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SENDFILE(2)		   Linux Programmer's Manual		   SENDFILE(2)

       sendfile	- transfer data	between	file descriptors

       #include	<sys/sendfile.h>

       ssize_t sendfile(int out_fd, int	in_fd, off_t *offset, size_t count);

       sendfile()  copies  data	 between one file descriptor and another.  Be-
       cause this copying is done within the kernel, sendfile()	is more	 effi-
       cient than the combination of read(2) and write(2), which would require
       transferring data to and	from user space.

       in_fd should be a file descriptor opened	for reading and	out_fd	should
       be a descriptor opened for writing.

       If  offset  is  not NULL, then it points	to a variable holding the file
       offset from which sendfile() will start reading data from in_fd.	  When
       sendfile() returns, this	variable will be set to	the offset of the byte
       following the last byte that was	read.  If offset  is  not  NULL,  then
       sendfile()  does	not modify the current file offset of in_fd; otherwise
       the current file	offset is adjusted to reflect the number of bytes read
       from in_fd.

       If  offset  is  NULL, then data will be read from in_fd starting	at the
       current file offset, and	the file offset	will be	updated	by the call.

       count is	the number of bytes to copy between the	file descriptors.

       The  in_fd  argument  must  correspond  to  a   file   which   supports
       mmap(2)-like operations (i.e., it cannot	be a socket).

       In  Linux  kernels before 2.6.33, out_fd	must refer to a	socket.	 Since
       Linux 2.6.33 it can be any file.	 If it is a regular file,  then	 send-
       file() changes the file offset appropriately.

       If  the	transfer was successful, the number of bytes written to	out_fd
       is returned.  On	error, -1 is returned, and errno is set	appropriately.

       EAGAIN Nonblocking I/O has been selected	using O_NONBLOCK and the write
	      would block.

       EBADF  The input	file was not opened for	reading	or the output file was
	      not opened for writing.

       EFAULT Bad address.

       EINVAL Descriptor is not	valid or locked, or an mmap(2)-like  operation
	      is not available for in_fd.

       EIO    Unspecified error	while reading from in_fd.

       ENOMEM Insufficient memory to read from in_fd.

       sendfile()  is a	new feature in Linux 2.2.  The include file _sys/send-
       file.h_ is present since	glibc 2.1.

       Not specified in	POSIX.1-2001, or other standards.

       Other UNIX systems implement sendfile() with  different	semantics  and
       prototypes.  It should not be used in portable programs.

       If  you	plan  to use sendfile()	for sending files to a TCP socket, but
       need to send some header	data in	front of the file contents,  you  will
       find  it	 useful	to employ the TCP_CORK option, described in tcp(7), to
       minimize	the number of packets and to tune performance.

       In Linux	2.4 and	earlier, out_fd	could also refer to  a	regular	 file,
       and sendfile() changed the current offset of that file.

       The  original  Linux  sendfile()	system call was	not designed to	handle
       large file offsets.  Consequently, Linux	2.4 added sendfile64(),	with a
       wider type for the offset argument.  The	glibc sendfile() wrapper func-
       tion transparently deals	with the kernel	differences.

       Applications may	wish to	fall back  to  read(2)/write(2)	 in  the  case
       where sendfile()	fails with EINVAL or ENOSYS.

       The  Linux-specific  splice(2)  call supports transferring data between
       arbitrary files (e.g., a	pair of	sockets).

       mmap(2),	open(2), socket(2), splice(2)

       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at

Linux				  2011-09-14			   SENDFILE(2)


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