sendfile

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SENDFILE(2)                Linux Programmer's Manual               SENDFILE(2)NAME       sendfile - transfer data between file descriptorsSYNOPSIS       #include <sys/sendfile.h>       ssize_t sendfile(int out_fd, int in_fd, off_t *offset, size_t count);DESCRIPTION       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.       Presently (Linux 2.6.9): in_fd, must correspond to a  file  which  sup-       ports mmap(2)-like operations (i.e., it cannot be a socket); and out_fd       must refer to a socket.       Applications may wish to fall back  to  read(2)/write(2)  in  the  case       where sendfile() fails with EINVAL or ENOSYS.RETURN VALUE       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.ERRORS       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.VERSIONS       sendfile()  is a new feature in Linux 2.2.  The include file <sys/send-       file.h> is present since glibc 2.1.CONFORMING TO       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.NOTES       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 refer to  a  regular  file,  and       sendfile() changed the current offset of that file.SEE ALSO       mmap(2), open(2), socket(2), splice(2)COLOPHON       This  page  is  part of release 3.27 of the Linux man-pages project.  A       description of the project, and information about reporting  bugs,  can       be found at http://www.kernel.org/doc/man-pages/.Linux                             2010-02-15                       SENDFILE(2)