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NAME
sendfile - transfer data between file descriptors
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.
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.
count is the number of bytes to copy between the file descriptors.
Presently (Linux 2.6.9): in_fd, must correspond to a file which sup‐
ports mmap(2)-like operations (i.e., it cannot be a socket); and out_fd
must refer to a socket.
Applications may wish to fall back to read(2)/write(2) in the case
where sendfile() fails with EINVAL or ENOSYS.
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.
ERRORS
EAGAIN Non-blocking 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.
EIO Unspecified error while reading from in_fd.
ENOMEM Insufficient memory to read from in_fd.
VERSIONS
sendfile() is a new feature in Linux 2.2. The include file <sys/send‐
file.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.
NOTES
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 refer to a regular file, and
sendfile() changed the current offset of that file.
open(2), mmap(2), socket(2), splice(2)