Thanks, but that is the very problem, I do not know the size of the data in the user space. One probable solution could be that i make two ioctl calls wherein i get the length of the data in the first call and then the data in the second call. But isnt there a better solution then this one!!! Regards Mandeep -----Original Message----- From: T.L.Madhu [mailto:madhu.tarikere@wipro.com] Sent: Thursday, January 16, 2003 5:19 PM To: Mandeep S. Brar Cc: kernelnewbies@nl.linux.org Subject: Re: memory allocation As i know, you have to allocate memory either statically or dynamically in user space for var.string and pass pointer thro. ioctl. copy_to_user() is not going to allocate memory, it just copies data from kernel space to user space. Ensure that allocated memory for string member will always be more than len+1 (of kernel) to avoid memory corruption. Regards, Madhu On Thu, 16 Jan 2003, Mandeep S. Brar wrote: > hi, > I am a newbie into linux programming. Ps help me with a small problem i am > facing while writing a sample program for a driver module. > I have a structure like > struct devstruct { > char* string; > int len; > }; > I want to return information from the driver to a user process which queries > me with ioctl. The code has the following outline... > > USER PROCESS > ........... > struct devstruct var; > ioctl(fd, READ_COMMAND, &var); > printf("%s",var.string); > ..................... > > > DRIVER Module > > char* data="hello"; > int len=5; > > int myioctl (struct inode *inode, struct file *filp, unsigned int cmd, > unsigned long arg){ > switch(cmd) { > ............. > > case READ_COMMAND: > { > ..... > struct devstruct* var = (struct > devstruct*)arg; > var->len = len; > /********* > allocate memory in user space? > **********/ > var->string = (allocate memory = len+1 in > user space) > copy_to_user(var->string,data,len+1); > ..... > } > break; > ............. > } > } > My question is that how do i allocate memory in user space when i am in > kernel space or will copy_to_user do it automatically?. is there a variant > of kmalloc or what?? > > > Thanks in advance > Mandeep > > > -- > 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/
