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, send-
       file() is more efficient 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.

       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.

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
       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 pro-
       grams.

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 refer to a regular file, and sendfile() changed the current offset of that file.

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


COLOPHON
       This page is part of release 3.25 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                                                      2010-02-15                                                SENDFILE(2)