1 .\" This man page is Copyright (C) 1998 Pawel Krawczyk.
3 .\" %%%LICENSE_START(VERBATIM_ONE_PARA)
4 .\" Permission is granted to distribute possibly modified copies
5 .\" of this page provided the header is included verbatim,
6 .\" and in case of nontrivial modification author and date
7 .\" of the modification is added to the header.
10 .\" $Id: sendfile.2,v 1.5 1999/05/18 11:54:11 freitag Exp $
11 .\" 2000-11-19 bert hubert <ahu@ds9a.nl>: in_fd cannot be socket
14 .\" updated description of in_fd and out_fd for 2.6
15 .\" Various wording and formatting changes
17 .\" 2005-03-31 Martin Pool <mbp@sourcefrog.net> mmap() improvements
19 .TH SENDFILE 2 2017-09-15 "Linux" "Linux Programmer's Manual"
21 sendfile \- transfer data between file descriptors
23 .B #include <sys/sendfile.h>
25 .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
26 offset ", size_t" " count" );
27 .\" The below is too ugly. Comments about glibc versions belong
28 .\" in the notes, not in the header.
30 .\" .B #include <features.h>
32 .\" .B #if (__GLIBC__==2 && __GLIBC_MINOR__>=1) || __GLIBC__>2
34 .\" .B #include <sys/sendfile.h>
38 .\" .B #include <sys/types.h>
40 .\" .B /* No system prototype before glibc 2.1. */
42 .\" .BI "ssize_t sendfile(int" " out_fd" ", int" " in_fd" ", off_t *" \
43 .\" offset ", size_t" " count" )
49 copies data between one file descriptor and another.
50 Because this copying is done within the kernel,
52 is more efficient than the combination of
56 which would require transferring data to and from user space.
59 should be a file descriptor opened for reading and
61 should be a descriptor opened for writing.
65 is not NULL, then it points
66 to a variable holding the file offset from which
68 will start reading data from
72 returns, this variable
73 will be set to the offset of the byte following the last byte that was read.
78 does not modify the file offset of
80 otherwise the file offset is adjusted to reflect
81 the number of bytes read from
86 is NULL, then data will be read from
88 starting at the file offset,
89 and the file offset will be updated by the call.
92 is the number of bytes to copy between the file descriptors.
96 argument must correspond to a file which supports
99 (i.e., it cannot be a socket).
101 In Linux kernels before 2.6.33,
103 must refer to a socket.
104 Since Linux 2.6.33 it can be any file.
105 If it is a regular file, then
107 changes the file offset appropriately.
109 If the transfer was successful, the number of bytes written to
112 Note that a successful call to
114 may write fewer bytes than requested;
115 the caller should be prepared to retry the call if there were unsent bytes.
118 On error, \-1 is returned, and
120 is set appropriately.
124 Nonblocking I/O has been selected using
126 and the write would block.
129 The input file was not opened for reading or the output file
130 was not opened for writing.
136 Descriptor is not valid or locked, or an
138 operation is not available for
149 This is not currently supported by
153 Unspecified error while reading from
157 Insufficient memory to read from
162 is too large, the operation would result in exceeding the maximum size of either
163 the input file or the output file.
167 is not NULL but the input file is not seekable.
170 first appeared in Linux 2.2.
173 is present since glibc 2.1.
175 Not specified in POSIX.1-2001, nor in other standards.
177 Other UNIX systems implement
179 with different semantics and prototypes.
180 It should not be used in portable programs.
183 will transfer at most 0x7ffff000 (2,147,479,552) bytes,
184 returning the number of bytes actually transferred.
185 .\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
186 (This is true on both 32-bit and 64-bit systems.)
190 for sending files to a TCP socket, but need
191 to send some header data in front of the file contents, you will find
192 it useful to employ the
196 to minimize the number of packets and to tune performance.
198 In Linux 2.4 and earlier,
200 could also refer to a regular file;
201 this possibility went away in the Linux 2.6.x kernel series,
202 but was restored in Linux 2.6.33.
206 system call was not designed to handle large file offsets.
207 Consequently, Linux 2.4 added
209 with a wider type for the
214 wrapper function transparently deals with the kernel differences.
216 Applications may wish to fall back to
217 .BR read (2)/ write (2)
227 refers to a socket or pipe with zero-copy support, callers must ensure the
228 transferred portions of the file referred to by
230 remain unmodified until the reader on the other end of
232 has consumed the transferred data.
236 call supports transferring data between arbitrary file descriptors
237 provided one (or both) of them is a pipe.
239 .BR copy_file_range (2),