.read?=?????simple_read, ?????.open?=?????simple_open, ?????.release?=??simple_release, ?}; ?
應用程序參考代碼如下：
void?main() ?{ ?????int?fd; ?????int?i; ?????char?data[256]; ?????int?retval; ?????fd=open("/dev/fgj",O_RDWR); ?????if(fd==-1) ?????{ ?????????perror("error?open\n"); ?????????exit(-1); ?????} ?????printf("open?/dev/fgj?successfully\n"); ?????retval=lseek(fd,5,0); ?????if(retval==-1) ?????{ ?????????perror("lseek?error\n"); ?????????exit(-1); ?????} ?????retval=read(fd,data,3); ?????if(retval==-1) ?????{ ?????????perror("read?error\n"); ?????????exit(-1); ?????} ?????data[retval]=0; ?????printf("read?successfully:%s\n",data); ?????//文件定位 ?????retval=lseek(fd,2,0); ?????if(retval==-1) ?????{ ?????????perror("lseek?error\n"); ?????????exit(-1); ?????} ?????retval=read(fd,data,3); ?????if(retval==-1) ?????{ ?????????perror("read?error\n"); ?????????exit(-1); ?????} ?????data[retval]=0; ?????printf("read?successfully:%s\n",data); ?????close(fd); ?} ?
本例運行結果如下：
[root@urbetter?/home]#?insmod?demo.ko ?[root@urbetter?/home]#?mknod?/dev/fgj?c?224?0 ?[root@urbetter?/home]#?./test ?open?/dev/fgj?successfully ?read?successfully:FGH ?read?successfully:CDE ?
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