1 .\" This man page is Copyright (C) 1998 Pawel Krawczyk.
2 .\" Permission is granted to distribute possibly modified copies
3 .\" of this page provided the header is included verbatim,
4 .\" and in case of nontrivial modification author and date
5 .\" of the modification is added to the header.
6 .\" $Id: sendfile.2,v 1.5 1999/05/18 11:54:11 freitag Exp $
7 .\" 2000-11-19 bert hubert <ahu@ds9a.nl>: in_fd cannot be socket
10 .\" updated description of in_fd and out_fd for 2.6
11 .\" Various wording and formatting changes
13 .\" 2005-03-31 Martin Pool <mbp@sourcefrog.net> mmap() improvements
15 .TH SENDFILE 2 2011-09-14 "Linux" "Linux Programmer's Manual"
17 sendfile \- transfer data between file descriptors
19 .B #include <sys/sendfile.h>
21 .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
22 offset ", size_t" " count" );
23 .\" The below is too ugly. Comments about glibc versions belong
24 .\" in the notes, not in the header.
26 .\" .B #include <features.h>
28 .\" .B #if (__GLIBC__==2 && __GLIBC_MINOR__>=1) || __GLIBC__>2
30 .\" .B #include <sys/sendfile.h>
34 .\" .B #include <sys/types.h>
36 .\" .B /* No system prototype before glibc 2.1. */
38 .\" .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
39 .\" offset ", size_t" " count" )
45 copies data between one file descriptor and another.
46 Because this copying is done within the kernel,
48 is more efficient than the combination of
52 which would require transferring data to and from user space.
55 should be a file descriptor opened for reading and
57 should be a descriptor opened for writing.
61 is not NULL, then it points
62 to a variable holding the file offset from which
64 will start reading data from
68 returns, this variable
69 will be set to the offset of the byte following the last byte that was read.
74 does not modify the current file offset of
76 otherwise the current file offset is adjusted to reflect
77 the number of bytes read from
82 is NULL, then data will be read from
84 starting at the current file offset,
85 and the file offset will be updated by the call.
88 is the number of bytes to copy between the file descriptors.
92 argument must correspond to a file which supports
95 (i.e., it cannot be a socket).
97 In Linux kernels before 2.6.33,
99 must refer to a socket.
100 Since Linux 2.6.33 it can be any file.
101 If it is a regular file, then
103 changes the file offset appropriately.
105 If the transfer was successful, the number of bytes written to
108 On error, \-1 is returned, and
110 is set appropriately.
114 Nonblocking I/O has been selected using
116 and the write would block.
119 The input file was not opened for reading or the output file
120 was not opened for writing.
126 Descriptor is not valid or locked, or an
128 operation is not available for
132 Unspecified error while reading from
136 Insufficient memory to read from
140 is a new feature in Linux 2.2.
143 is present since glibc 2.1.
145 Not specified in POSIX.1-2001, or other standards.
147 Other UNIX systems implement
149 with different semantics and prototypes.
150 It should not be used in portable programs.
154 for sending files to a TCP socket, but need
155 to send some header data in front of the file contents, you will find
156 it useful to employ the
160 to minimize the number of packets and to tune performance.
162 In Linux 2.4 and earlier,
164 could also refer to a regular file, and
166 changed the current offset of that file.
170 system call was not designed to handle large file offsets.
171 Consequently, Linux 2.4 added
173 with a wider type for the
178 wrapper function transparently deals with the kernel differences.
180 Applications may wish to fall back to
181 .BR read (2)/ write (2)
191 call supports transferring data between arbitrary files
192 (e.g., a pair of sockets).