sendfile - phpMan

SENDFILE(2) Linux Programmer’s Manual SENDFILE(2)

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
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 sendfile() will start reading data. When sendfile() returns, this vari-
able 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.

NOTES
Sendfile does not modify the current file pointer of in_fd, but does for out_fd.

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 performance.

Presently the descriptor from which data is read cannot correspond to a socket, it
must correspond to a file which supports mmap()-like operations.

RETURN VALUE
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
EBADF The input file was not opened for reading or the output file was not opened
for writing.

EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and the write would
block.

EINVAL Descriptor is not valid or locked.

ENOMEM Insufficient memory to read from in_fd.

EIO Unspecified error while reading from in_fd.

EFAULT Bad address.

VERSIONS
sendfile is a new feature in Linux 2.2. The include file <sys/sendfile.h> is
present since glibc2.1.

Other Unixes often implement sendfile with different semantics and prototypes. It
should not be used in portable programs.