# splice

## NAME



           splice - splice data to/from a pipe
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#define _FILE_OFFSET_BITS 64**
           **#include <fcntl.h>**
    
           **ssize_t splice(int** _fd_in_**, off_t *_Nullable** _off_in_**,**
                          **int** _fd_out_**, off_t *_Nullable** _off_out_**,**
                          **size_t** _size_**, 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 _size_ 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.
    
           •  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, and the offset pointed
              to by _off_in_ is adjusted appropriately instead.
    
           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:
    
           **SPLICE_F_MOVE**
                  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.
    
           **SPLICE_F_NONBLOCK**
                  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).
    
           **SPLICE_F_MORE**
                  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)).
    
           **SPLICE_F_GIFT**
                  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 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.
    


## ERRORS



           **EAGAIN SPLICE_F_NONBLOCK** was specified in _flags_ or one of the file
                  descriptors had been marked as nonblocking (**O_NONBLOCK**)**,**
                  and the operation would block.
    
           **EBADF** One or both file descriptors are not valid, or do not have
                  proper read-write mode.
    
           **EINVAL** The target filesystem doesn't support splicing.
    
           **EINVAL** The target file is opened in append mode.
    
           **EINVAL** Neither of the file descriptors refers to a pipe.
    
           **EINVAL** An offset was given for nonseekable device (e.g., a pipe).
    
           **EINVAL** _fd_in_ and _fd_out_ refer to the same pipe.
    
           **ENOMEM** Out of memory.
    
           **ESPIPE** Either _off_in_ or _off_out_ was not NULL, but the
                  corresponding file descriptor refers to a pipe.
    


## STANDARDS



           Linux.
    


## 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.
    
           **_FILE_OFFSET_BITS** should be defined to be 64 in code that uses
           non-null _off_in_ or _off_out_ or that takes the address of **splice** , if
           the code is intended to be portable to traditional 32-bit x86 and
           ARM platforms where **off_t** 's width defaults to 32 bits.
    


## EXAMPLES



           See tee(2) for another example.
    
           #define _GNU_SOURCE
           #define _FILE_OFFSET_BITS 64
           #include <err.h>
           #include <fcntl.h>
           #include <stdint.h>
           #include <stdio.h>
           #include <stdlib.h>
           #include <sys/types.h>
           #include <unistd.h>
    
           int
           main(void)
           {
                   int         fd;
                   int         pfd[2];
                   off_t       off;
                   const char  s[12] = "Hello, world";
    
                   fd = open("out", O_WRONLY | O_CREAT | O_EXCL, 0666);
                   if (fd == -1)
                           err(EXIT_FAILURE, "open");
    
                   if (pipe(pfd) == -1)
                           err(EXIT_FAILURE, "pipe");
    
                   if (write(pfd[1], s, sizeof(s)) != sizeof(s))
                           err(EXIT_FAILURE, "write");
                   if (close(pfd[1]) == -1)
                           err(EXIT_FAILURE, "close");
    
                   off = 10;
                   if (splice(pfd[0], NULL, fd, &off, sizeof(s), 0) != sizeof(s))
                           err(EXIT_FAILURE, "splice");
                   if (close(pfd[0]) == -1)
                           err(EXIT_FAILURE, "close");
    
                   printf("New offset is %jd\n", (intmax_t) off);
    
                   if (close(fd) == -1)
                           err(EXIT_FAILURE, "close");
                   exit(EXIT_SUCCESS);
           }
    


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