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       If  offset  is  NULL, then data will be read from in_fd starting at the
31       current file offset, and the file offset will be updated by the call.
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33       count is the number of bytes to copy between the file descriptors.
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35       Presently (Linux 2.6.9): in_fd, must correspond to a  file  which  sup‐
36       ports mmap(2)-like operations (i.e., it cannot be a socket); and out_fd
37       must refer to a socket.
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39       Applications may wish to fall back  to  read(2)/write(2)  in  the  case
40       where sendfile() fails with EINVAL or ENOSYS.
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RETURN VALUE

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

47       EAGAIN Nonblocking I/O has been selected using O_NONBLOCK and the write
48              would block.
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50       EBADF  The input file was not opened for reading or the output file was
51              not opened for writing.
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53       EFAULT Bad address.
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55       EINVAL Descriptor is not valid or locked, or an mmap(2)-like  operation
56              is not available for in_fd.
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58       EIO    Unspecified error while reading from in_fd.
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60       ENOMEM Insufficient memory to read from in_fd.
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VERSIONS

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

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

73       If  you  plan  to use sendfile() for sending files to a TCP socket, but
74       need to send some header data in front of the file contents,  you  will
75       find  it  useful to employ the TCP_CORK option, described in tcp(7), to
76       minimize the number of packets and to tune performance.
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78       In Linux 2.4 and earlier, out_fd could refer to  a  regular  file,  and
79       sendfile() changed the current offset of that file.
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SEE ALSO

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

86       This  page  is  part of release 3.25 of the Linux man-pages project.  A
87       description of the project, and information about reporting  bugs,  can
88       be found at http://www.kernel.org/doc/man-pages/.
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92Linux                             2010-02-15                       SENDFILE(2)