sendfile() - Unix, Linux System Call
sendfile - transfer data between file descriptors
ssize_t sendfile(int out_fd, int in_fd, off_t *
offset ", size_t" " count" );
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
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.
offset is not NULL, then it points
to a variable holding the file offset from which
sendfile() will start reading data from
sendfile() returns, this variable
will be set to the offset of the byte following the last byte that was read.
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
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
(i.e., it cannot be a socket);
out_fd must refer to a socket.
Applications may wish to fall back to
in the case where
sendfile() fails with EINVAL or ENOSYS.
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
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.
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.
Non-blocking 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.
Descriptor is not valid or locked, or an
mmap()-like operation is not available for
Unspecified error while reading from
Insufficient memory to read from
sendfile() is a new feature in Linux 2.2.
The include file <sys/sendfile.h> is present since glibc2.1.
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.