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
       current  file  offset  of  in_fd;  otherwise the current 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 current 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.  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.

       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 also refer to a regular file, and sendfile() changed the current
       offset of that file.

       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.

       The  Linux-specific  splice(2)  call  supports transferring data between arbitrary files (e.g., a pair of
       sockets).

SEE ALSO

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.