splice and pipes¶

splice API¶

splice is a method for moving blocks of data around inside the kernel, without continually transferring them between the kernel and user space.

ssize_t splice_to_pipe(struct pipe_inode_info * pipe, struct splice_pipe_desc * spd)¶: fill passed data into a pipe

Parameters

struct pipe_inode_info * pipe: pipe to fill
struct splice_pipe_desc * spd: data to fill

Description

spd contains a map of pages and len/offset tuples, along with the struct pipe_buf_operations associated with these pages. This function will link that data to the pipe.

ssize_t generic_file_splice_read(struct file * in, loff_t * ppos, struct pipe_inode_info * pipe, size_t len, unsigned int flags)¶: splice data from file to a pipe

Parameters

struct file * in: file to splice from
loff_t * ppos: position in in
struct pipe_inode_info * pipe: pipe to splice to
size_t len: number of bytes to splice
unsigned int flags: splice modifier flags

Description

Will read pages from given file and fill them into a pipe. Can be used as long as it has more or less sane ->read_iter().

int splice_from_pipe_feed(struct pipe_inode_info * pipe, struct splice_desc * sd, splice_actor * actor)¶: feed available data from a pipe to a file

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct splice_desc * sd: information to actor
splice_actor * actor: handler that splices the data

Description

This function loops over the pipe and calls actor to do the actual moving of a single struct pipe_buffer to the desired destination. It returns when there’s no more buffers left in the pipe or if the requested number of bytes (sd->total_len) have been copied. It returns a positive number (one) if the pipe needs to be filled with more data, zero if the required number of bytes have been copied and -errno on error.

This, together with splice_from_pipe_{begin,end,next}, may be used to implement the functionality of __splice_from_pipe() when locking is required around copying the pipe buffers to the destination.

int splice_from_pipe_next(struct pipe_inode_info * pipe, struct splice_desc * sd)¶: wait for some data to splice from

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct splice_desc * sd: information about the splice operation

Description

This function will wait for some data and return a positive value (one) if pipe buffers are available. It will return zero or -errno if no more data needs to be spliced.

void splice_from_pipe_begin(struct splice_desc * sd)¶: start splicing from pipe

Parameters

struct splice_desc * sd: information about the splice operation

Description

This function should be called before a loop containing splice_from_pipe_next() and splice_from_pipe_feed() to initialize the necessary fields of sd.

void splice_from_pipe_end(struct pipe_inode_info * pipe, struct splice_desc * sd)¶: finish splicing from pipe

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct splice_desc * sd: information about the splice operation

Description

This function will wake up pipe writers if necessary. It should be called after a loop containing splice_from_pipe_next() and splice_from_pipe_feed().

ssize_t __splice_from_pipe(struct pipe_inode_info * pipe, struct splice_desc * sd, splice_actor * actor)¶: splice data from a pipe to given actor

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct splice_desc * sd: information to actor
splice_actor * actor: handler that splices the data

Description

This function does little more than loop over the pipe and call actor to do the actual moving of a single struct pipe_buffer to the desired destination. See pipe_to_file, pipe_to_sendpage, or pipe_to_user.

ssize_t splice_from_pipe(struct pipe_inode_info * pipe, struct file * out, loff_t * ppos, size_t len, unsigned int flags, splice_actor * actor)¶: splice data from a pipe to a file

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct file * out: file to splice to
loff_t * ppos: position in out
size_t len: how many bytes to splice
unsigned int flags: splice modifier flags
splice_actor * actor: handler that splices the data

Description

See __splice_from_pipe. This function locks the pipe inode, otherwise it’s identical to __splice_from_pipe().

ssize_t iter_file_splice_write(struct pipe_inode_info * pipe, struct file * out, loff_t * ppos, size_t len, unsigned int flags)¶: splice data from a pipe to a file

Parameters

struct pipe_inode_info * pipe: pipe info
struct file * out: file to write to
loff_t * ppos: position in out
size_t len: number of bytes to splice
unsigned int flags: splice modifier flags

Description

Will either move or copy pages (determined by flags options) from the given pipe inode to the given file. This one is ->write_iter-based.

ssize_t generic_splice_sendpage(struct pipe_inode_info * pipe, struct file * out, loff_t * ppos, size_t len, unsigned int flags)¶: splice data from a pipe to a socket

Parameters

struct pipe_inode_info * pipe: pipe to splice from
struct file * out: socket to write to
loff_t * ppos: position in out
size_t len: number of bytes to splice
unsigned int flags: splice modifier flags

Description

Will send len bytes from the pipe to a network socket. No data copying is involved.

