sendfile - transfer data between file descriptors
ssize_t sendfile(int out_fd, int in_fd, off_t
*offset, size_t count);
() 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)
, which would
require transferring data to and from user space.
should be a file descriptor opened for reading and out_fd
should be a descriptor opened for writing.
is not NULL, then it points to a variable holding the file
offset from which sendfile
() will start reading data from in_fd
() 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
() 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
is NULL, then data will be read from in_fd
the current file offset, and the file offset will be updated by the call.
is the number of bytes to copy between the file descriptors.
argument must correspond to a file which supports
-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
() changes the file offset appropriately.
If the transfer was successful, the number of bytes written to out_fd
returned. On error, -1 is returned, and errno
is set appropriately.
- Nonblocking I/O has been selected using O_NONBLOCK and the write
- The input file was not opened for reading or the output file was not
opened for writing.
- Bad address.
- Descriptor is not valid or locked, or an mmap(2)-like operation is
not available for in_fd.
- Unspecified error while reading from in_fd.
- Insufficient memory to read from in_fd.
() is a new feature in Linux 2.2. The include file
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.
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)
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
() 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
function transparently deals with the kernel differences.
Applications may wish to fall back to read(2)
in the case
() fails with EINVAL
The Linux-specific splice(2)
call supports transferring data between
arbitrary files (e.g., a pair of sockets).
This page is part of release 3.74 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at