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       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 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


       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.


       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

       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.

              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.


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


       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.


       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.


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


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