Hi , if you print the parameters that are passed to "moduleRead" when you issue cat command, it'll show len=4096 (thats what it gives on my machine) , and so the function will be called repeatedly till that count is reached or EOF is reached. i think in function "moduleRead" when you come out of while loop check if 'cnt' is zero, i yes then return EOF or else return 'len' Dhanashri > -----Original Message----- > From: kernelnewbies-bounce@xxxxxxxxxxxx > [mailto:kernelnewbies-bounce@xxxxxxxxxxxx]On Behalf Of > Siddharth Taneja > Sent: Monday, March 21, 2005 2:54 PM > To: kernelnewbies@xxxxxxxxxxxx > Subject: Character device driver problem > > > Hi, > > I am trying to build a simple character device driver which supports > only the open, read and release interface. The code compiles fine and > the module gets inserted also fine but when I do cat <device > name>, the > read function is entered multiple times, resulting in > multiple and same > outputs("we did it" repeated continously). I am using Suse 9.2 with > kernel 2.6.8-24.11. The following is the code and outputs. > > Thanks > > Siddharth > > > //Building a character device driver > > #include <linux/module.h> > #include <linux/kernel.h> > #include <linux/fs.h> > #include <asm/uaccess.h> > > static int Major; > static int devOpen; > static char msg[80]; > static char*msgPtr; > static int moduleOpen(struct inode *,struct file *); > static ssize_t moduleRead(struct file *,char *,size_t,loff_t *); > static int moduleRelease(struct inode *,struct file *); > > #define DEV_NAME "myModule" > > static struct file_operations fops = { > .read=moduleRead, > .open=moduleOpen, > .release=moduleRelease > }; > > int startup(void) { > printk(KERN_EMERG "I am in\n"); > devOpen=0; > Major=register_chrdev(0,DEV_NAME,&fops); > if (Major <0) { > printk(KERN_EMERG "wtf!?\n"); > return Major; > } > printk(KERN_EMERG "Heres ur maj no:%d\n",Major); > return 0; > } > > > void cleanup(void) { > printk(KERN_EMERG "Have to go..\n"); > unregister_chrdev(Major,DEV_NAME); > } > > static int moduleOpen(struct inode *in,struct file *fi) { > if(devOpen==1) > return -EBUSY; > devOpen++; > sprintf(msg,"we did it"); > msgPtr=msg; > printk(KERN_EMERG "Open seeeaa..\n"); > return 0; > } > > static ssize_t moduleRead(struct file *fi,char *buff, size_t > len, loff_t > * offs){ > static int cnt=8; > printk(KERN_EMERG "Reading %d\n",len); > > while(cnt>=0&&len) { > put_user(*msgPtr,buff); > msgPtr++; > buff++; > cnt--; > } > //put_user('\0',buff); > return len; > } > > static int moduleRelease(struct inode *in,struct file* fi) { > printk(KERN_EMERG "Release me...\n"); > devOpen--; > return 0; > } > > module_init(startup); > module_exit(cleanup); > MODULE_LICENSE("GPL and additional rights"); > > -------------------------------------------------------------- > -------------------------------------------------------------- > ---------------------- > > Output (log messages) > > Mar 21 01:19:32 linux kernel: module first_module unsupported by > SUSE/Novell, tainting kernel. > Mar 21 01:19:32 linux kernel: I am in > Mar 21 01:19:32 linux kernel: Heres ur maj no:253 > Mar 21 01:19:46 linux kernel: Open seeeaa.. > Mar 21 01:19:46 linux kernel: Reading 4096 > Mar 21 01:19:46 linux last message repeated 177 times > Mar 21 01:19:46 linux kernel: Release me... > Mar 21 01:19:56 linux kernel: Have to go.. > > > > > > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > FAQ: http://kernelnewbies.org/faq/ > > -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
