splice - splice data to/from a pipe
#define _GNU_SOURCE /* See feature_test_macros(7) */
ssize_t splice(int fd_in, loff_t *off_in, int fd_out,
loff_t *off_out, size_t len, unsigned int flags);
splice() moves data between two file descriptors without copying between kernel address space and user address space. It transfers up to len bytes of data from the file descriptor fd_in to the file descriptor fd_out, where one of the file descriptors must refer to a pipe.
The following semantics apply for fd_in and off_in:
- If fd_in refers to a pipe, then off_in must be NULL.
- If fd_in does not refer to a pipe and off_in is NULL, then bytes are read from fd_in starting from the file offset, and the file offset is adjusted appropriately.
Analogous statements apply for fd_out and off_out.
The flags argument is a bit mask that is composed by ORing together zero or more of the following values:
- If fd_in does not refer to a pipe and off_in is not NULL, then off_in must point to a buffer which specifies the starting offset from which bytes will be read from fd_in; in this case, the file offset of fd_in is not changed.
- Attempt to move pages instead of copying. This is only a hint to the kernel: pages may still be copied if the kernel cannot move the pages from the pipe, or if the pipe buffers don't refer to full pages. The initial implementation of this flag was buggy: therefore starting in Linux 2.6.21 it is a no-op (but is still permitted in a splice() call); in the future, a correct implementation may be restored.
- Do not block on I/O. This makes the splice pipe operations nonblocking, but splice() may nevertheless block because the file descriptors that are spliced to/from may block (unless they have the O_NONBLOCK flag set).
- More data will be coming in a subsequent splice. This is a helpful hint when the fd_out refers to a socket (see also the description of MSG_MORE in send(2), and the description of TCP_CORK in tcp(7)).
Upon successful completion, splice() returns the number of bytes spliced to or from the pipe.
A return value of 0 means end of input. If fd_in refers to a pipe, then this means that there was no data to transfer, and it would not make sense to block because there are no writers connected to the write end of the pipe.
On error, splice() returns -1 and errno is set to indicate the error.
- Unused for splice(); see vmsplice(2).
- SPLICE_F_NONBLOCK was specified in flags, and the operation would block.
- One or both file descriptors are not valid, or do not have proper read-write mode.
- The target filesystem doesn't support splicing.
- The target file is opened in append mode.
- Neither of the file descriptors refers to a pipe.
- An offset was given for nonseekable device (e.g., a pipe).
- fd_in and fd_out refer to the same pipe.
- Out of memory.
The splice() system call first appeared in Linux 2.6.17; library support was added to glibc in version 2.5.
This system call is Linux-specific.
The three system calls splice(), vmsplice(2), and tee(2), provide user-space programs with full control over an arbitrary kernel buffer, implemented within the kernel using the same type of buffer that is used for a pipe. In overview, these system calls perform the following tasks:
- Either off_in or off_out was not NULL, but the corresponding file descriptor refers to a pipe.
- moves data from the buffer to an arbitrary file descriptor, or vice versa, or from one buffer to another.
- "copies" the data from one buffer to another.
Though we talk of copying, actual copies are generally avoided. The kernel does this by implementing a pipe buffer as a set of reference-counted pointers to pages of kernel memory. The kernel creates "copies" of pages in a buffer by creating new pointers (for the output buffer) referring to the pages, and increasing the reference counts for the pages: only pointers are copied, not the pages of the buffer.
In Linux 2.6.30 and earlier, exactly one of fd_in and fd_out was required to be a pipe. Since Linux 2.6.31, both arguments may refer to pipes.
- "copies" data from user space into the buffer.