This call copies data between one file descriptor and another. Either or both of
these file descriptors may refer to a socket (but see below). in_fd
should be a file descriptor opened for reading and out_fd should be a
descriptor opened for writing. offset is a pointer to a variable
holding the input file pointer position from which sendfile() will
start reading data. When sendfile() returns, this variable will be set
to the offset of the byte following the last byte that was read. count
is the number of bytes to copy between file descriptors.
Because this copying is done within the kernel, sendfile() does not need
to spend time transferring data to and from user space.
Sendfile does not modify the current file pointer of in_fd, but does for
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, please see the TCP_CORK
option in tcp(7) to minimize the number of packets and to tune
Presently the descriptor from which data is read cannot correspond to a socket,
it must correspond to a file which supports mmap()-like operations.