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

       sendfile()  copies  data between one file descriptor and another.  Because this copying is done within the kernel, sendfile() is more effi-
       cient 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 send-
       file() changes the file offset appropriately.

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  appropri-
       ately.

ERRORS

       EAGAIN Nonblocking 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/sendfile.h> is present since glibc 2.1.

CONFORMING TO

       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 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).

SEE ALSO

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

COLOPHON

       This  page is part of release 3.44 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2011-09-14							       SENDFILE(2)