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       sendfile - transfer data between file descriptors


       Standard C library (libc, -lc)


       #include <sys/sendfile.h>

       ssize_t sendfile(int out_fd, int in_fd, off_t *_Nullable 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).

       Before Linux 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.


       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 to indicate the error.


       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 seekable.


       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) and 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)