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 rd_wait, wr_wait;
  unsigned int head;
  unsigned int tail;
  unsigned int max_usage;
  unsigned int ring_size;
  unsigned int readers;
  unsigned int writers;
  unsigned int files;
  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
rd_wait
reader wait point in case of empty pipe
wr_wait
writer wait point in case of full pipe
head
The point of buffer production
tail
The point of buffer consumption
max_usage
The maximum number of slots that may be used in the ring
ring_size
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)
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_empty(unsigned int head, unsigned int tail)

Return true if the pipe is empty

Parameters

unsigned int head
The pipe ring head pointer
unsigned int tail
The pipe ring tail pointer
unsigned int pipe_occupancy(unsigned int head, unsigned int tail)

Return number of slots used in the pipe

Parameters

unsigned int head
The pipe ring head pointer
unsigned int tail
The pipe ring tail pointer
bool pipe_full(unsigned int head, unsigned int tail, unsigned int limit)

Return true if the pipe is full

Parameters

unsigned int head
The pipe ring head pointer
unsigned int tail
The pipe ring tail pointer
unsigned int limit
The maximum amount of slots available.
unsigned int pipe_space_for_user(unsigned int head, unsigned int tail, struct pipe_inode_info * pipe)

Return number of slots available to userspace

Parameters

unsigned int head
The pipe ring head pointer
unsigned int tail
The pipe ring tail pointer
struct pipe_inode_info * pipe
The pipe info structure
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.