.\" This man page is Copyright (C) 1998 Pawel Krawczyk.
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
.\" $Id: sendfile.2,v 1.5 1999/05/18 11:54:11 freitag Exp $
.\" 2000-11-19 bert hubert <ahu@ds9a.nl>: in_fd cannot be socket
.TH SENDFILE 2 1998-12-01 "Linux Man Page" "Linux Programmer's Manual" 
.SH NAME
sendfile \- transfer data between file descriptors
.SH SYNOPSIS
.B #include <sys/sendfile.h>
.sp
.BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" offset ", size_t" " count" );
.\" The below is too ugly. Comments about glibc versions belong
.\" in the notes, not in the header.
.\"
.\" .B #include <features.h>
.\" .br
.\" .B #if (__GLIBC__==2 && __GLIBC_MINOR__>=1) || __GLIBC__>2 
.\" .br
.\" .B #include <sys/sendfile.h>
.\" .br
.\" #else
.\" .br
.\" .B #include <sys/types.h>
.\" .br
.\" .B /* No system prototype before glibc 2.1. */ 
.\" .br
.\" .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" offset ", size_t" " count" )
.\" .br
.\" .B #endif
.\"
.SH DESCRIPTION
This call copies data between one file descriptor and another.
Either or both of these file descriptors may refer to a socket (but see below).
.I in_fd
should be a file descriptor opened for reading and
.I out_fd
should be a descriptor opened for writing.
.I offset
is a pointer to a variable holding the input file pointer position from
which
.BR sendfile ()
will start reading data.  When 
.BR sendfile ()
returns, this variable
will be set to the offset of the byte following the last byte that was read.
.I count
is the number of bytes to copy between file descriptors.

Because this copying is done within the kernel,
.BR sendfile ()
does not need to spend time transferring data
to and from user space.

.SH NOTES
Sendfile does not modify the current file pointer of 
.IR in_fd ,
but does for
.IR out_fd .

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, please see
the 
.B TCP_CORK 
option in
.BR tcp (7)
to minimize the number of packets and to tune performance.

Presently the descriptor from which data is read cannot
correspond to a socket, it must correspond to a file
which supports mmap()-like operations.

.SH "RETURN VALUE"
If the transfer was successful, the number of bytes written to
.I out_fd
is returned.  On error, \-1 is returned, and
.I errno
is set appropriately.

.SH ERRORS
.TP
.B EAGAIN
Non-blocking I/O has been selected using
.B O_NONBLOCK
and the write would block.
.TP
.B EBADF
The input file was not opened for reading or the output file
was not opened for writing.
.TP
.B EFAULT
Bad address.
.TP
.B EINVAL
Descriptor is not valid or locked.
.TP
.B EIO
Unspecified error while reading from
.IR in_fd .
.TP
.B ENOMEM
Insufficient memory to read from
.IR in_fd .
.SH VERSIONS
.B sendfile
is a new feature in Linux 2.2.
The include file <sys/sendfile.h> is present since glibc2.1.

Other Unixes often implement 
.B sendfile 
with different semantics and prototypes. It should
not be used in portable programs.
.SH "SEE ALSO"
.BR open (2),
.BR socket (2)