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 copy_splice_read(struct file *in, loff_t *ppos, struct pipe_inode_info *pipe, size_t len, unsigned int flags)¶: Copy data from a file and splice the copy into a pipe

Parameters

struct file *in: The file to read from
loff_t *ppos: Pointer to the file position to read from
struct pipe_inode_info *pipe: The pipe to splice into
size_t len: The amount to splice
unsigned int flags: The SPLICE_F_* flags

Description

This function allocates a bunch of pages sufficient to hold the requested amount of data (but limited by the remaining pipe capacity), passes it to the file's ->read_iter() to read into and then splices the used pages into the pipe.

Return

On success, the number of bytes read will be returned and *ppos will be updated if appropriate; 0 will be returned if there is no more data to be read; -EAGAIN will be returned if the pipe had no space, and some other negative error code will be returned on error. A short read may occur if the pipe has insufficient space, we reach the end of the data or we hit a hole.

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_sendmsg, 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 splice_to_socket(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.

long vfs_splice_read(struct file *in, loff_t *ppos, struct pipe_inode_info *pipe, size_t len, unsigned int flags)¶: Read data from a file and splice it into a pipe

Parameters

struct file *in: File to splice from
loff_t *ppos: Input file offset
struct pipe_inode_info *pipe: Pipe to splice to
size_t len: Number of bytes to splice
unsigned int flags: Splice modifier flags (SPLICE_F_*)

Description

Splice the requested amount of data from the input file to the pipe. This is synchronous as the caller must hold the pipe lock across the entire operation.

If successful, it returns the amount of data spliced, 0 if it hit the EOF or a hole and a negative error code otherwise.

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;
#ifdef CONFIG_WATCH_QUEUE;
    bool note_loss;
#endif;
    unsigned int nr_accounted;
    unsigned int readers;
    unsigned int writers;
    unsigned int files;
    unsigned int r_counter;
    unsigned int w_counter;
    bool poll_usage;
    struct page *tmp_page;
    struct fasync_struct *fasync_readers;
    struct fasync_struct *fasync_writers;
    struct pipe_buffer *bufs;
    struct user_struct *user;
#ifdef CONFIG_WATCH_QUEUE;
    struct watch_queue *watch_queue;
#endif;
};

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)
note_loss: The next read() should insert a data-lost message
nr_accounted: The amount this pipe accounts for in user->pipe_bufs
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
poll_usage: is this pipe used for epoll, which has crazy wakeups?
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
watch_queue: If this pipe is a watch_queue, this is the stuff for that

bool pipe_has_watch_queue(const struct pipe_inode_info *pipe)¶: Check whether the pipe is a watch_queue, i.e. it was created with O_NOTIFICATION_PIPE

Parameters

const struct pipe_inode_info *pipe: The pipe to check

Return

true if pipe is a watch queue, false otherwise.

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.

struct pipe_buffer *pipe_buf(const struct pipe_inode_info *pipe, unsigned int slot)¶: Return the pipe buffer for the specified slot in the pipe ring

Parameters

const struct pipe_inode_info *pipe: The pipe to access
unsigned int slot: The slot of interest

struct pipe_buffer *pipe_head_buf(const struct pipe_inode_info *pipe)¶: Return the pipe buffer at the head of the pipe ring

Parameters

const struct pipe_inode_info *pipe: The pipe to access

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

bool pipe_buf_try_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

bool generic_pipe_buf_try_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 the tee() system call, when we duplicate the buffers in one pipe into another.

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.

The Linux Kernel

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splice and pipes¶

splice API¶

pipes API¶