sendfile

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



NAME
       sendfile - transfer data between file descriptors

SYNOPSIS
       #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 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).

       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
       sendfile() changes the file offset appropriately.

RETURN VALUE
       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.

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, 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.

       EOVERFLOW
              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.

VERSIONS
       sendfile() first appeared in Linux 2.2.  The include file
       <sys/sendfile.h> is present since glibc 2.1.

CONFORMING TO
       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.

NOTES
       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)/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.

SEE ALSO
       copy_file_range(2), mmap(2), open(2), socket(2), splice(2)

COLOPHON
       This page is part of release 5.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                        SENDFILE(2)