sendfile(2) is a UNIX system call which provides a "zero-copy" way of copying data from one file descriptor (a file) to another (a socket). Because this copying is done entirely within the kernel, sendfile(2) is more efficient than the combination of file.read() and socket.send(), which requires transferring data to and from user space. This copying of the data twice imposes some performance and resource penalties which sendfile(2) syscall avoids; it also results in a single system call (and thus only one context switch), rather than the series of read(2) / write(2) system calls (each system call requiring a context switch) used internally for the data copying. A more exhaustive explanation of how sendfile(2) works is available here, but long story short is that sending a file with sendfile() is usually twice as fast than using plain socket.send(). Typical applications which can benefit from using sendfile() are FTP and HTTP servers.
I recently contributed a patch for Python's socket module which adds a high-level socket.sendfile() method (see full discussion at BPO-17552). socket.sendfile() will transmit a file until EOF is reached by attempting to use os.sendfile(), if available, else it falls back on using plain socket.send(). Internally, it takes care of handling socket timeouts and provides two optional parameters to move the file offset or to send only a limited amount of bytes. I came up with this idea because getting all of that right is a bit tricky, so a generic wrapper seemed to be convenient to have. socket.sendfile() will make its appearance in Python 3.5.
sendfile and Python¶
sendfile(2) made its first appearance into the Python stdlib kind of late: Python 3.3. It was contributed by Ross Lagerwall and me in BPO-10882. Since the patch didn't make it into python 2.X and I wanted to use sendfile() in pyftpdlib I later decided to release it as a stand alone module working with older (2.5+) Python versions (see pysendfile project). Starting with version 3.5, Python will hopefully start using sendfile() more extensively, in details:
- BPO-13563: ftplib
- BPO-13559: httplib
- asyncio: there are some plans for this even though no actual patch yet, see discussion and BDFL involvement.
Also, Windows provides something similar to sendfile(2): TransmitFile. Now that socket.sendfile() is in place it seems natural to add support for it as well (see BPO-21721).
Backport to Python 2.6 and 2.7¶
For those of you who are interested in using socket.sendfile() with older Python 2.6 and 2.7 versions here's a backport. It requires pysendfile module to be installed. Full code including tests is hosted here.
#!/usr/bin/env python """ This is a backport of socket.sendfile() for Python 2.6 and 2.7. socket.sendfile() will be included in Python 3.5: http://bugs.python.org/issue17552 Usage: >>> import socket >>> file = open("somefile.bin", "rb") >>> sock = socket.create_connection(("localhost", 8021)) >>> sendfile(sock, file) 42319283 >>> """ import errno import io import os import select import socket try: memoryview # py 2.7 only except NameError: memoryview = lambda x: x if os.name == 'posix': import sendfile as pysendfile # requires "pip install pysendfile" else: pysendfile = None _RETRY = frozenset((errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK, errno.EINPROGRESS)) class _GiveupOnSendfile(Exception): pass if pysendfile is not None: def _sendfile_use_sendfile(sock, file, offset=0, count=None): _check_sendfile_params(sock, file, offset, count) sockno = sock.fileno() try: fileno = file.fileno() except (AttributeError, io.UnsupportedOperation) as err: raise _GiveupOnSendfile(err) # not a regular file try: fsize = os.fstat(fileno).st_size except OSError: raise _GiveupOnSendfile(err) # not a regular file if not fsize: return 0 # empty file blocksize = fsize if not count else count timeout = sock.gettimeout() if timeout == 0: raise ValueError("non-blocking sockets are not supported") # poll/select have the advantage of not requiring any # extra file descriptor, contrarily to epoll/kqueue # (also, they require a single syscall). if hasattr(select, 'poll'): if timeout is not None: timeout *= 1000 pollster = select.poll() pollster.register(sockno, select.POLLOUT) def wait_for_fd(): if pollster.poll(timeout) == : raise socket._socket.timeout('timed out') else: # call select() once in order to solicit ValueError in # case we run out of fds try: select.select(, [sockno], , 0) except ValueError: raise _GiveupOnSendfile(err) def wait_for_fd(): fds = select.select(, [sockno], , timeout) if fds == (, , ): raise socket._socket.timeout('timed out') total_sent = 0 # localize variable access to minimize overhead os_sendfile = pysendfile.sendfile try: while True: if timeout: wait_for_fd() if count: blocksize = count - total_sent if blocksize <= 0: break try: sent = os_sendfile(sockno, fileno, offset, blocksize) except OSError as err: if err.errno in _RETRY: # Block until the socket is ready to send some # data; avoids hogging CPU resources. wait_for_fd() else: if total_sent == 0: # We can get here for different reasons, the main # one being 'file' is not a regular mmap(2)-like # file, in which case we'll fall back on using # plain send(). raise _GiveupOnSendfile(err) raise err else: if sent == 0: break # EOF offset += sent total_sent += sent return total_sent finally: if total_sent > 0 and hasattr(file, 'seek'): file.seek(offset) else: def _sendfile_use_sendfile(sock, file, offset=0, count=None): raise _GiveupOnSendfile( "sendfile() not available on this platform") def _sendfile_use_send(sock, file, offset=0, count=None): _check_sendfile_params(sock, file, offset, count) if sock.gettimeout() == 0: raise ValueError("non-blocking sockets are not supported") if offset: file.seek(offset) blocksize = min(count, 8192) if count else 8192 total_sent = 0 # localize variable access to minimize overhead file_read = file.read sock_send = sock.send try: while True: if count: blocksize = min(count - total_sent, blocksize) if blocksize <= 0: break data = memoryview(file_read(blocksize)) if not data: break # EOF while True: try: sent = sock_send(data) except OSError as err: if err.errno in _RETRY: continue raise else: total_sent += sent if sent < len(data): data = data[sent:] else: break return total_sent finally: if total_sent > 0 and hasattr(file, 'seek'): file.seek(offset + total_sent) def _check_sendfile_params(sock, file, offset, count): if 'b' not in getattr(file, 'mode', 'b'): raise ValueError("file should be opened in binary mode") if not sock.type & socket.SOCK_STREAM: raise ValueError("only SOCK_STREAM type sockets are supported") if count is not None: if not isinstance(count, int): raise TypeError( "count must be a positive integer (got %s)" % repr(count)) if count <= 0: raise ValueError( "count must be a positive integer (got %s)" % repr(count)) def sendfile(sock, file, offset=0, count=None): """sendfile(sock, file[, offset[, count]]) -> sent Send a *file* over a connected socket *sock* until EOF is reached by using high-performance sendfile(2) and return the total number of bytes which were sent. *file* must be a regular file object opened in binary mode. If sendfile() is not available (e.g. Windows) or file is not a regular file socket.send() will be used instead. *offset* tells from where to start reading the file. If specified, *count* is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is updated on return or also in case of error in which case file.tell() can be used to figure out the number of bytes which were sent. The socket must be of SOCK_STREAM type. Non-blocking sockets are not supported. """ try: return _sendfile_use_sendfile(sock, file, offset, count) except _GiveupOnSendfile: return _sendfile_use_send(sock, file, offset, count)