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 current file offset of
in_fd;
otherwise the current 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 current 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. 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.
- 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.
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 also refer to a regular file, and
sendfile() changed the current offset of that file.
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.
The Linux-specific
splice(2) call supports transferring data between
arbitrary files (e.g., a pair of sockets).
SEE ALSO¶
mmap(2),
open(2),
socket(2),
splice(2)
COLOPHON¶
This page is part of release 3.74 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
http://www.kernel.org/doc/man-pages/.