ssize_t splice_direct_to_actor(struct file * in, struct splice_desc * sd, splice_direct_actor * actor)¶: splices data directly between two non-pipes

Parameters

struct file * in: file to splice from
struct splice_desc * sd: actor information on where to splice to
splice_direct_actor * actor: handles the data splicing

Description

This is a special case helper to splice directly between two points, without requiring an explicit pipe. Internally an allocated pipe is cached in the process, and reused during the lifetime of that process.

long do_splice_direct(struct file * in, loff_t * ppos, struct file * out, loff_t * opos, size_t len, unsigned int flags)¶: splices data directly between two files

Parameters

struct file * in: file to splice from
loff_t * ppos: input file offset
struct file * out: file to splice to
loff_t * opos: output file offset
size_t len: number of bytes to splice
unsigned int flags: splice modifier flags

Description

For use by do_sendfile(). splice can easily emulate sendfile, but doing it in the application would incur an extra system call (splice in + splice out, as compared to just sendfile()). So this helper can splice directly through a process-private pipe.

pipes API¶

Pipe interfaces are all for in-kernel (builtin image) use. They are not exported for use by modules.

struct pipe_buffer¶: a linux kernel pipe buffer

Definition

struct pipe_buffer {
  struct page *page;
  unsigned int offset, len;
  const struct pipe_buf_operations *ops;
  unsigned int flags;
  unsigned long private;
};

Members

page: the page containing the data for the pipe buffer
offset: offset of data inside the page
len: length of data inside the page
ops: operations associated with this buffer. See pipe_buf_operations.
flags: pipe buffer flags. See above.
private: private data owned by the ops.

struct pipe_inode_info¶: a linux kernel pipe

Definition

struct pipe_inode_info {
  struct mutex mutex;
  wait_queue_head_t wait;
  unsigned int nrbufs, curbuf, buffers;
  unsigned int readers;
  unsigned int writers;
  unsigned int files;
  unsigned int waiting_writers;
  unsigned int r_counter;
  unsigned int w_counter;
  struct page *tmp_page;
  struct fasync_struct *fasync_readers;
  struct fasync_struct *fasync_writers;
  struct pipe_buffer *bufs;
  struct user_struct *user;
};

Members

mutex: mutex protecting the whole thing
wait: reader/writer wait point in case of empty/full pipe
nrbufs: the number of non-empty pipe buffers in this pipe
curbuf: the current pipe buffer entry
buffers: total number of buffers (should be a power of 2)
readers: number of current readers of this pipe
writers: number of current writers of this pipe
files: number of struct file referring this pipe (protected by ->i_lock)
waiting_writers: number of writers blocked waiting for room
r_counter: reader counter
w_counter: writer counter
tmp_page: cached released page
fasync_readers: reader side fasync
fasync_writers: writer side fasync
bufs: the circular array of pipe buffers
user: the user who created this pipe

bool pipe_buf_get(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: get a reference to a pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to get a reference to

Return

true if the reference was successfully obtained.

void pipe_buf_release(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: put a reference to a pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to put a reference to

int pipe_buf_confirm(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: verify contents of the pipe buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to confirm

int pipe_buf_steal(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: attempt to take ownership of a pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to attempt to steal

int generic_pipe_buf_steal(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: attempt to take ownership of a pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to attempt to steal

Description

This function attempts to steal the struct page attached to buf. If successful, this function returns 0 and returns with the page locked. The caller may then reuse the page for whatever he wishes; the typical use is insertion into a different file page cache.

bool generic_pipe_buf_get(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: get a reference to a struct pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to get a reference to

Description

This function grabs an extra reference to buf. It’s used in in the tee() system call, when we duplicate the buffers in one pipe into another.

int generic_pipe_buf_confirm(struct pipe_inode_info * info, struct pipe_buffer * buf)¶: verify contents of the pipe buffer

Parameters

struct pipe_inode_info * info: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to confirm

Description

This function does nothing, because the generic pipe code uses pages that are always good when inserted into the pipe.

void generic_pipe_buf_release(struct pipe_inode_info * pipe, struct pipe_buffer * buf)¶: put a reference to a struct pipe_buffer

Parameters

struct pipe_inode_info * pipe: the pipe that the buffer belongs to
struct pipe_buffer * buf: the buffer to put a reference to

Description

This function releases a reference to buf.

void pipe_buf_mark_unmergeable(struct pipe_buffer * buf)¶: mark a struct pipe_buffer as unmergeable

Parameters

struct pipe_buffer * buf: the buffer to mark

Description

This function ensures that no future writes will be merged into the given struct pipe_buffer. This is necessary when multiple pipe buffers share the same backing page.