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SuSE 11.3 - man page for sendfile (suse section 2)

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

sendfile - transfer data between file descriptors
#include <sys/sendfile.h> ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
sendfile() copies data between one file descriptor and another. Because this copying is done within the ker- nel, sendfile() is more efficient than the combination of read(2) and write(2), which would require transfer- ring 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 follow- ing the last byte that was read. If offset is not NULL, then sendfile() does not modify the current file off- set 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. Presently (Linux 2.6.9): in_fd, must correspond to a file which supports 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.
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.
sendfile() is a new feature in Linux 2.2. The include file <sys/sendfile.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.
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.
mmap(2), open(2), socket(2), splice(2)
This page is part of release 3.25 of the Linux man-pages project. A description of the project, and informa- tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2010-02-15 SENDFILE(2)

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