Section: Linux Programmer's Manual (2)
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,
is more efficient than the combination of
which would require transferring data to and from user space.
should be a file descriptor opened for reading and
should be a descriptor opened for writing.
is not NULL, then it points
to a variable holding the file offset from which
will start reading data from
returns, this variable
will be set to the offset of the byte following the last byte that was read.
is not NULL, then
does not modify the file offset of
otherwise the file offset is adjusted to reflect
the number of bytes read from
is NULL, then data will be read from
starting at 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
(i.e., it cannot be a socket).
In Linux kernels before 2.6.33,
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
Note that a successful call to
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
is set appropriately.
Nonblocking I/O has been selected using
and the write would block.
The input file was not opened for reading or the output file
was not opened for writing.
Descriptor is not valid or locked, or an
operation is not available for
This is not currently supported by
Unspecified error while reading from
Insufficient memory to read from
is too large, the operation would result in exceeding the maximum size of either
the input file or the output file.
is not NULL but the input file is not seekable.
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
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
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
option, described in
to minimize the number of packets and to tune performance.
In Linux 2.4 and earlier,
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
system call was not designed to handle large file offsets.
Consequently, Linux 2.4 added
with a wider type for the
wrapper function transparently deals with the kernel differences.
Applications may wish to fall back to
in the case where
refers to a socket or pipe with zero-copy support, callers must ensure the
transferred portions of the file referred to by
remain unmodified until the reader on the other end of
has consumed the transferred data.
call supports transferring data between arbitrary file descriptors
provided one (or both) of them is a pipe.
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