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 current file offset of
in_fd;
otherwise the current file offset is adjusted to reflect
the number of bytes read from
in_fd.
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.
RETURN VALUE
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.
ERRORS
EAGAIN
Non-blocking 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.
EIO
Unspecified error while reading from
in_fd.
ENOMEM
Insufficient memory to read from
in_fd.
VERSIONS
sendfile()
is a new feature in Linux 2.2.
The include file
<sys/sendfile.h>
is present since glibc 2.1.
CONFORMING TO
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.
NOTES
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 3.14 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/.
