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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 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.

       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 can-
       not 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 Non-blocking 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 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 3.22 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                             2004-12-17                       SENDFILE(2)