SENDFILE(2) Linux Programmer's Manual SENDFILE(2)
NAME
sendfile - transfer data between file descriptors 在兩個文件描述符之間傳輸數據
SYNOPSIS
#include <sys/sendfile.h>
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
DESCRIPTION
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.
拷貝文件，因爲此拷貝過程在內核態完成，所以效率比使用讀寫函數更好，讀取函數通過用戶空間傳輸數據。
in_fd should be a file descriptor opened for reading and out_fd should
be a descriptor opened for writing.
in_fd指定以讀打開的文件，out_fd指定以寫打開。
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.
如果offset不爲空，從此偏移開始讀取，返回時設置此讀取後的偏移。
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.
如果偏移爲空，將從文件當前偏移讀取，完成調用後更新文件調用。
count is the number of bytes to copy between the file descriptors.
拷貝的字節數
The in_fd argument must correspond to a file which supports
mmap(2)-like operations (i.e., it cannot be a socket).
in_fd文件描述符必須支持內存映射，不能是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 send‐
file() changes the file offset appropriately.
2.6.33之前out_fd必須爲socket，之後可以爲任何文件。如果是常規文件，sendfile必變文件描述的偏移。
RETURN VALUE
If the transfer was successful, the number of bytes written to out_fd
is 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.
傳輸成功，字節數寫入out_fd,sendfile調用成功寫入的字節數可能少於請求的。如果有末發送的字節，應該重發。
On error, -1 is returned, and errno is set appropriately.
失敗返回-1，並設置errno
ERRORS
EAGAIN Nonblocking I/O has been selected using O_NONBLOCK and the write
would block.
EBADF The input file was not opened for reading or the output file was
not opened for writing.
EFAULT Bad address.
EINVAL Descriptor is not valid or locked, or an mmap(2)-like operation
is not available for in_fd, or count is negative.
EINVAL out_fd has the O_APPEND flag set. This is not currently sup‐
ported by sendfile().
EIO Unspecified error while reading from in_fd.
ENOMEM Insufficient memory to read from in_fd.
EOVERFLOW
count is too large, the operation would result in exceeding the
maximum size of either the input file or the output file.
ESPIPE offset is not NULL but the input file is not seek(2)-able.
VERSIONS
sendfile() first appeared in Linux 2.2. The include file <sys/send‐
file.h> is present since glibc 2.1.
CONFORMING TO
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.
NOTES
sendfile() 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.)
sendfile最多傳輸0x7ffff000字節
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(7), to
minimize the number of packets and to tune performance.
如果計劃使用sendfile將文件發送到TCP socket，但是需要在內容前發達一些頭數據。參考TCP_CORK選項。
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 Linux 2.6.33.
2.6.33之後可以爲普通文件
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() wrapper func‐
tion transparently deals with the kernel differences.
2.4之後，sendfile64，用來支大文件傳輸
Applications may wish to fall back to read(2)/write(2) in the case
where sendfile() fails with EINVAL or ENOSYS.
If out_fd refers to a socket or pipe with zero-copy support, callers
must ensure the transferred portions of the file referred to by in_fd
remain unmodified until the reader on the other end of out_fd has con‐
sumed the transferred data.
如果out_fd是socket或pipe，零拷貝支持，調用方必需確保in_fd保持末修改的狀態，直至out_fd寫完
The Linux-specific splice(2) call supports transferring data between
arbitrary file descriptors provided one (or both) of them is a pipe.
特定內核 splice調用支持在任務文件描述符之前傳遞數據
SEE ALSO
copy_file_range(2), mmap(2), open(2), socket(2), splice(2)
COLOPHON
This page is part of release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SENDFILE(2)