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 2004-12-17 "Linux Man Page" "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.
60 is a pointer to a variable holding the file offset from
63 will start reading data from
67 returns, this variable
68 will be set to the offset of the byte following the last byte that was read.
70 does not modify the current file offset of
74 is the number of bytes to copy between the file descriptors.
76 Presently (Linux 2.6.9):
78 must correspond to a file which supports mmap()-like operations
79 (i.e., it cannot be a socket);
82 must refer to a socket.
84 Applications may wish to fall back to
85 .BR read (2)/ write (2)
88 fails with EINVAL or ENOSYS.
90 If you plan to use sendfile for sending files to a TCP socket, but need
91 to send some header data in front of the file contents, you will find
92 it useful to employ the
96 to minimize the number of packets and to tune performance.
98 In Linux 2.4 and earlier,
100 could refer to a regular file, and
102 changed the current offset of that file.
104 If the transfer was successful, the number of bytes written to
106 is returned. On error, \-1 is returned, and
108 is set appropriately.
112 Non-blocking I/O has been selected using
114 and the write would block.
117 The input file was not opened for reading or the output file
118 was not opened for writing.
124 Descriptor is not valid or locked, or an mmap()-like
125 operation is not available for
129 Unspecified error while reading from
133 Insufficient memory to read from
137 is a new feature in Linux 2.2.
138 The include file <sys/sendfile.h> is present since glibc2.1.
140 Other Unixes implement
142 with different semantics and prototypes. It should
143 not be used in portable programs.