[C] How to create file under proc

# Create a file
vim /tmp/proc.c

#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/proc_fs.h>
#include <linux/string.h>
#include <linux/vmalloc.h>
#include <asm/uaccess.h>

#define MAX_LEN       4096
int read_info( char *page, char **start, off_t off,int count, int *eof, void *data );
ssize_t write_info( struct file *filp, const char __user *buff,unsigned long len, void *data );

static struct proc_dir_entry *proc_entry;
static char *info;
static int write_index;
static int read_index;

int init_module( void )
{
    int ret = 0;
    info = (char *)vmalloc( MAX_LEN );
    memset( info, 0, MAX_LEN );
    proc_entry = create_proc_entry( "procEntry123", 0644, NULL );

    if (proc_entry == NULL)
    {
        ret = -1;
        vfree(info);
        printk(KERN_INFO "procEntry123 could not be created\n");
    }
    else
    {
        write_index = 0;
        read_index = 0;
        proc_entry->read_proc = read_info;
        proc_entry->write_proc = write_info;
        printk(KERN_INFO "procEntry123 created.\n");
    }

    return ret;
}

void cleanup_module( void )
{
    remove_proc_entry("procEntry123", proc_entry);
    printk(KERN_INFO "procEntry123 unloaded.\n");
    vfree(info);
}

ssize_t write_info( struct file *filp, const char __user *buff, unsigned long len, void *data )
{
    int capacity = (MAX_LEN-write_index)+1;
    if (len > capacity)
    {
        printk(KERN_INFO "No space to write in procEntry123!\n");
        return -1;
    }
    if (copy_from_user( &info[write_index], buff, len ))
    {
        return -2;
    }

    write_index += len;
    info[write_index-1] = 0;
    return len;
}

int read_info( char *page, char **start, off_t off, int count, int *eof, void *data )
{
    int len;
    if (off > 0)
    {
        *eof = 1;
        return 0;
    }

    if (read_index >= write_index)
    read_index = 0;

    len = sprintf(page, "%s\n", &info[read_index]);
    read_index += len;
    return len;
}

# Start to compile
Create a Makefile

$ cat Makefile
obj-m += proc.o
all:
        sudo make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules
clean:
        sudo make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

make
make -C /lib/modules/2.6.32-50-generic-pae/build M=/tmp/test2 modules
make[1]: Entering directory /usr/src/linux-headers-2.6.32-50-generic-pae'

CC [M] /tmp/test2/proc.o
Building modules, stage 2.
MODPOST 1 modules
CC /tmp/test2/proc.mod.o
LD [M] /tmp/test2/proc.ko
make[1]: Leaving directory/usr/src/linux-headers-2.6.32-50-generic-pae’

# Insert module
sudo insmod proc.ko

# Testing
ls /proc/
/proc/procEntry123

echo “happy” > /proc/procEntry123
cat /proc/procEntry123
happy

Reference:

• How to Create Linux Proc Files in C Program using LKM
• Access the Linux kernel using the /proc filesystem