STEP1:open
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
STEP2:实现open(pathname, flags, mod) open.c
SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
{
if (force_o_largefile())
flags |= O_LARGEFILE;
return do_sys_open(AT_FDCWD, filename, flags, mode);
}
STEP3: do_sys_open(AT_FDCWD, filename, flags, mode);
filename : const char *pathname
flags: flags
mode : mode
add: AT_FDCWD
#define AT_FDCWD -100 /* Special value used to indicate
openat should use the current
working directory. */
long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode)
{
struct open_flags op;
int fd = build_open_flags(flags, mode, &op);//mode-->op
struct filename *tmp;
if (fd)
return fd;
tmp = getname(filename);//path(filename)--->filename
if (IS_ERR(tmp))
return PTR_ERR(tmp);
fd = get_unused_fd_flags(flags);//分配一个可用的fd
if (fd >= 0) {
struct file *f = do_filp_open(dfd, tmp, &op);//dfd,tmp,op==>分配file
if (IS_ERR(f)) {
put_unused_fd(fd);
fd = PTR_ERR(f);
} else {
fsnotify_open(f);//f关联到inode.
fd_install(fd, f);//补冲说明fd和file的关系.
}
}
putname(tmp);
return fd;
}
add: fd_install(fd, f)
-->__fd_install(current->files, fd, file);
/*
* Install a file pointer in the fd array.
*
* The VFS is full of places where we drop the files lock between
* setting the open_fds bitmap and installing the file in the file
* array. At any such point, we are vulnerable to a dup2() race
* installing a file in the array before us. We need to detect this and
* fput() the struct file we are about to overwrite in this case.
*
* It should never happen - if we allow dup2() do it, _really_ bad things
* will follow.
*
* NOTE: __fd_install() variant is really, really low-level; don't
* use it unless you are forced to by truly lousy API shoved down
* your throat. 'files' *MUST* be either current->files or obtained
* by get_files_struct(current) done by whoever had given it to you,
* or really bad things will happen. Normally you want to use
* fd_install() instead.
*/
__fd_install(current->files, fd, file);
void __fd_install(struct files_struct *files, unsigned int fd,
struct file *file)
{
struct fdtable *fdt;
spin_lock(&files->file_lock);
fdt = files_fdtable(files);
BUG_ON(fdt->fd[fd] != NULL);
rcu_assign_pointer(fdt->fd[fd], file);//file关联到fd
spin_unlock(&files->file_lock);
}
STEP4:
open();
do_sys_open(AT_FDCWD, filename, flags, mode);
do_filp_open(dfd, tmp, &op);
-->path_openat(dfd, pathname, &nd, op, flags | LOOKUP_RCU);
-->error = do_last(nd, &path, file, op, &opened, pathname);
-->error = vfs_open(&nd->path, file, current_cred());
-->do_dentry_open(filp, NULL, cred); 注意第二个参数
STEP5: 如何调用到底层的dev->open
static int do_dentry_open(struct file *f,
int (*open)(struct inode *, struct file *),//会回调设备的open
const struct cred *cred)
{
if (!open)
open = f->f_op->open;//设备对应的open
if (open) {
error = open(inode, f);//对应我们写驱动时，
.............
}
我们使用char.c中的chardev_open,查看其参数.现在真相已经很显然，只要我们 f->f_op->open = char_open 就会调用.
static int chrdev_open(struct inode *inode, struct file *filp)
1.设备初始化时，就会绑定fops
cdev->ops = fops;
2.初始化fops
const struct file_operations def_chr_fops = {
.open = chrdev_open,//因此系统调用open就会一直调用到底层的chrdev_open.
.llseek = noop_llseek,
};
STEP:END
1.int open(const char *pathname, int flags, mode_t mode);
2.SYSCALL_DEFINE3(open, const char __user *, filename, int, flags, umode_t, mode)
3.do_sys_open(AT_FDCWD, filename, flags, mode);
4.
do_sys_open(AT_FDCWD, filename, flags, mode);
do_filp_open(dfd, tmp, &op);
-->path_openat(dfd, pathname, &nd, op, flags | LOOKUP_RCU);
-->error = do_last(nd, &path, file, op, &opened, pathname);
-->error = vfs_open(&nd->path, file, current_cred());
-->do_dentry_open(filp, NULL, cred); 注意第二个参数
调用设备的open;
