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,  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 file offset of in_fd; otherwise the 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
   file offset, and the file offset will be updated by the call.

   count is the number of bytes to copy between the file descriptors.

   The   in_fd   argument   must  correspond  to  a  file  which  supports
   mmap(2)-like operations (i.e., it cannot be a socket).

   In Linux kernels before 2.6.33, out_fd must refer to a  socket.   Since
   Linux  2.6.33  it  can  be  any  file.   If  it is a regular file, then
   sendfile() changes the file offset appropriately.

RETURN VALUE

   If the transfer was successful, the number of bytes written  to  out_fd
   is returned.  Note that a successful call to sendfile() may write fewer
   bytes than requested; the caller should be prepared to retry  the  call
   if there were unsent bytes.  See also NOTES.

   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, or count is negative.

   EINVAL out_fd has  the  O_APPEND  flag  set.   This  is  not  currently
          supported by sendfile().

   EIO    Unspecified error while reading from in_fd.

   ENOMEM Insufficient memory to read from in_fd.

   EOVERFLOW
          count  is too large, the operation would result in exceeding the
          maximum size of either the input file or the output file.

   ESPIPE offset is not NULL but the input file is not seek(2)-able.

VERSIONS

   sendfile()  first  appeared   in   Linux   2.2.    The   include   file
   <sys/sendfile.h> is present since glibc 2.1.

CONFORMING TO

   Not specified in POSIX.1-2001, nor in other standards.

   Other  UNIX  systems  implement sendfile() with different semantics and
   prototypes.  It should not be used in portable programs.

NOTES

   sendfile() will transfer  at  most  0x7ffff000  (2,147,479,552)  bytes,
   returning  the  number of bytes actually transferred.  (This is true on
   both 32-bit and 64-bit systems.)

   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 also refer to a regular file;
   this possibility went away in the Linux 2.6.x kernel  series,  but  was
   restored in Linux 2.6.33.

   The  original  Linux  sendfile() system call was not designed to handle
   large file offsets.  Consequently, Linux 2.4 added sendfile64(), with a
   wider  type  for  the  offset  argument.   The glibc sendfile() wrapper
   function transparently deals with the kernel differences.

   Applications may wish to fall back  to  read(2)/write(2)  in  the  case
   where sendfile() fails with EINVAL or ENOSYS.

   If  out_fd  refers  to a socket or pipe with zero-copy support, callers
   must ensure the transferred portions of the file referred to  by  in_fd
   remain  unmodified  until  the  reader  on  the other end of out_fd has
   consumed the transferred data.

   The Linux-specific splice(2) call supports  transferring  data  between
   arbitrary file descriptors provided one (or both) of them is a pipe.

SEE ALSO

   copy_file_range(2), mmap(2), open(2), socket(2), splice(2)

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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