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 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.
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 writ
ten 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.
EINVAL Descriptor is not valid or locked.
ENOMEM Insufficient memory to read from in_fd.
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.
SEE ALSO
socket(2), open(2)
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