Linux man pages : sendfile (2)
SENDFILE(2) Linux Programmer's Manual SENDFILE(2)
sendfile - transfer data between file descriptors
ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
This call copies data between one file descriptor and another. Either
or both of these file descriptors may refer to a socket (but see
below). in_fd should be a file descriptor opened for reading and
out_fd should be a descriptor opened for writing. offset is a pointer
to a variable holding the input file pointer position from which send-
file() will start reading data. When sendfile() returns, this variable
will be set to the offset of the byte following the last byte that was
read. count is the number of bytes to copy between file descriptors.
Because this copying is done within the kernel, sendfile() does not
need to spend time transferring data to and from user space.
Sendfile does not modify the current file pointer of in_fd, but does
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, please see the
TCP_CORK option in tcp(7) to minimize the number of packets and to tune
Presently the descriptor from which data is read cannot correspond to a
socket, it must correspond to a file which supports mmap()-like opera-
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.
EBADF The input file was not opened for reading or the output file was
not opened for writing.
EINVAL Descriptor is not valid or locked.
ENOMEM Insufficient memory to read from in_fd.
EIO Unspecified error while reading from in_fd.
sendfile is a new feature in Linux 2.2. The include file <sys/send-
file.h> is present since glibc2.1.
Other Unixes often implement sendfile with different semantics and pro-
totypes. It should not be used in portable programs.
Linux Man Page 1998-12-01 SENDFILE(2)