NAME
splice - splice data to/from a pipe SYNOPSIS
#define GNUSOURCE /* See featuretestmacros(7) */
#include
ssizet splice(int fdin, lofft *offin, int fdout, lofft *offout, sizet len, unsigned int flags); DESCRIPTION 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 fdin to the file descriptor fdout, where one of the descriptors must refer to a pipe. If fdin refers to a pipe, then offin must be NULL. If fdin does not refer to a pipe and offin is NULL, then bytes are read from fdin starting from the current file offset, and the current file offset is adjusted appropriately. If fdin does not refer to a pipe and offin is not NULL, then offin must point to a buffer which specifies the starting offset from which bytes will be read from fdin; in this case, the current file offset of fdin is not changed. Analogous statements apply for fdout and offout. The flags argument is a bit mask that is composed by ORing together zero or more of the following values: SPLICEFMOVE 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. SPLICEFNONBLOCK Do not block on I/O. This makes the splice pipe operations nonblocking, but splice() may neverthe‐ less block because the file descriptors that are spliced to/from may block (unless they have the ONONBLOCK flag set). SPLICEFMORE More data will be coming in a subsequent splice. This is a helpful hint when the fdout refers to a socket (see also the description of MSGMORE in send(2), and the description of TCPCORK in tcp(7)) SPLICEFGIFT Unused for splice(); see vmsplice(2). RETURN VALUE Upon successful completion, splice() returns the number of bytes spliced to or from the pipe. A return value of 0 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 referred to by fdin.
On error, splice() returns -1 and errno is set to indicate the error. ERRORS EBADF One or both file descriptors are not valid, or do not have
proper read-write mode. EINVAL Target file system doesn't support splicing; target file is opened in append mode; neither of the descriptors refers to a pipe; or offset given for nonseekable device. ENOMEM Out of memory. ESPIPE Either offin or offout was not NULL, but the corresponding file descriptor refers to a pipe. VERSIONS The splice() system call first appeared in Linux 2.6.17; library sup‐ port was added to glibc in version 2.5. CONFORMING TO
This system call is Linux-specific. NOTES
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: splice() moves data from the buffer to an arbitrary file descriptor, or vice versa, or from one buffer to another. tee(2) "copies" the data from one buffer to another. vmsplice(2) "copies" data from user space into the buffer. 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. EXAMPLE See tee(2). SEE ALSO sendfile(2), tee(2), vmsplice(2) COLOPHON
This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.
Linux 2012-05-04 SPLICE(2)