Re: What's wrong with this really simple function?

[Juergen Quade <quade@hsnr.de>]

On Sun, Nov 27, 2005 at 12:57:47PM -0600, Mohamed El Dawy wrote:
> Hi,
>  I have created this 5-liner system call, which basically opens a
> file, write "Hello World" to it, and then returns. That's all.
> 
> Now, when I actually call it, it creates the file successfully but
> writes nothing to it. The file is created and is only zero bytes. So,
> either write didn't write, or close didn't close. Any help would be
> greatly appreciated.
> ...

The following (module-) code will create and write a file from
inside a kernel. Ok -- you know -- you should not use it
without really good reasons ...

          Juergen.

#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/fs.h>
#include <asm/uaccess.h>

static char filename[255];
module_param_string( filename, filename, sizeof(filename), 666 );
struct file *log_file;

static int __init mod_init(void)
{
	mm_segment_t oldfs;

	if( filename[0]=='\0' )
		strncpy( filename, "/tmp/kernel_file", sizeof(filename) );
	printk("opening filename: %s\n", filename);
	log_file = filp_open( filename, O_WRONLY | O_CREAT, 0644 );
	printk("log_file: %p\n", log_file );
	if( IS_ERR( log_file ) )
		return -EIO;

	oldfs = get_fs();
	set_fs( KERNEL_DS );
	vfs_write( log_file, "hallo\n", 6, &log_file->f_pos );
	set_fs( oldfs );
	filp_close( log_file, NULL );
	return 0;
}

static void __exit mod_exit(void)
{
}
module_init( mod_init );
module_exit( mod_exit );
MODULE_LICENSE("GPL");
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