Linux Kernel Architecture

Chapter 10: Filesystems without Persistent Storage

Writing SequentialFile Handlers

Basically, an instance ofstruct file_operationsthat provides pointers to someseq_routines must be

implemented to benefit from the sequential file standard implementation. The kprobes subsystem does

this as follows:

kernel/kprobes.c

static struct file_operations debugfs_kprobes_operations = {

.open = kprobes_open,

.read = seq_read,

.llseek = seq_lseek,

.release = seq_release,

};

This instance offile_operationscan be associated with a file by the methods discussed in Chapter 8. In

the case of kprobes, the file will be created in the debugging filesystem; see Section 10.2.3.

The only method that needs to be implemented isopen. Not much effort is required for the function,

though: A simple one-liner connects the file with the sequential file interface:

kernel/kprobes.c

static struct seq_operations kprobes_seq_ops = {

.start = kprobe_seq_start,

.next = kprobe_seq_next,

.stop = kprobe_seq_stop,

.show = show_kprobe_addr

};

static int __kprobes kprobes_open(struct inode *inode, struct file *filp)

{

return seq_open(filp, &kprobes_seq_ops);

}

struct file struct seq_file struct seq_operations

private op

start

stop

next

show

Figure 10-8: Data structures for sequential files.

seq_opensets up the data structures required by the sequential file mechanism. The result is shown in

Figure 10-8. Recall from Chapter 8 that theprivate_dataelement ofstruct filecan point to arbitrary

data that are private to the file and not touched by the generic VFS functions. In this case,seq_openuses

the pointer to establish a connection with an instance ofstruct seq_filethat contains status information

about the sequential file:

<seq_file.h>

struct seq_file {

char *buf;