sendfile man page on Oracle

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SENDFILE(2)		   Linux Programmer's Manual		   SENDFILE(2)

       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 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	 send‐
       file() changes the file offset appropriately.

       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.

       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.

       sendfile()  is a new feature in Linux 2.2.  The include file <sys/send‐
       file.h> is present since glibc 2.1.

       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.

       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 func‐
       tion 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).

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

       This page is part of release 3.53 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2011-09-14			   SENDFILE(2)

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