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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 copy-
       ing is done within the kernel, sendfile() 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 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 cannot be a socket); and out_fd must refer to a

       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 Non-blocking I/O has been selected using  O_NONBLOCK  and  the  write  would

       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 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-

Linux                             2004-12-17                       SENDFILE(2)

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