sendfile(2)

1SENDFILE(2)                Linux Programmer's Manual               SENDFILE(2)
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NAME

6       sendfile - transfer data between file descriptors
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SYNOPSIS

9       #include <sys/sendfile.h>
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11       ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);
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DESCRIPTION

14       sendfile()  copies  data  between  one  file  descriptor  and  another.
15       Because this copying is done within  the  kernel,  sendfile()  is  more
16       efficient  than  the  combination  of read(2) and write(2), which would
17       require transferring data to and from user space.
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19       in_fd should be a file descriptor opened for reading and out_fd  should
20       be a descriptor opened for writing.
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22       If  offset  is  not NULL, then it points to a variable holding the file
23       offset from which sendfile() will start reading data from in_fd.   When
24       sendfile() returns, this variable will be set to the offset of the byte
25       following the last byte that was read.  If offset  is  not  NULL,  then
26       sendfile()  does not modify the current file offset of in_fd; otherwise
27       the current file offset is adjusted to reflect the number of bytes read
28       from in_fd.
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30       count is the number of bytes to copy between the file descriptors.
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32       Presently  (Linux  2.6.9):  in_fd, must correspond to a file which sup‐
33       ports mmap()-like operations (i.e., it cannot be a socket); and  out_fd
34       must refer to a socket.
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36       Applications  may  wish  to  fall  back to read(2)/write(2) in the case
37       where sendfile() fails with EINVAL or ENOSYS.
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NOTES

40       If you plan to use sendfile() for sending files to a  TCP  socket,  but
41       need  to  send some header data in front of the file contents, you will
42       find it useful to employ the TCP_CORK option, described in  tcp(7),  to
43       minimize the number of packets and to tune performance.
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45       In  Linux  2.4  and  earlier, out_fd could refer to a regular file, and
46       sendfile() changed the current offset of that file.
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RETURN VALUE

49       If the transfer was successful, the number of bytes written  to  out_fd
50       is returned.  On error, -1 is returned, and errno is set appropriately.
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ERRORS

53       EAGAIN Non-blocking  I/O  has  been  selected  using O_NONBLOCK and the
54              write would block.
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56       EBADF  The input file was not opened for reading or the output file was
57              not opened for writing.
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59       EFAULT Bad address.
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61       EINVAL Descriptor  is  not valid or locked, or an mmap()-like operation
62              is not available for in_fd.
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64       EIO    Unspecified error while reading from in_fd.
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66       ENOMEM Insufficient memory to read from in_fd.
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VERSIONS

69       sendfile() is a new feature in Linux 2.2.  The include file  <sys/send‐
70       file.h> is present since glibc2.1.
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CONFORMING TO

73       Not specified in POSIX.1-2001, or other standards.
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75       Other  Unix  systems  implement sendfile() with different semantics and
76       prototypes.  It should not be used in portable programs.
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