sendfile — transfer data between file descriptors


#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 current file offset of in_fd; otherwise the current 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 current 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.


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.



Nonblocking 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.


Bad address.


Descriptor is not valid or locked, or an mmap(2)-like operation is not available for in_fd.


Unspecified error while reading from in_fd.


Insufficient memory to read from in_fd.


sendfile() is a new feature in Linux 2.2. The include file <sys/sendfile.h> is present since glibc 2.1.


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.


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.

In Linux 2.4 and earlier, out_fd could also refer to a regular file, and sendfile() changed the current offset of that file.

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 function transparently deals with the kernel differences.

Applications may wish to fall back to read(2)/write(2) in the case where sendfile() fails with EINVAL or ENOSYS.

The Linux-specific splice(2) call supports transferring data between arbitrary files (e.g., a pair of sockets).


mmap(2), open(2), socket(2), splice(2)


This page is part of release 3.52 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  This man page is Copyright (C) 1998 Pawel Krawczyk.

Permission is granted to distribute possibly modified copies
of this page provided the header is included verbatim,
and in case of nontrivial modification author and date
of the modification is added to the header.

$Id: sendfile.2,v 1.5 1999/05/18 11:54:11 freitag Exp $
2000-11-19 bert hubert <>: in_fd cannot be socket

2004-12-17, mtk
updated description of in_fd and out_fd for 2.6
Various wording and formatting changes

2005-03-31 Martin Pool <> mmap() improvements