目前来看没有系统对应的系统调用支持。
sendfile的原型如下，第一个参数必须是文件类型的描述符。只支持文件到其他buffer的zero copy传输。
参见：http://man7.org/linux/man-pages/man2/sendfile.2.html
#include <sys/sendfile.h>
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 read(2) and write(2), 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.
If offset is not NULL, then it points to a variable holding the file
offset from which sendfile() will start reading data from in_fd.
When sendfile() 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 sendfile() does not modify the file offset of in_fd;
otherwise the file offset is adjusted to reflect the number of bytes
read from in_fd.
If offset is NULL, then data will be read from in_fd starting at the
file offset, and the file offset will be updated by the call.
count is the number of bytes to copy between the file descriptors.
The in_fd argument must correspond to a file which supports
mmap(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
sendfile() changes the file offset appropriately.
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