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. Because this copying
is done within the kernel, sendfile() is more efficient 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.
Presently (Linux 2.6.9): in_fd, must correspond to a
file which supports mmap(2)-like operations
(i.e., it cannot be a socket); and out_fd must refer to a
socket.
Applications may wish to fall back to read(2)/write(2) in the case where
sendfile() fails with
EINVAL or ENOSYS.
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.
Nonblocking I/O has been selected using O_NONBLOCK and the write would
block.
The input file was not opened for reading or the output file was not opened for writing.
Bad address.
Descriptor is not valid or locked, or an mmap(2)-like
operation is not available for in_fd.
Unspecified error while reading from in_fd.
Insufficient memory to read from in_fd.
sendfile() is a new feature
in Linux 2.2. The include file <sys/sendfile.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 refer to a regular
file, and sendfile() changed
the current offset of that file.
This page is part of release 3.24 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/.
|
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 <ahuds9a.nl>: in_fd cannot be socket 2004-12-17, mtk updated description of in_fd and out_fd for 2.6 Various wording and formatting changes 2005-03-31 Martin Pool <mbpsourcefrog.net> mmap() improvements |