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 writ­
       ten 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.

       EINVAL Descriptor is not valid or locked.

       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 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)


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