SENDFILE
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
sendfile - transfer data between file descriptors
SYNOPSIS
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
DESCRIPTION
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 file offset of
in_fd;
otherwise the 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 file offset,
and the file offset will be updated by the call.
count
is the number of bytes to copy between the file descriptors.
The
in_fd
argument must correspond to a file which supports
mmap(2)-like
operations
(i.e., it cannot be a socket).
In Linux kernels before 2.6.33,
out_fd
must refer to a socket.
Since Linux 2.6.33 it can be any file.
If it is a regular file, then
sendfile()
changes the file offset appropriately.
RETURN VALUE
If the transfer was successful, the number of bytes written to
out_fd
is returned.
Note that a successful call to
sendfile()
may write fewer bytes than requested;
the caller should be prepared to retry the call if there were unsent bytes.
See also NOTES.
On error, -1 is returned, and
errno
is set appropriately.
ERRORS
- EAGAIN
-
Nonblocking I/O has been selected using
O_NONBLOCK
and the write would block.
- EBADF
-
The input file was not opened for reading or the output file
was not opened for writing.
- EFAULT
-
Bad address.
- EINVAL
-
Descriptor is not valid or locked, or an
mmap(2)-like
operation is not available for
in_fd,
or
count
is negative.
- EINVAL
-
out_fd
has the
O_APPEND
flag set.
This is not currently supported by
sendfile().
- EIO
-
Unspecified error while reading from
in_fd.
- ENOMEM
-
Insufficient memory to read from
in_fd.
- EOVERFLOW
-
count
is too large, the operation would result in exceeding the maximum size of either
the input file or the output file.
- ESPIPE
-
offset
is not NULL but the input file is not
seek(2)-able.
VERSIONS
sendfile()
first appeared in Linux 2.2.
The include file
<sys/sendfile.h>
is present since glibc 2.1.
CONFORMING TO
Not specified in POSIX.1-2001, nor in other standards.
Other UNIX systems implement
sendfile()
with different semantics and prototypes.
It should not be used in portable programs.
NOTES
sendfile()
will transfer at most 0x7ffff000 (2,147,479,552) bytes,
returning the number of bytes actually transferred.
(This is true on both 32-bit and 64-bit systems.)
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 also refer to a regular file;
this possibility went away in the Linux 2.6.x kernel series,
but was restored in Linux 2.6.33.
The original Linux
sendfile()
system call was not designed to handle large file offsets.
Consequently, Linux 2.4 added
sendfile64(),
with a wider type for the
offset
argument.
The glibc
sendfile()
wrapper function transparently deals with the kernel differences.
Applications may wish to fall back to
read(2)/write(2)
in the case where
sendfile()
fails with
EINVAL
or
ENOSYS.
If
out_fd
refers to a socket or pipe with zero-copy support, callers must ensure the
transferred portions of the file referred to by
in_fd
remain unmodified until the reader on the other end of
out_fd
has consumed the transferred data.
The Linux-specific
splice(2)
call supports transferring data between arbitrary file descriptors
provided one (or both) of them is a pipe.
SEE ALSO
copy_file_range(2),
mmap(2),
open(2),
socket(2),
splice(2)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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