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) and write
(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 file offset of in_fd
the file offset is adjusted to reflect the number of bytes read from
is NULL, then data will be read from in_fd
the 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
(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
() changes the file offset appropriately.
If the transfer was successful, the number of bytes written to out_fd
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.
- Nonblocking I/O has been selected using O_NONBLOCK
and the write would block.
- 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, or
count is negative.
- out_fd has the O_APPEND flag set. This is not
currently supported by sendfile().
- Unspecified error while reading from in_fd.
- Insufficient memory to read from in_fd.
- count is too large, the operation would result in
exceeding the maximum size of either the input file or the output
- offset is not NULL but the input file is not
() first appeared in Linux 2.2. The include file
is present since glibc 2.1.
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.
() 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
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
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
refers to a socket or pipe with zero-copy support, callers must
ensure the transferred portions of the file referred to by in_fd
unmodified until the reader on the other end of out_fd
has consumed the
The Linux-specific splice
(2) call supports transferring data between
arbitrary file descriptors provided one (or both) of them is a pipe.
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