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
       This  call  copies  data between one file descriptor and another.  Either or both of these
       file descriptors may refer to a socket (but see below).  in_fd should be a file descriptor
       opened  for  reading  and  out_fd  should be a descriptor opened for writing.  offset is a
       pointer to a variable holding the input file pointer position from which  sendfile()  will
       start  reading  data.  When sendfile() returns, this variable will be set to the offset of
       the byte following the last byte that was read.  count is the  number  of  bytes  to  copy
       between file descriptors.

       Because  this  copying  is  done within the kernel, sendfile() does not need to spend time
       transferring data to and from user space.


NOTES
       Sendfile does not modify the current file pointer of in_fd, but does for out_fd.

       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, please see the TCP_CORK option in tcp(7) to
       minimize the number of packets and to tune performance.

       Presently the descriptor from which data is read cannot correspond to a  socket,  it  must
       correspond to a file which supports mmap()-like operations.


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
       EBADF  The input file was not opened for reading or the output file  was  not  opened  for
              writing.

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

       EINVAL Descriptor is not valid or locked.

       ENOMEM Insufficient memory to read from in_fd.

       EIO    Unspecified error while reading from in_fd.

       EFAULT Bad address.

VERSIONS
       sendfile  is  a  new  feature  in Linux 2.2.  The include file <sys/sendfile.h> is present
       since glibc2.1.

       Other Unixes often implement sendfile with different semantics and prototypes.  It  should
       not be used in portable programs.

SEE ALSO
       socket(2), open(2)



Linux Man Page                              1998-12-01                                SENDFILE(2)